topic 4 Flashcards
What is a gene?
Base sequence of DNA that codes for:
- amino acid sequence of a polypeptide
- functional RNA
How do we know amino acids are coded in triplets?
- There are 20 common amino acids in proteins.
- Each amino acid must have a code in DNA.
- DNA has only 4 bases.
- 4^2 is less than 20 but 4^3 is greater than 20.
- Therefore, 64 possible triplets for 20 amino acids meaning some codes code for the same amino acid.
Features of the genetic code
- Triplets are read in one direction.
- The start of a DNA sequence is always the same triplet; if this amino acid is not needed, it is later removed.
- 3 triplets are stop codes, not coding for an amino acid.
- Code is degenerate.
- Code is non-overlapping.
- Code is universal.
Why is the genetic code degenerate?
Most amino acids are coded for by more than one triplet.
Why is the genetic code universal?
The same codon makes the same amino acid in different organisms. (evolution evidence)
DNA in prokaryotes
- Short, circular and not associated with proteins.
- Found in cytoplasm.
DNA in eukaryotes
- DNA molecules are very long, linear and associated with proteins called histones which forms chromosomes.
- Found in nucleus.
How is the DNA in mitochondria and chloroplasts most similar?
Prokaryotic cells.
What is a locus?
The precise location of a gene on a chromosome.
What are exons?
Coding base sequences of DNA that code for a sequence of amino acids.
What are introns?
Non-coding base sequences.
What is the genome?
The complete set of genes in a cell.
What is the proteome?
The full range of proteins that a cell is able to produce.
What is a homologous pair of chromosomes?
Two chromosomes that carry the same genes.
What is an allele?
One of a number of alternative forms of a gene.
How is a DNA molecule formed into a chromosome?
1) DNA molecule coils and is combined with histones.
2) DNA-histone complex is coiled.
3) Coils fold to form loops.
4) Loops coil and pack together to form the chromosome.
Structure of mRNA and tRNA
- Single polynucleotide chain.
- A pentose sugar, ribose, and the bases adenine, uracil, guanine, and cytosine.
How is mRNA different to tRNA?
- Longer polynucleotide chain.
- Single stranded molecule.
- Chemically unstable (only present when needed for protein synthesis).
How is tRNA different to mRNA?
- Shorter polynucleotide chain.
- Cloverleaf shaped (amino acid attachment site and anticodon binding site).
- Chemically stable.
What does mRNA do?
Transfers genetic information from DNA to the ribosomes in a process called transcription.
- Moves out of nucleus via nuclear pores into cytoplasm where its codons act as a template for protein synthesis.
What is a codon?
Sequence of 3 bases on mRNA that codes for a single amino acid.
What does tRNA do?
Anticodon on tRNA binds to complementary codon on mRNA carrying specific amino acid to growing polypeptide chain.
What is protein synthesis?
The mechanism in cells whereby:
- The DNA template is transcribed into a mRNA molecule in the nucleus.
- The mRNA base sequence is then translated into an amino acid sequence in association with tRNA on ribosomes in the cytoplasm.
Process of Transcription
The production of mRNA from DNA:
1) DNA helicase breaks the hydrogen bonds between bases, causing DNA to unwind and expose bases.
2) Only one chain of DNA acts as a template.
3) The enzyme RNA polymerase moves along the.
4) Free RNA nucleotides align themselves opposite complementary DNA bases.
5) RNA polymerase joins together mRNA nucleotides and makes short strands of pre-mRNA until it comes to a terminator sequence.
6) Sections of pre-mRNA are spliced together to remove intron sequences and join exon sequences to form mRNA.
In which organisms does splicing occur?
Eukaryotes.
- In prokaryotes transcription results directly into mRNA as most don’t include introns.
Process of Translation
The production of polypeptides from the sequence of codons carried by mRNA:
1) mRNA moves from the nucleus through a nuclear pore to the cytoplasm and associates with ribosome which moves to the start codon on mRNA.
2) A tRNA (carrying an amino acid) with a complementary anticodon moves to ribosome and pairs with the first mRNA codon.
3) The ribosome moves along the mRNA, pairing a complementary tRNA to the mRNA.
4) Energy released from ATP is used to form a polypeptide bond between amino acids.
5) When the ribosome reaches a stop codon, the polypeptide is complete and the RNAs are released and the ribosome can be reused.
What is a mutation?
Any change in the DNA sequence.
What is a gene mutation?
A change in the base sequence of a gene and can happen spontaneously during DNA replication.
Examples of gene mutation
Base substitution and deletion.
What is a base substitution mutation?
A single replacement of one nucleotide for another which contains a different base.
- Potentially coding for a new amino acid and disrupting polypeptide chain.
Why do not all base substitutions impact the sequence of amino acids?
Due to the degenerate nature of the genetic code, where many triplets code for the same amino acid, and so depending on which base changes, there may be no impact.
What is base deletion?
One or more nucleotides get deleted from the DNA sequence (result in a frameshift mutation).
- Often has significant effect on encoded protein because it can alter the sequence of all the codons following the lost base.
What is a chromosome mutation?
A change in the structure or number of chromosomes.
Examples of chromosome mutation?
1) Change in whole sets of chromosome - when organisms have three or more sets of chromosomes rather than the standard pair.
2) Change in number of individual chromosomes - when homologous pairs fail to separate during meiosis (non-disjunction) leading to an organism having 1 more or 1 less chromosome.
What is meiosis?
Cell division that produces 4 genetically different daughter haploid cells from a diploid parent cell.
Process of meiosis
1) Genetic information duplicates and the chromosomes form pairs.
2) First division - homologous pairs pair up and undergo crossing over and then are separated and each pair goes into one of the 2 daughter cells.
3) Second division - the chromatids move apart and 4 genetically different daughter cells are formed.
How does meiosis create genetic variation?
- Independent segregation of homologous chromosomes.
- New combinations of maternal and paternal alleles by crossing over.
What is independent segregation?
The random division of homologous chromosomes into separate daughter cells during meiosis.
What is crossing over?
The exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring.
Process of crossing over
1) Homologous chromosomes pair up.
2) The chromatids of each pair become twisted round each other.
3) Tensions are created and portions of chromatids break off.
4) These portions rejoin the chromatid of its homologous partner (recombination).
5) Therefore new genetic combinations of alleles are formed.
When does crossing over occur?
Prophase I of meiosis.
When does independent segregation occur?
Metaphase I of meiosis.
How to calculate number of different combinations of chromosomes in a gamete?
2^n.
How to calculate number of different combinations of alleles in an organism?
(2^n)^2.
What is genetic diversity?
The number of different alleles of genes in a species or population.
What does genetic diversity enable?
Natural selection.
What is a gene pool?
All the alleles in a population.
Process of natural selection and evolution
1) Within any population of a species, there will be a gene pool containing many different alleles.
2) Random mutations can result in a new allele which in most cases will be harmful.
3) However, in certain environments, this mutation could be beneficial to the organism - becoming better adapted and outliving competition.
4) It is these adapted organisms that then reproduce successfully.
5) The advantageous allele is inherited by members of the next generation.
6) Therefore, over many generations, the new allele will increase in frequency in the population.
What is selection?
Process by which organisms that are better adapted tend to survive and breed whilst those less adapted tend not to.
Types of selection
- Directional.
- Stabilising.
- Disruptive.
What is directional selection?
Selection that favors individuals that vary in one direction from the mean of the population e.g. antibiotic resistance.
What is stabilising selection?
Selection that favours more average individuals e.g. human birth weights.
Types of adaptation
- Anatomical.
- Physiological.
- Behavioural.
What is anatomical adaptation?
Features an organism’s body has that increase chance of survival e.g. webbed feet in beavers.
What is physiological adaptation?
Are those that relate to how an organism functions and increase its chances of survival in its natural environment.
What is behavioural adaptation?
Ways in which an organism acts that increases its chance of survival and reproduction. For example, playing dead to escape attack.
What is disruptive selection?
Describes changes in population genetics in which extreme values for a trait are favored over intermediate values - leads to speciation.
What is a species?
Organisms that are capable of breeding and producing living fertile offspring.
What system names species?
The binomial system.
How the binomial system works
- Universal, based on Latin and Greek.
- First name is their generic name/genus.
- Second name is their specific name/species.
How do we write the binomial system names?
- Italics (underlined if handwritten).
- First letter of generic name is capital.
- First letter of specific name is lower case.
- Unknown specific name = sp.
What is courtship behaviour?
Behaviour carried out by organisms to attract a mate of the same species.
How does courtship behaviour help to ensure successful mating?
It enables individuals to:
- Recognise members of their own species to make sure the mating is between two members of the same species.
- Identify a mate that is capable of breeding as they need to be sexually mature and fertile.
- Form a pair bond that will lead to successful mating and raising of offspring.
- Synchronise mating so it takes place when there is the max probability of sperm and egg meeting.
What is classification?
Organising living organisms into groups.
What is taxonomy?
The theory and practice of biological classification.
What are the 2 forms of biological classification?
- Artificial classification.
- Phylogenetic classification.
What is artificial classification?
Artificial classification involves arbitrarily selecting unifying characteristics first and then grouping organisms accordingly - do not have the same evolutionary origins.
What is phylogenetic classification?
- Based on evolutionary relationships between organisms and their ancestors.
- Classification of organisms using shared features derived from ancestors.
- Arranging into a hierarchy where the groups are contained within larger composite groups with no overlap.
What is a taxon?
A taxonomic group of any rank, such as a species, family, or class.
What is the highest taxonomic rank?
Domain.
What are the 3 domains?
Bacteria, Archaea, Eukarya.
What defines the bacteria domain?
- Absence of membrane bound organelles.
- Unicellular.
- 70s ribosomes.
- Murein cell walls.
- Single loop of DNA with no histones.
What defines the archaea domain?
Single cells prokaryotes which differ from bacteria because:
- Genes and protein synthesis much similar to eukaryotes.
- Membranes contain fatty acid chains attached to glycerol by ether linkages.
- No murein in cell walls.
- More complex form of RNA polymerase.
What defines the eukarya domain?
Multicellular organisms:
- Membrane bound organelles.
- Membranes containing fatty acid chains attached to glycerol by ester.
- Not usually cell walls and if so no murein.
- 80s ribosomes.
What are the 4 eukarya kingdoms?
Animalia, Plantae, Fungi, Protista.
Classification rank order
Kingdom, Phylum, Class, Order, Family, Genus, Species.
What is meant by phylogeny?
The evolutionary relationship between organisms.
What is meant by biodiversity?
The number and variety of living organisms in a particular area.
What are the 3 components of biodiversity?
- Species diversity.
- Genetic diversity.
- Ecosystem 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 genetic diversity?
Number of different alleles of each gene.
What is ecosystem diversity?
Refers to the range of different habitats, from a small local habitat to the whole of the earth.
What is species richness?
The number of species in a community.
How do we measure the species diversity?
Calculate index of diversity.
What does the index of diversity describe?
The relationship between the number of species in a community and the number of individuals in each species.
Impact of agriculture on biodiversity
Intensive farming clears woodland and hedges to grow one crop species which is known as monoculture. This increases area of farmland to produce ideal condition to grow the crop and supply a high yield. Pesticides used to kill organisms that feed on crops and herbicides used to kill weeds. This decreases biodiversity as low variety of habitats, few plant species, few species of herbivores and carnivores so little variety of food.
Practices with a direct impact on biodiversity
- Removal of hedgerows.
- Creating monocultures.
- Filling in ponds and draining marsh.
- Over grazing of land preventing regeneration.
What is intercropping?
Two or more crop species planted in the same field at the same time.
Practices with an indirect effect on biodiversity
- Use of pesticides.
- Escape of effluent from silage stores into water courses.
- Absence of crop rotation and lack of intercropping.
Examples of balancing conservation and farming
- Plant hedges instead of fences.
- Use crop rotation and increase intercropping.
- Maintain existing ponds.
- Reduce pesticide use.
Techniques used to compare genetic diversity
Comparing:
- Frequency of measurable or observable characteristics.
- Base sequence of DNA.
- Base sequence of mRNA.
- The amino acid sequence of proteins in an organism.
Disadvantages of comparing observable characteristics?
- Many observable characteristics are coded by more than one gene (polygenic) so hard to distinguish specific genes.
- Can also be impacted by environment and so may be a measure of environmental conditions rather than alleles.
Why do more distantly related organisms have a more different DNA?
More mutations.
What is interspecific variation?
Variation between different species.
What is intraspecific variation?
Variation within a species.
Reasons why samples aren’t representative?
- Sampling bias e.g. only collecting flowers from a nice dry field rather than a wet muddy field.
- Chance e.g. getting the tallest flowers.
How to avoid sampling bias?
Random sampling.
Random sampling method
- Divide study area into a grid of numbered lines.
- Use random numbers from a generator to obtain coordinates.
- Take samples at these coordinates.
How can we minimise chance?
- Use a large sample size; anomalies have less influence.
- Analysis of the data collected e.g. statistical tests to determine if variation is a result of chance.
What is a skewed normal distribution?
When the curve is shifted to one side - averages.
What is sampling bias?
Only collecting flowers from a nice dry field rather than a wet muddy field.
Chance, e.g., getting the tallest flowers.
How to avoid sampling bias?
Use random sampling.
What is the random sampling method?
Divide study area into a grid of numbered lines, use random numbers from a generator to obtain coordinates, and take samples at these coordinates.
How can we minimise chance?
Use a large sample size; anomalies have less influence. Analyze the data collected, e.g., statistical tests to determine if variation is a result of chance.
What is a skewed normal distribution?
When the curve is shifted to one side - averages have different values.
How to measure the mean from a normal distribution curve?
The mean is the measurement at the maximum height of the curve.
How to measure the standard deviation from a normal distribution curve?
The standard deviation is the measure of the width of the curve - the distance from the mean to the point of inflection is a standard deviation.
Must have a minimum number of values to have any accuracy.
What is the mean?
The sum of the observations divided by the number of observations.
What is the mode?
Most frequently occurring value.
What is the median?
Middle value in a set of values.
What is conservation?
Concept and method of preserving and maintaining an ecosystem and the organisms which live there sustainably.
What is a hierarchy?
Smaller groups arranged within larger groups; no overlap between groups.
What is a non-disjunction mutation?
Homologous chromosomes or sister chromatids fail to separate correctly in meiosis. Gametes have at least one more (polysomy) or one less (monosomy) chromosomes than normal.
