Genetic Information, variation and relationships between them Flashcards
DNA function
Used to store all the genetic information (all the instructions an organism needs to grow and develop from a fertilised egg to a fully grown adult)
DNA molecule
Double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs
3 components of a nucleotide
Phosphate, deoxyribose sugar, nitrogenous base
What are the 4 nitrogenous bases in DNA?
Adenine, thymine, cytosine and guanine
Which nitrogenous bases are complementary to each other?
Adenine + thymine
Cytosine + guanine
What are the 2 types of nitrogenous bass?
Purines and pyridamines
What’s a purine and some examples?
Nitrogenous bases with 2 carbon rings such as adenine and guanine
What’s a pyridamine and some examples?
Nitrogenous base with 1 carbon ring such as thymine and cytosine
2 mononucleotides join together to make…
Dinucleotide
Nucleotides join together to form polynucleotides through what type of reaction?
Condensation reaction between the phosphate group of one and the pentose sugar of another
What type of bond is created between nucleotides after a condensation reaction?
Phosphodiester
The chain of sugars and phosphate is known as what?
Sugar phosphate backbone of the DNA molecule
Two polynucleotide strands are joined together to form what?
A double helix
Two polynucleotide strands are joined together due to what?
Hydrogen bonds between nitrogenous bases and due to complementary base pairing, adenine pairs with thymine and cytosine pairs with guanine
How many hydrogen bonds form between adenine and thymine?
2
How many hydrogen bonds form between cytosine and guanine?
3
Functions of part of DNA molecule
- sugar phosphate backbone= provides strength and stability along with protecting the more reactive organic bases inside the double helix
- DNA coiled up into double helix= compact = lots of information can be stored in the molecule
- order of nitrogenous bases= carry information for protein synthesis
- complementary base pairing= molecule replicate itself accurately
- weak hydrogen bonds between bases = can easily be broken = little energy needed = easier for replication
RNA function
Transfer genetic information from DNA to ribosome for protein synthesis (only carry information for one gene)
RNA structure
Single stranded nucleic acid that’s small enough to fit through nuclear pores- composed of phosphate group, ribose sugar, adenine, uracil, cytosine and guanine
Differences between DNA and RNA
- DNA is double stranded whereas RNA is single stranded
- DNA contains complementary base pairing between nitrogenous bases whereas RNA doesn’t contain complementary base pairing
- DNA contains the nitrogenous bases adenine, thymine, cytosine and guanine whereas RNA contains adenine, uracil, cytosine and guanine
- DNA contains deoxyribose sugar whereas RNA contains ribose sugar
- different functions- DNA is responsible for storing genetic information whereas RNA is responsible for transferring genetic information from DNA to the ribosome for protein synthesis
What is a gene?
A section of DNA at a particular position, a locus, on a DNA molecule
Function of genes?
A base sequence of DNA that codes for the amino acid sequence of a polypeptide or functional RNA, including ribosomal RNA (rRNA) and transfer RNA (tRNA)
In trying to discover how DNA bases coded for amino acids, why did scientists suggested that there must be a minimum of 3 bases that coded for each amino acid?
- only 20 amino acids regularly occur in proteins
- each amino acid must have its own code of bases on the DNA
- only 4 different bases present in DNA
- if each base coded for a different amino acid, only 4 different amino acids could be coded for
- using a pair of bases (4x4= 16) different codes are possible but inadequate
- three bases (4X4X4= 64) produces different codes, more than enough to satisfy the requirement for 20 amino acids
As the genetic code has 3 bases for each amino acid, each one is called
A triplet
What are the main features of the genetic code?
1- degenerate= many amino acids are coded for by more than one triplet
2- non-overlapping= each base in the sequence is only read only once
3- universal= with a few minor exceptions, each triplet codes for the same amino acid in all organisms
Which feature of the genetic code is indirect evidence for evolution?
The genetic code being universal
Much of the DNA in eukaryotes doesn’t code for polypeptides, what are some reasons for this?
- between genes there are non-coding sequences made up of multiple repeats of base sequences
- even within genes, only certain sequences code for amino acids called exons, within the gene, these exons are separated by further non-coding sequences called introns
- some genes code for rRNA and tRNA
Explain how a change in one base along a DNA molecule may result in a non-functional enzyme
- base sequence change= may code for a different amino acid
- the sequence of amino acids in polypeptide produced will be different
- change to primary structure of a protein
- might result in different tertiary shape
- enzyme’s active site may be different and might not fit the substrate -enzyme-substrate complex cannot be formed so the enzyme is non-functional
Eukaryotic DNA characteristics
- long
- linear
- occur in association with protein molecules called histones to form chromosomes
- DNA contains introns
Histone function
DNA in chromosomes is held by histones
Prokaryotic DNA characteristics
- short
- circular
- not associated with protein molecules
- so no chromosomes
- DNA doesn’t contain introns
Which organelles contain their own DNA?
Mitochondria and chloroplasts which is like prokaryote DNA: short, circular and not associated with proteins
Why are chromosomes not visible in non-dividing cells?
Because the chromatin are spread through the nucleus
How is DNA packed into a chromosome?
- DNA is combined with histones
- DNA-histone complex is coiled
- coils fold to form loops
- loops coil and pack together to form the chromosome
2 types of RNA that are important in protein synthesis
- mRNA (messenger RNA)
- tRNA (transfer RNA)
mRNA characteristics
- consisting of thousands of mononucleotides
- long strand that is arranged into a single helix
- base sequence of mRNA is determined by the sequence of bases on a length of DNA in transcription
- once mRNA is formed, mRNA leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm where it associates with ribosomes
How is mRNA adapted to its function?
It possesses information in the form of codons- the sequence of codons determines the amino acid sequence of a specific polypeptide that will be made
What is tRNA’s function?
Carries amino acids to the ribosome for protein synthesis
What are tRNA’s characteristics?
- relatively small molecule made up of around 80 nucleotides
- single stranded chain folded into a clover-leaf shape (hydrogen bonds between specific base pairs hold the molecule in this shape) , with one end of the chain extending beyond the other- part of the tRNA molecule to which an amino acid can easily attach (amino acid binding site)
- every tRNA molecule has a specific sequence of three bases at opposite end to amino acid binding site called an anticodon
- many types of tRNA, each of which bind to a specific amino acid
- chemically more stable than mRNA but less stable than DNA
Where is tRNA manufactured in eukaryotic and prokaryotic cells?
Eukaryotic = nucleus Prokaryotic= cytoplasm
Each tRNA molecule is specific to
One amino acid and has an anticodon that is specific to that amino acid
What are the two main processes involved in protein synthesis called?
- transcription
- translation
What is transcription?
The process of making pre-mRNA using a strand of DNA as a template
Describe the stages of transcription
- RNA polymerase, binds to a sequence of DNA called the promotor causing the 2 strands of DNA to separate and expose their nucleotide bases. RNA polymerase builds on a strand in the 5’ to the 3’ direction, adding each new nucleotide to the 3’ end of the strand
- one of the DNA strands is then used as a template to make an mRNA copy
- RNA polymerase lines up free nucleotides alongside the exposed bases on the template strand. Specific complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand, except uracil pairs with adenine
- once the RNA nucleotides have paired up with their specific bases on the DNA strand, they’re joined together by RNA polymerase forming a pre-mRNA molecule
- when the RNA polymerase reaches a particular sequence of bases on the DNA (terminators), that it recognises as a ‘stop’ triplet code, it detaches and the production of pre-mRNA is complete- in eukaryotes pre-mRNA moves out of the nucleus through a nuclear pore
Why is splicing necessary in eukaryotic cells?
The DNA of a gene from eukaryotic cells contains introns and exons. These intervening introns would prevent the synthesis of a polypeptide. The mRNA molecules are too large to diffuse out of the nucleus, and so once they’ve been spliced, they leave via the nuclear pore.
What is translation?
Synthesis of a polypeptide from an mRNA molecule
There are about how many different tRNA?
60
A particular tRNA has a specific
Anticodon and attaches to a specific amino acid
What is splicing?
Where introns are removed and exons are joined together
Where does translation occur in eukaryotes and prokaryotes?
At the ribosome in the cytoplasm
Explain the stages of translation
1- the mRNA molecule attaches to the ribosome and tRNA molecules carry amino acids to it- ATP provides the energy needed for the bond between amino acid and tRNA molecule to form
2- a tRNA molecule (carrying an amino acid) with an anticodon that’s complementary to the first codon on the mRNA, attaches itself to the mRNA by specific base pairing
3- a second tRNA molecule attaches itself to the next codon in the same way
4- the two amino acids attached to the tRNA molecules are joined by a peptide bond. The first tRNA molecule moves away leaving behind its amino acid
5- a third tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first 2 amino acids and the the tRNA molecule moves away
6- this process continues, producing a chain of linked amino acids, a polypeptide chain, until there’s a stop codon on the mRNA molecule
7- the polypeptide chain moves away from the ribosome and translation is complete
A strand of mRNA has 64 codons but the protein produced from it has only 63 amino acids. Suggest a reason for this difference
One of the codons is a stop codon that indicated the end of protein synthesis and stop codons don’t code for amino acids, so there’s one less amino acid than codons
What is a mutation?
Any change to the quantity or the base sequence of the DNA of an organism
What is a gene mutation?
Any change to one or more nucleotide bases or a change in the sequence of bases
Mutations that affect large sections of DNA are lethal because
The protein isn’t expressed
Many mutations affect the tertiary structure of a protein molecule meaning that
The protein can’t function properly
Give an example of when a mutation may be beneficial?
A more efficient enzyme
2 types of mutation we need to know are
Substitution and deletion
What is substitution?
Replacement of one nucleotide with another that contains a different base
What is deletion?
The loss of a nucleotide, also known as frame-shift mutation
Which type of mutation may often be more dangerous?
Deletion due to the effect of frame-shift mutation as all the codons after the mutation will be affected
Why may a mutation have no effect?
- it occurs in a non-coding region of DNA
- a different codon may produce the same amino acid (genetic code is degenerate)
- altered amino acid may not affect the protein’s shape or function
How do mutations arise?
- spontaneously through DNA replication when a cell divides by mitosis or meiosis
- by mutagenic agents
What’s a mutagenic agent?
Anything that can increase the rate of gene mutation?
Mutagens are also what in higher organisms?
Carcinogenic
Give some examples of mutagenic agents
- ionising radiation e.g. alpha and gamma rays
- UV radiation
- certain chemicals e.g. mustard gas, thalidomide, benzo (a) pyrene in cigarette smoke
What are chromosome mutations?
Changes in the structure or number of whole chromosomes- they can arise spontaneously and take 2 forms
What are the 2 forms of chromosome mutations?
Changes in whole sets of chromosomes and changes in the number of individual chromosomes
Changes in whole sets of chromosomes occur when
Organisms have 3 or more sets of chromosomes rather than the usual 2; this condition is called polyploidy
Polyploidy occurs mostly in
Plants
Explain why sometimes changes in the number of individual chromosomes occur
Sometimes, individual pairs of homologous chromosomes fail to separate during meiosis; known as non-disjunction
What result does non-disjunction have on gametes?
Non-disjunction usually results in a gamete having either one more or one fewer chromosome
What’s an example of non-disjunction in humans?
Down’s syndrome- individuals have an additional chromosome 21
Cell division occurs in one of two ways
Mitosis and meiosis
What is the result of mitosis vs meiosis?
Mitosis- results in 2 daughter cells with the same number of chromosomes as the parent cells and each other
Meiosis- results in 4 daughter cells, each with half the number of chromosomes as the parent cell
Explain the importance of meiosis
In sexual reproduction, 2 gametes fuse together to give rise to new offspring. If each gamete had a diploid number of chromosomes, then the cell they would produce would have double this number; this doubling of the number of chromosomes would continue at each generation. It follows that, in order to maintain a constant number of chromosomes in the adult of a species, the number of chromosomes must be halved at some stage in the life cycle- this halving occurs as a result of meiosis
In most animals, meiosis occurs in the formation of
Gametes
What is a gamete?
A reproductive sex cell that contains haploid number of chromosomes and fuses with another gamete during fertilisation
Give an example of a plant where gametes are produced by mitosis
Fern
In the fern life cycle, meiosis occurs in the formation of
Spores
Every diploid cell of an organism has 2 complete sets of chromosomes, one set produced by
Each parent
Meiosis involves how many nuclear divisions?
2
Describe the first division (meiosis 1) of meiosis
Homologous pairs of chromosomes pair up and their chromatids wrap around each other- equivalent portions of these chromatids may be exchanged in a process called crossing over. By the end of this division, the homologous chromosomes pairs have separated with one chromosome from each pair going into 2 daughter cells
Describe the second division (meiosis 2) of meiosis
The chromatids move apart. At the end of meiosis 2, four cells have usually been formed- in humans, each of these cells contains 23 chromosomes
Meiosis brings about genetic variation in 2 ways
- independent segregation of homologous chromosomes
- new combinations of maternal and paternal alleles by crossing over
What are homologous chromosomes?
A pair of chromosomes, one maternal and one paternal, that have the same gene loci, and therefore determine the same characteristics
What is an allele?
One number of alternative forms of a gene e.g. Blood group
How does independent segregation lead to genetic variation?
Each homologous pair of chromosomes in our cells is made up of one maternal chromosome and one paternal. When homologous pairs separate in meiosis 1 it is completely random which chromosome from each pair ends up in the daughter cell. So the 4 daughter cells produced by meiosis have completely different combinations of those maternal and those paternal
Explain how crossing over leads to variation
- During meiosis 1, each chromosome lines up against its homologous partner
- the chromatids of each pair become twisted about one another
- during this twisting process, tensions are created and portions of chromatids break off
- these broken portions might then rejoin with the chromatids of its homologous partner.
- usually it is equivalent portions of homologous chromosomes that are exchanged
- in this way, new genetic combinations of maternal and paternal alleles are produced
Mathematical equation based on the number of chromosomes in an organism to determine the number of possible combinations of chromosomes for each daughter cell
2 (to the power of n)
N= number of pairs of homologous pairs of chromosomes
An organism with 4 homologous pairs = 16 different combinations of chromosomes of maternal and paternal origin are possible
Variety from random pairing of male and female gametes mathematical equation
(2 to the power of n) (2)
N= number of pairs of homologous chromosomes
Using the example of 4 homologous chromosomes there are 256 different combinations of chromosomes in the offspring produced as the result of sexual reproduction
A mule is a cross between a horse (64 chromosomes) and a donkey (62 chromosomes). Mules therefore have 63 chromosomes. From your knowledge of meiosis suggest why mules are sterile
Gametes are produced by meiosis. In meiosis, homologous pairs of chromosomes line up. With 63, chromosome precise pairings are impossible. This prevents meiosis and gamete production
What is a species?
A group of similar organisms that can breed together to produce fertile offspring- can consist of one or more populations
What is a population?
Group of individuals of the same species that live in a similar place and interbreed
All members of the same species have the same genes e.g. all humans have a gene for blood group- which blood group humans have depends on
Which two alleles for blood group they inherited
Organisms of the same species differ in their _______ not their genes
Combination of alleles
What is genetic diversity?
The total number of different alleles in a population
The greater the number of different alleles that all members of the species posses, the greater the
Genetic diversity of that species
Genetic diversity is reduced when
A species has fewer different alleles
The greater the genetic diversity, the more likely that some individuals in a population will survive an environmental change. Why is this?
Because of a wider range of alleles and therefore a wider range of characteristics. This gives a greater probability that some individuals will posses a characteristic that suits it to the new environmental conditions
Genetic diversity is a factor that enables what to occur?
Natural selection
Not all alleles of a population are equally likely to be passed to the next generation; this is because only certain individuals are
Reproductively successful
Differences between the reproductive success of individuals affects allele frequency in a population. Explain the process:
- within any population of a species, there will be a gene pool containing a wide variety of alleles
- random mutations of alleles within this gene pool may result in a new allele or a gene, which in most cases will be harmful
- however, in certain environments, the new allele or a gene might give its possessor an advantage over other individuals in the population
- these individuals will be better adapted and therefore more likely to survive in competition with others
- these individuals are more likely to obtain the available resources and so grow more rapidly and live longer. As a result, they will have a better chance of breeding successfully and producing more offspring
- only those individuals that reproduce successfully will pass on their alleles to the next generation
- therefore it is the new allele that gave the parents an advantage in the competition for survival, that is most likely to be passed onto the next generation
- as these new individuals also possess the new ‘advantageous allele’, they in turn are more likely to survive and reproduce successfully
- over many generations, the number of individuals with the ew ‘advantageous allele’ will increase at the expense of the individuals with the ‘less advantageous’ allele
- over time, the frequency of the new advantageous allele in the population increases while that of the ‘non-advantageous’ decreases
Explain how a difference in its DNA might lead to an organism having a different appearance and hence the species showing greater diversity
Different DNA = different amino acids coded for
Different amino acids= different protein shape
Different protein function= change in feature determined by that protein = altered appearance = greater genetic diversity
What is selection?
Process by which organisms that are better adapted to their environment tend to survive and breed, while those that are less well adapted tend not to
What is directional selection?
Selection may favour individuals that vary in one direction from the mean of the population and changes the characteristics of the population
What is stabilising selection?
Selection may favour average individuals and means characteristics of the population are reserved
Most characteristics are influenced by more than one gene (polygenic); these types of characteristics are more influential by the ____ than ones determined by a single gene
Environment
The effect of the environment on polygenes produces individuals in a population that vary about a
Mean
When you plot the effect of environment on polygenes producing individuals in a population that vary about a mean you get a
Normal distribution curve
Explain how directional selection works
if the environmental conditions change, the phenotypes that are best suited to the new conditions are most likely to survive and breed. They will therefore contribute more offspring, and the alleles these offspring possess to the next generation, than other individuals. Over time, the mean will move in the direction of these individuals
Directional selection- example of development of antibiotic resistance in bacteria (not due to tolerance of antibiotics but random mutations). Explain the penicillin example
- spontaneous mutation in allele of gene in bacterium = allowed it to make new protein. New enzyme (penicillinase) broke down penicillin before it was able to kill bacterium
- bacterium happened by chance to be in a situation where penicillin was being used to treat an individual. In these circumstances, mutation gave bacterium an advantage in using penicillinase to break down the antibiotic and so survive while the rest of population killed
- surviving bacterium divided by binary fission to build up a small population of penicillin- resistance
- members of this small population more able to survive and multiply in presence of penicillin than members of non-resistant population
- the population of penicillin-resistant bacteria increased at the expense of the non-resistant population
- consequently, the frequency of the allele that had enabled the production of penicillinase increased in the population
- population’s distribution curve shifted in the direction of a population having greater resistance to penicillin
Stabilising selection tends to eliminate
Phenotypes at the extremes
Stabilising selection reduces range of
Characteristics
Explain how human birth weight is an example of stabilising selection
1- humans have a range of birth weights
2- very small babies are less likely to survive (partly because they find it very difficult to maintain their body temperature)
3- giving birth to large babies is often extremely difficult and to some extent impossible, so large babies are less likely to survive too
4- conditions are most favoured for medium-sized babies- so weight of human babies tends to shift towards the medium of the range
How does human birth weight illustrate directional stabilisation?
Because the mortality rate is higher at the two extremes so the population’s characteristics are preserved rather than changed
Natural selection results in species that are better adapted to
The environment they live in
3 adaptions involved in natural selection (and examples)
1- anatomical e.g. shorter ears and thicker fur in Arctic foxes compared to foxes in warmer climates
2- physiological e.g. oxidising of fat rather than carbohydrates in kangaroo rats to produce additional water in a dry deserted area
3- behavioural e.g. Autumn migration of swallows from the UK to Africa to avoid food shortages over winter
What is classification?
Organisation of living organisms into groups
A species is the basic unit of
Classification
What is the binomial system? (Naming of species)
- Organisms are identified by 2 names
- it is a universal system based upon Latin or Greek names
- first name called the generic name, denotes the genus to which the organism belongs
- the second name called the specific name denotes the species to which an organism belongs
State the 3 rules that are applied to the use of the binomial system in scientific writing
1- the names are printed in italics, or if it is handwritten,they are underlined to indicate that they are scientific names
2- first letter of generic name is in upper case, but the specific name is lower case
3- if the specific name is not known it can be written as ‘sp’ e.g. Felix sp
Why was the binomial system created?
At one time, scientists often gave new organisms a name that described their features e.g. ‘Blackbird’ or ‘rainbow trout’. This practice resulted in the same name being used in different parts of the world for very different species. Therefore, it was very difficult for scientists to be sure they were referring to the same organism.
(Courtship behaviour)how does members of the same species having similar genes help them to distinguish members of their own species from those of other species?
Members of the same species have the same/ similar genes, therefore they resemble one another physically and biochemically
(Courtship) the behaviour of members of the same species is more alike than behaviour from other species so
Individuals can therefore recognise members of their own species by the way they act
Like the physical and biochemical factors of a species, the ability to display a behaviour is ___ determined
Genetically
When it comes to the survival of a species, what 2 things are essential?
Courtship and mating
Courtship behaviour helps to ensure that mating is successful and that offspring has the maximum chance of survival by enabling individuals to:
- recognise members of their own species (to ensure that mating only takes place between members of the same species because only members of the same species can reproduce and produce fertile offspring)
- identify a mate that is capable of mating ( both partners need to be sexually mature, fertile and receptive to mating)
- form a pair bond (that will lead to successful mating and raising of offspring
- synchronise mating (so that it takes place when there is maximum probability of the sperm and egg meeting)
- become able to breed ( by bringing a member of the opposite sex into a physiological state that allows breeding to occur)
The females of many species undergo a cycle of sexual activity in which they can only conceive during a very short time. They are often only receptive to mating for a period around the time when they produce eggs. Courtship behaviour is used by males to
Determine whether the female is at this receptive stage; if she responds with the appropriate behavioural response, courtship continues and is likely to result in the production of offspring
Describe the processes that lead to a reduction in the genetic diversity of a population
- founder effect
- genetic bottlenecks
- selective breeding/artificial selection
What is the founder effect and how does it effect genetic diversity?
Occurs when a few individuals from a population colonise a new region
This means that:
- few individuals from population become isolated
- these individuals carry with them only a small fraction of the alleles of the population as a whole = reduction in variety of alleles so a reduced gene pool
What is a genetic bottleneck and how does it effect genetic diversity?
An event that causes a significant reduction in population I.e. volcanic eruption
This means that there a few survivors left that will possess a much smaller variety of alleles than the original population = reduction in number of alleles in the gene pool= reduction in genetic diversity
During courtship animals use signals to communicate with a potential mate and typically there is a _____ between the male and the female
Chain of actions
The chain of actions is the same for all members of the same species but different for members of different species. In this way, both individuals recognise
That their partner is of the same species and that they may be prepared to mate
Explain why species recognition is important in courtship
To ensure that mating only takes leave between members of the same species as only they can produce fertile offspring
Suggest a way in which the courtship behaviour of one species might be used to determine which of two other species is most closely related to it
The courtship display that most closely resembles that of the first species is likely to be most related
Classification vs taxonomy
Classification is simply the grouping of organisms whereas taxonomy is the science of classification-it involves naming organisms and organising them into groups that make sit easier to identify and study them. Scientists now take into account phylogeny when classifying organisms and group organisms according to their evolutionary relationships
What are the two forms of biological classification?
Artificial classification and phylogenetic classification
What is artificial classification?
Divides organisms according to differences that are useful at the time e.g. colour, size, number of legs, leaf size which are described as analogous characteristics where they have the same function but do not have evolutionary origin e.g. wings of birds and butterflies are both used for flight but originated in different ways
What is phylogenetic classification?
- Classification based upon evolutionary relationships between organisms and their ancestors
- classifies species into groups using shared features derived from their ancestors
- arranges the groups into a hierarchy in which the groups are contained within larger composite groups with no overlap
Relationships in phylogenetic classification are partly based on homologous characteristics. What are these?
Homologous characteristics have similar evolutionary origins regardless of their function e.g. the wing of a bird, the arm of a human and the front leg of a horse all have the same basic structure and evolutionary origin, therefore homologous
Each group within a phylogenetic biological classification is called a
Taxon
Taxonomy straight definition
The study of taxons and their positions in hierarchical order, where they are known as taxonomic ranks
What are taxonomic ranks based upon?
The evolutionary line of descent of group members
A domain is the highest is the highest taxonomic rank and 3 are recognised:
Bacteria, Archaea and Eukarya
What are the characteristics of ‘bacteria’?
Absence of membrane-bound organelles e.g. nucleus and mitochondria, ribosomes are smaller than in eukaryotic, cell’s cell wall present and made of murein, single loop of naked DNA molecule made up of nucleic acids,but no histones
What are the characteristics of ‘Archaea’?
Group of single-celled prokaryotes that were classified as bacteria
They differ from bacteria because
- their genes and protein synthesis are more similar to eukaryotes, no murein in their cell walls, they have a more complex form of RNA polymerase
What are the characteristics of ‘Eukarya’?
A group of organisms made up of one or more eukaryotic cells
- cell possesses membrane bound organelles such as mitochondria, possess cells with a cell wall that don’t contain murein, ribosomes are larger than in bacteria and Archarea
Classification order
1- domain 2- kingdom 3- phylum 4- class 5- order 6- family 7- genus 8- species
Way to remember classification order
Some girls favour orange cellophane panties kinky darling
The phylogenetic (evolutionary relationship) relationship of different species are usually represented by what kind of diagram?
Tree-line diagram called a phylogenetic tree
What are the 4 main stages in the immune response?
1- phagocytes engulf pathogens by phagocytosis
2- phagocytes activate T-cells
3- T-cells activate B-cells, which divide into plasma cells
4- plasma cells make more antibodies to a specific antigen
Who worked out the structure of DNA?
Watson and Crick, 1953, followed by the pioneering work of Rosalind
Why is DNA a stable molecule?
1- the phosphodiester backbone protects the more chemically reactive organic bases inside the double helix
2- hydrogen bonds link the nitrogenous base pairs- as there are 3 hydrogen bonds between cytosine and guanine, the higher the proportion of C-G pairings, the more stable the DNA molecule
How is DNA molecule adapted to carry out its function?
1- very stable molecule which normally passes from generation to generation without change; only rarely does it mutate
2- its two separate strands are held together only by hydrogen bonds, which allow the two strands to separate quickly and easily during DNA replication and protein synthesis
3- extremely large molecule and therefore carries an immense amount of genetic information
4- by having the base pairs within the phosphodiester backbone, the genetic information, is to some extent, protected from being corrupted by outside chemical and physical forces
5- base pairing leads to DNA being able to replicate and to transfer information as mRNA
The cells that make up organisms are always derived from existing cells by the process of division. Cell division occurs in two ways:
1- Nuclear division: process by which nucleus divides- there are two types of nuclear division: mitosis and meiosis
2- cytokinesis: follows nuclear division and is the process by which the whole cell divides
Before a nucleus divides,its DNA must be replicated- this is to ensure
All the daughter cells have the genetic information to produce produce the enzymes and other proteins that they need
What is the name of DNA replication?
Semi-conservative replication
For semi-conservative replication to take place, there are 4 requirements:
1- 4 types of nucleotide each with their bases of adenine, thymine, cytosine or guanine must be present
2- both strands of the DNA molecule act as a template for the attachment of these nucleotides
3- the enzyme DNA polymerase
4- a source of chemical energy is used to drive the process
Explain the stages of semi-conservative DNA replication
1- the enzyme DNA helicase breaks the hydrogen bonds linking the nitrogenous bases
2- as a result, the double helix separates into its two strands and unwinds
3- each exposed polynucleotide strand then acts as a template to which complementary free nucleotides bind due to complementary base pairing e.g. A-T and C-G
4- nucleotides are joined together in a condensation reaction by the enzyme DNA polymerase to form the ‘missing’ polynucleotide strand on each of the two original polynucleotide strands of DNA
5- each of the new DNA molecules contains one of the original DNA strands = semi-conservative
Explain why DNA polymerase moves in opposite ways along the antiparallel DNA strand
- each end of a DNA strand is slightly different in its structure. One end is called the 3’ end (three prime) and one end is called the 5’ end (five prime) end. In a DNA helix, the strands run in opposite directions- they’re antiparallel
- the active site of DNA polymerase is only complementary to the 3’ end of the newly forming DNA strand- so the enzyme can only add nucleotides to the new strand at the 3’ end
- this means that the new strand is made in a 5’ to 3’ direction and that DNA polymerase moves down the template strand in a 3’ to 5’ direction. As the strands in the double-helix are antiparallel, DNA polymerase working on one of the template strands moves in the opposite direction to the DNA polymerase working on the other template strand
Meselson and Stahl showed that DNA is replicated using the semi-conservative method. Their experimenting used to isotopes of nitrogen:
Heavy nitrogen 15N and its isotope light nitrogen 14N
(Small 15 above N)
How did Meselson and Stahl provide evidence for semi-conservative replication?
1- two samples of bacteria were grown- one in a nutrient broth containing light nitrogen and one in a broth with heavy nitrogen. As the bacteria reproduced, they took up nitrogen from the broth to help make nucleotides for new DNA; so the nitrogen gradually became part of the bacteria’s DNA
2- a sample of DNA was taken from each batch of bacteria and spun in a centrifuge. The DNA from the heavy nitrogen bacteria settled lower down the centrifuge than the light nitrogen bacteria, because it’s heavier
3- then the bacteria grown in heavy nitrogen broth were taken out and put in a broth containing only light nitrogen. The bacteria were left for one round of DNA replication, and then another DNA sample was out and spun in the centrifuge
4- if replication was conservative, the original heavy DNA, would settle at the bottom and the new light DNA would settle at the top
5- if replication was semi-conservative the new bacterial DNA molecules would contain one strand of the old DNA containing heavy nitrogen and one strand of new DNA containing light nitrogen, so the DNA would settle out between where the light nitrogen settled out and where heavy nitrogen settled out
6- as it turned out, the DNA settled out in the middle, snowing that the DNA molecules contained a mixture of heavy and light nitrogen- the bacterial DNA had replicated semi-conservatively in the light nitrogen
Once Meselson and Stahl has confirmed that DNA replication in bacteria was semi-conservative
, other scientists carried out experiments to show that it was the universal method for DNA replication in all living things
What’s the key difference between conservative and semi-conservative theories of DNA replication?
Semiconservative replication would produce two copies that each contained one of the original strands and one new strand. Conservative replication would leave the two original template DNA strands together in a double helix and would produce a copy composed of two new strands containing all of the new DNA base pairs.
What is biodiversity?
General term used to describe variety in the living world
Biodiversity refers to the number and variety of living organisms in a particular area and has 3 components:
- species diversity
- genetic diversity
- ecological diversity
What is species diversity?
Refers to the number of different species and the number of individuals of each species within any one community
What is ecosystem diversity?
Refers to the range of different habitats, from a small local habitat to the whole of the Earth
One measure of species diversity is
Species richness
What is species richness?
The number of different species in a community at a given time
NOTE: two communities may have the same number of species, but the proportions of the community made up of each species may differ e.g. natural meadow and a field of wheat may both have 25 species, but in the meadow, all 25 species might be equally abundant, whereas in the wheat field, over 95% of plants may be a single species of wheat
Another way to calculate species diversity is to use
Index of diversity
Why is it more useful to calculate a species index than just to record the number of species present?
It measures both the number of species and the number of individuals so therefore takes account of species that are only present in small numbers
Regarding species diversity and human activities, what do we need to know the effect of?
Impacts of agriculture
As natural ecosystems develop over time, they become complex communities I.e. they have a high
Index of diversity
Agriculture ecosystems are controlled by humans and are different to natural ecosystems:
- farmers often select species for particular qualities that make them productive (selective breeding) = number of species and the genetic variety of alleles they posses, is reduced to the few that exhibit the desired features
- to be economic, the number of individuals of these desirable species needs to be large
- any particular can only support a certain amount of biomass
- if most of the area is taken up by the one species farmers consider desirable, it follows that there is a smaller area available for all the other species and these many other species will have to compete for what little space and resources are available = may not survive competition
- even if defies evolved to adapt to the changes, the population of the species would be considerably reduced
- pesticides used to exclude these species because they compete for light, mineral ions, water and food required by farmed species
= OVERALL EFFECT = reduction in species diversity = index of species low in agriculture ecosystem
A balance needs to be struck between conservation and
Farming
Food is essential for life and with global exponential population growth, pressure to produce is extremely intense- it has however been achieved by:
- use of improved genetic varieties of plant and animal species
- greater use of chemical fertilisers and pesticides
- greater use of biotech
- changes in farm practices, leading to larger farms and conversion of land supporting natural communities in farmland
These changes have had many ecological impacts but the overriding effect of intense food production has been to diminish the variety of habitats within the ecosystem and consequently reduced species diversity
Farmers try to maximise the amount of food they can produce from a given area of land- but many of the methods they use reduce biodiversity e.g.:
- woodland clearance = increase area of farmland- directly reduces number of trees and often the number of different tree species and also destroys habitats so some organisms may lose their habitats and thus niches, further reducing biodiversity
- hedgerow removal (same reasons)
- pesticides- chemicals that kill organisms deemed as pests that feed on crops reducing diversity by directly eliminating pests and species that feed on these pests will diminish = further reduced biodiversity
- monoculture- single type of plant grown reduces biodiversity directly and will support fewer range of organisms
- escape of effluent from silage stores and slum tanks into water sources
Despite conflicts between intensive food production and conservation, there are a number of management techniques that can be applied to increase species and habitat diversity without unduly raising food costs or lowering yields:
1- maintaining existing hedgerows at the most beneficial height shape (A-shape provides better habitats than rectangular ones)
2- plant native trees on land with a low species diversity rather than species-rich areas
3- reduce use of pesticides- use biological control where possible or genetically modified organisms that are resistant to pests
4- introduce conservation headlands- areas at the edges of fields where pesticides are used restrictively so that wild insects and flowers can breed
5- creare protected areas such as SSSIs (sites of special scientific interests) and AONBs (areas of outstanding natural beauty) that restrict further development including agricultural development
It is recognised that these practices will make good slightly more expensive to produce, and therefore to encourage farmers, there are a number of financial incentives from DEFRA and the EU
Regarding conservation vs farming debate why is maintaining biodiversity very important?
If reduced, the global living system becomes increasingly unstable and we all rely on the global system for a variety of resources, most importantly, food!
How did diversity used to be investigated?
Comparison of observable characteristics
Comparing genetic diversity within and between species helps scientists to determine the
Evolutionary relationships between them
Explain theory behind comparison of observable characteristics
- traditionally, genetic diversity was measured by observing the characteristics of organisms
- this method was based on the fact that each observable characteristic is determined by a gene or genes (with environmental influences)
- the variety within a characteristic depends on the number and variety of alleles of that gene (plus environmental influences)
What were the issues associated with investigating diversity by comparison of observable characteristics?
- large number of characteristics are coded for by more than one gene I.e. polygenic = they are not discrete from one antihero, but rather vary continuously = hard to distinguish between them
- characteristics can also be modified by the environment = differences may therefore be the result of different environmental conditions rather than different alleles
- for instance, height in humans is polygenic I.e. coded for by multiple genes however environmental factors like diet can influence the actual height of an individual
- for these reasons, inferring DNA differences from observable characteristics has been replaced by directly observing DNA sequences themselves (made possible through advances in gene technology made over recent years)
What current methods are used to investigate diversity?
- comparison of DNA base sequences
- comparison of base sequences of mRNA
- comparison of amino acids in certain proteins
Explain how comparison of DNA base sequences is done
- DNA sequencing now routinely done by automatic machines and the DNA produced is analysed by computers
- in these computerised systems, each nucleotide base can be tagged with a different coloured fluorescent dye: A(green), T(red), C(blue) and G (yellow) = produced a series of coloured bands, each of which represents one of the four nucleotide bases
How do we measure the genetic diversity of a species by sampling the DNA of its members and sequencing it to produce patterns of coloured bands?
- analysis of these patterns allows us to compare one species with another or one individual with another of the same species to determine how diverse they are
- process would be slow using the human eye and so the patterns are scanned by lasers and interpreted by computer software to give the DNA nucleotide base sequence in a fraction of the time
How can we use comparison of DNA base sequences to determine the evolutionary relationship between species?
- when one species gives rise to another species during evolution, the DNA of the new species will initially be very similar to that of the species that gave rise to it
- due to mutations, the sequences of nucleotide bases in the DNA of a new species will change
- consequently, over time, the new species will accumulate more differences in its DNA
- as a result, we would expect species that are more closely related to show more similarity in their DNA base sequences than species that are more distantly related
Explain methodology behind comparison of base sequences of mRNA
- mRNA code for by DNA
- base sequences on mRNA are complementary to those of the strand of DNA from which they were synthesised
- it follows that we can measure DNA diversity and therefore genetic diversity by comparing the base sequences of mRNA
Explain how diversity is investigated through the use of comparison of amino acids in proteins
- sequence of amino acids in proteins (primary structure) is determined by mRNA which, in turn, is determined by DNA
- genetic diversity within and between species can therefore be measured by comparing the amino acid sequences of their proteins
- the degree of similarity in the amino acid sequence of same protein in two species will also reflect how closely related the two species are
- once amino acid sequence for a chosen protein has been determined for the two sequences, they are compared and can be done by counting number of similarities or number of differences in each sequence
