3.4 genetic information, variation and relationships between organisms

Question 1

what is the DNA in eukaryotes

Answer 1

Long
linear ( not circular)
Associated with proteins called histones to form chromosomes
The mitochondria and chloroplasts also contain DNA, which is like DNA in prokaryotes,

Question 2

what is the DNA in prokaryotes

Answer 2

Are shorter
Form a circle
Not associated with protein molecules
Therefore don’t have chromosomes

Question 3

what is genes

Answer 3

A section of DNA that contains the coded information for making polypeptides and functional RNA.
The coded information is in the form of a specific sequence of bases along the DNA molecule.
Polypeptides make up proteins and so genes determine the proteins of an organism. Enzymes are proteins and as they control chemical reactions, they are responsible for an organism’s development and activities.

Question 4

what is the genetic code

Answer 4

In trying to discover how DNA bases code for amino acids, scientists suggested that there must be a minimum of 3 bases that coded for each amino acid,

Question 5

why must there be 3 bases

Answer 5

• Only 20 amino acids regularly occurred in proteins and must have its own code of bases on the DNA
• Only 4 different bases are present in DNA so if each base coded for a different amino acid, only 4 different amino acids could be coded for.
• Using a pair of bases 16 (42) different codes are possible, which is inadequate
• 3 bases produce 64 (43) different codes, which is more than enough to satisfy the requirements of 20 amino acids

Question 6

what are features of the genetic code

Answer 6

A few amino acids are coded for by only a single triplet.
The remaining amino acids are coded for by between 2 and 6 triplets each
The code is known as a ‘degenerate’ code because most amino acids are coded for by more than 1 triplet
A triplet is always read in 1 particular direction
The start of the DNA strand that codes for a polypeptide is always the same triplet. This codes for the amino acid methionine. If the 1st methionine doesn’t form part of the final polypeptide, it is later removed

Question 7

homologous chromosomes

Answer 7

Homologous pair is always 2 chromosomes that carry the same genes but not necessarily the same allele of the gene.

Question 8

chromosomes

Answer 8

Only visible as distinct structures when a cell us dividing.
For the rest of the time, they are widely dispersed throughout the nucleus
Then they first become visible at the start of cell division chromosomes appear as
2 threads, joined at a single point. Each thread is called a chromatid as DNA has
already replicated to give 2 identical DNA molecules.
The DNA in chromosomes is held by histones and the DNA inside is highly coiled and folded

Question 9

alleles

Answer 9

An allele is 1 of the number of alternative forms of a gene.
Each gene exists in 2, occasionally more, different form. Each of it’s different forms is called an allele.
Each individual inherits 1 allele from a parent and is the alleles are different, each allele has a different base sequence and so creates a different amino acid sequence and thus polypeptide

Question 10

RNA

Answer 10

Is a polymer made up of repeating mononucleotide sub-units. Forms a single strand in which each nucleotide is made up of:
The pentose sugar ribose
One of the organic bases A,U,C,G
A phosphate group
The 2 types of RNA that are important in protein synthesis:
Messenger RNA
Transfer RNA

Question 11

mRNA

Answer 11

Consists of thousands of mononucleotides
A long strand arranged in a single helix
The base sequence is determined by the sequence of bases on the length of DNA in a process called transcription
Once formed, it leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm, where it associates with ribosomes.
Acts as a template for protein synthesis. Process information in the form of codons.
The sequence of codons determines the amino acid sequence of a specific polypeptide

Question 12

tRNA

Answer 12

A relatively small molecule made up of around 80 nucleotides.
Is a single-stranded chain folded into a clover-leaf shape, with 1 end extending beyond the other. This the part of tRNA that an amino acid can easily be attached.
At the opposite end of tRNA is 3 other organic bases known as the anticodon

Question 13

transcription

Answer 13

In order for transcription to occur, the DNA double helix must be unzipped and the hydrogen bonds broken
The reaction is catalysed by the enzyme DNA helicase which moves along the sugar-phosphate backbone, beginning at the start codon. This exposes a section of both the coding sense strand and the template antisense strand
DNA antisense strand acts a template for mRNA as free nucleotide pair with exposed complementary bases
RNA nucleotides are then joined to adjacent nucleotides by phosphodiester bonds to form the mRNA strand
The reaction is catalysed by the enzyme RNA polymerase which travels along the sugar-phosphate backbone in the 3’ to 5’ direction
When transcription ends, the miRNA strand detaches from the DNA, allowing the double-helix to reform

Question 14

sense and antisense strands

Answer 14

Process of transcription involves forming a single-stranded mRNA from soluble-stranded DNA
Only 1 of the 2 strands of DNA contains the codons that are used to code for amino acids. This strand is known as the sense strand of the coding strand and runs from 5’ to 3’.
The complementary strand is known as the antisense strand or the template strand and runs from 3’ to 5’.
Antisense strand is used as a template so mRNA synthesised has the same sequence as the sense strand

Question 15

splicing

Answer 15

In prokaryotes, the process of transcription results in the direct synthesis of mRNA
In eukaryotes, the process of transcription results in the synthesis of pre-mRNA which must be modified to form mature mRNA
This is because eukaryotic genes contain introns and exons.
Exons- sections of DNA that code for proteins
Introns- sections of DNA that doesn’t code for proteins
Before the pre-mRNA can be used to synthesis polypeptides, the introns must be removed
Exons are then joined back together to form mature mRNA (splicing)
Splicing- the process where introns are removed from pre-mRNA and the exons join together to form mature mRNA

Question 16

translation

Answer 16

After mRNA enters the cytoplasm, it associates with a ribosome, bind to a small subunit of the start codon
The 1st tRNA molecule with a complementary anticodon then binds to the start codon with hydrogen bonding
The large subunit of the ribosome binds and translation begins
Once the ribosome is fully assembled, a 2nd tRNA molecule with complementary anticodon is able to bind to the next mRNA
A peptide bond is then formed between the 2 amino acids forming a dipeptide and releasing methionine
The ribosome then moves along the mRNA strand by exactly 3 base pairs and the 1st tRNA molecule is released
A 3rd tRNA with a complementary anticodon and carrying a specific amino acid is now able to bind to the next mRNA codon.
The cycle repeats itself, forming a large polypeptide
Once a ribosome has moved along the mRNA strand away from the start, another ribosome is able to attach to the start codon
This results in may identical polypeptides being synthesised simultaneously from the same polypeptide chain.
There are no tRNA molecules with complementary anticodons for stop codons

Question 17

gene mutations

Answer 17

Gene mutations involve a change in the base sequence of chromosomes. They can arise spontaneously during DNA replication and include base deletion and base substitution. Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids. Mutagenic agents can increase the rate of gene mutation.

Question 18

gene mutation- base deletion

Answer 18

Arise when a nucleotide is lost from the normal DNA sequence.
Usually the amino acid sequence of the polypeptide is entirely different and so the polypeptide is unlikely to function correctly.
Due to the fact that the sequences of bases in DNA is read in units of 3 bases. 1 deleted nucleotide causes all triplets to shift to the left by 1 base.

Question 19

genetic mutation- base substitution

Answer 19

The type of gene mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that has a different base.
May cause the protein to become a different shape and therefore not function properly.
May still code for the same amino acid which is possible due to the degenerate nature of the genetic code, in which a lot of amino acids have more than 1 code.

Question 20

chromosome mutation

Answer 20

Changes in the structure or number of whole chromosomes are called chromosome mutations.
Can arise spontaneously and take 2 forms:
Changes in whole sets of chromosomes- occur when organisms have 3 or more sets of chromosomes rather than the usually 2.
Changes in the number of individual chromosomes.- sometimes individual homologous chromosomes fail to separate during meiosis. Known as non-disjunction and usually results in a gamete having either 1 more or 1 fewer chromosome

Question 21

genetic diversity

Answer 21

Described as the total number of different alleles in a population.
A population is a group of individuals of the same species that live in the same place and can interbreed.
A species consists of 1, or more, populations.
The greater number of different alleles that all members of a species poses, the greater the genetic diversity of that species.
The greater the genetic diversity, the more likely that some individuals in a population will survive an environmental change. This is due to 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 natural selection.

Question 22

natural selection in population evolution

Answer 22

Described as the total number of different alleles in a population.
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 and so pass on their alleles.
Reproduction success and allele frequency
Differences between the reproductive success of individuals affects the allele frequency in populations, the process works like:
Within a population of species there will be a gene pool containing a wide variety of alleles.
Random mutation of alleles within this gene pool may result in a new allele of a gene which in most cases will be harmful
However in certain environments, the new allele of a gene might give its possessor an advantage
These individuals will be better adapted and therefore more likely to survive
Are more likely to obtain the available resources and so grow more rapidly and live longer. Have a greater chance of breeding and produce more offspring
Only these who reproduce successfully will pass on their alleles.
The offspring will have the advantageous allele, they will also be more likely to survive and pass on that gene.
Overtime, the frequency of the new advantageous allele un the population will increase as the disadvantageous one decreases

Question 23

directional selection

Answer 23

If environmental conditions change the phenotypes that are best suited to the new conditions are most likely to survive.
Some individuals who fall either side of the mean, will posses a phenotype more suited to the new conditions.
These individuals will be more likely to survive and breed and will contribute more offspring.
Over time, the mean will move to the direction of those individuals
Will result in phenotypes at 1 extreme of the population being selected for and those at the other extreme being selected against

Question 24

stabilising selection

Answer 24

If environmental conditions remain stable, it is the individuals with phenotypes closest to the mean that are favoured.
These individuals are more likely to pass their alleles on the next generation.
The individuals with phenotypes at extremes are less likely to pass on their alleles.
Tends to eliminate the phenotypes at extremes

Question 25

species

Answer 25

Are capable of breeding to produce living, fertile offspring. They are then able to produce more offspring.

Question 26

binomial naming system features

Answer 26

Is a universal system  based upon Latin  or Greek names 
The 1st name (generic name) denotes the genus to which the organism belongs. Equivalent to their surname
Second name (specific name), denotes the species to which an organism belongs. Is equivalent to given names used to identify people.

Question 27

binomial naming system rules

Answer 27

The names are printed in italics, or if handwritten, they are underlined to indicate that they are scientific names
The 1st letter of the generic name is uppercase but the specific name is lowercase
If the specific name is unknown, then it is known as ‘sp’

Question 28

phylogenetic classification

Answer 28

Is based upon evolutionary relationships between organisms and their ancestors
Classifies species into groups using shared features derived from their ancestors
Arranges the group into a hierarchy, in which the groups are contained within larger composite groups with no overlap.
Relationships are partly based on homologous characteristics.

Question 29

organising the groups of a species- taxonomy

Answer 29

Each group within a phylogenetic biological classification is called a taxon.
Taxonomy is the study of these groups and their positions in a hierarchal order, where they are known as taxonomic ranks. These are based upon evolutionary line of descent if the group.

Question 30

courtship behaviour

Answer 30

Helps achieve successful mating and the offspring has the maximum chance of survival
Enables individuals to:
Recognise members of their own species
Identify a mate that is capable of breeding- both 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 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

Question 31

types of diversity

Answer 31

Species diversity- refers to the number of different species and the number of individuals of each species within any 1 community
Genetic diversity- refers to the variety of genes possessed by the individuals that makes up a population of a species
Ecosystem diversity- refers to the range of different habitats, from a small local habitat to the whole of earth

Question 32

species richness

Answer 32

The number of different species in a particular area at a given time ( community).
2 communities may have the same number of species but the proportions of the community made up of each species may differ markedly.

Question 33

index of diversity

Answer 33

Takes into account both the number of species in a community and the abundance of each species ( population)
N- number of organisms of all species
n- number of individuals

Question 34

impact of agriculture

Answer 34

As natural ecosystem develop over time, they become complex communities with many individuals of a large number of different species. (high index of diversity)
Agricultural ecosystems are controlled by humans and are different.
Farmers often select species for particular qualities that make them more productive. As a result, the number of species and the genetic variety of alleles they posses is reduced to the few that exhibit desirable features
To be economical., the individuals of these desirable species needs to be large
If most of the area is taken up by 1 species, then there is a smaller area available for all other species. They may have to compete.

Question 35

balance between conservation and farming

Answer 35

Things that have removed habitations and reduced species diversity:
Removal of hedgerows and grubbing out woodland
Creating monocultures
Filling ponds and daring marshland/wetland
Over-grazing of land
Pesticides/inorganic fertilisers
Absence of crop rotation and lack of intercropping/under sowing

Question 36

conservation

Answer 36

Maintain existing hedgerows as the most beneficial height and shape. A-shapes provides more habitats than rectangles
Plant hedges rather than erect fences as field boundaries
Leave wet corners on fields
Plant native trees on areas with low species diversity
Reduce the use of pesticides
Use organic fertilisers
Use crop rotation that includes a nitrogen-fixing crop
Use intercropping to control weeds and other pests
Create natural meadows and use hay for silage
Leave the cutting of verges had field edges until after flowering and when seeds have dispersed
Introduce conservation headlands

Question 37

comparison of observable techniques

Answer 37

Traditionally genetic diversity was measured by observing characteristics of organisms. Has its limitations as a large number of them is coded for by more than 1 gene. This means that they aren’t discrete from one another by rather vary continuously.
Characteristics can also be modified by the environment.
Direct observing DNA sequences has replaced it.

Question 38

comparison of DNA base sequences

Answer 38

We can now determine the number of nucleotides on DNA. DNA sequencing is now routinely done by automatic machines and the data produced is analysed by computers.
In these computerised systems, each nucleotide base can be tagged with different coloured fluorescent dye- adenine ( green), thymine( red), cytosine(blue) and guanine ( yellow).
This produces a series of coloured bands, each of which represents one of the 4 nucleotide bases.

We can measure the genetic diversity of a species by sampling the DNA of its members and sequencing it to produce a pattern of coloured bands. Analysis of these patterns allows us to compare 1 species with another or one individual with another of the same species to determine how diverse they are.

Question 39

comparison of base sequence of mRNA

Answer 39

The base sequence of mRNA are complementary to those of the strand of DNA from which they are made. It follows that we can measure DNA diversity and therefore genetic diversity, by comparing the base sequence of mRNA

Question 40

comparison of amino acid sequence

Answer 40

The sequence of amino acids in proteins 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 sequence of their proteins. The degree of similarity in the amino acid sequence of the same protein in 2 species will also reflect how closely related the 2 species are.
Once the amino acid sequence for a chosen protein has been determined for 2 species, the 2 sequences are compared. This can be done by counting either the number of similarities or the number of differences in each sequence

Question 41

random sampling

Answer 41

Why measurements may not be representative:
Sampling bias- selection process may be biased
Chance- individuals chosen by chance may not be representative
Method to carry out random sampling:
Divide the study into a grid of numbered lines
Use random numbers, from a table or generated from a computer to obtain a group of co-ordinates
Take samples at the intersection at the intersection at each co-ordinate
Minimise the effect of chance by:
Using a large sample size- more individuals reduces the probability that chance will influence the results.
Analysis of data collected- data can be analysed to determine the extent to which chance may have an influence on the results.

Question 42

normal distribution curve

Answer 42

Bell-shape curve is typical for a feature that shows continuous variation.
Occasionally, the curve is shifted to 1 side, which is called a skewed distribution.
Are 3 terms associated:
Mean – sum of sampled values divided by the number of items
Mode- the single value of a sample that occurs the most often
Median- central or middle value of a set of values when the values are arranged in ascending order.

Question 43

normal distribution curve

Answer 43

Bell-shape curve is typical for a feature that shows continuous variation.
Occasionally, the curve is shifted to 1 side, which is called a skewed distribution.
Are 3 terms associated:
Mean – sum of sampled values divided by the number of items
Mode- the single value of a sample that occurs the most often
Median- central or middle value of a set of values when the values are arranged in ascending order.

Question 44

mean and standard deviation

Answer 44

Mean is the measurement of the maximum height of the curve. Provides an average value and is useful in comparing 1 sample with another. Doesn’t provide data about the range of values 
Standard deviation (s) is a measure of the width of the curve. Gives an indication of the range of values either side of the mean. A standard deviation is the distance from the mean to the point where the curve changes from being convex to concave.