Genes

Question 1

Features of the genetic code

Answer 1

Degenerate - different triplets can code for the same amino acid
Non-overlapping - each base in the sequence is only part of one triplet and is only read once
Universal - each triplet codes for the same amino acid in all organisms

Question 2

What are introns and exons?

Answer 2

Introns are non-coding parts of DNA.

Exons are coding parts of DNA.

Question 3

DNA in prokaryotic and eukaryotic cells

Answer 3

Prokaryotic cells have DNA in the form of a single circular loop of free floating DNA. Some also have small rings of DNA called plasmids.
Eukaryotic cells have DNA folded around proteins called histones. The DNA is supercoiled and form structures called chromosomes.

Question 4

Chromosome structure

Answer 4

Chromosomes are made up of coiled up DNA and histones and they are only visible as distinct structures when a cell is dividing. Chromosomes appear as two threads, called chromatids, joined at a single point, the centromere.

Question 5

Structure of mRNA and tRNA

Answer 5

mRNA is a long, single strand of nucleotides.

tRNA is made up of around 80 nucleotides in a clover-like shape. The ‘clover leaves’ have unpaired bases sticking out of them, and one of the leaves is the anticodon loop.

Question 6

Transcription

Answer 6

DNA helicase unwinds part of the DNA
Free RNA nucleotides pair up with the exposed DNA
RNA polymerase joins the RNA nucleotides together to form pre-mRNA
The DNA re-forms and the pre-mRNA leaves the nucleus from a pore in the nuclear envelope
The pre-mRNA is spliced, removing introns so that it becomes mRNA

Question 7

Translation

Answer 7

After leaving the nucleus, mRNA attaches to a ribosome
Free tRNA molecules with the right anticodon, and their amino acid, bind with the complementary codon on the mRNA
Peptide bonds form between the amino acids and the tRNA is released.
The second and third step repeats until a stop codon is reached

Question 8

Base mutations

Answer 8

Substitution - where one nucleotide is swapped for another. Usually has a small or no impact due to the degenerate nature of the genetic code.

Deletion - where one nucleotide is lost from the sequence. Can have a huge impact due to the non-overlapping nature of the genetic code.

Question 9

Chromosome mutations

Answer 9

Individual homologous pairs of chromosomes can fail to separate in meiosis, resulting in a gamete having one more or one less chromosome. This is known as non-disjunction.

Question 10

Meiosis

Answer 10

During meiosis 1, the first division, homologous chromosomes line up and twist around each other. Equivalent portions of each break off and recombine with the other chromatid; this is known as crossing over (and recombination) and is important for genetic diversity as it creates unique combinations of alleles.

In meiosis 1, the separation of homologous chromosomes into daughter cells is random; this is known as the independent segregation of homologous chromosomes and is important for genetic diversity as it creates genetically different daughter cells.

Question 11

Calculating chromosome combinations

Answer 11

An organism can produce 2ⁿ possible different combinations of chromosomes as a result of meiosis.

Two organisms can produce (2ⁿ)² possible different combinations of chromosomes in offspring as a result of sexual reproduction.

Question 12

Types of selection

Answer 12

Directional selection - phenotypes at one extreme of the population being selected for and those at the other extreme being selected against.

Stabilising selection - phenotypes around the mean of the population being selected for and those at the extremes being selected against.

Question 13

Definition of a species

Answer 13

Members of a species can produce fertile offspring.

Question 14

Phylogenetic classification

Answer 14

Phylogenetic classification is a form of classification that classifies species into groups based on evolutionary relationships and shared features between organisms and their ancestors. The groups are arranged into a non-overlapping hierarchy.

Question 15

Taxonomic hierarchy

Answer 15

Domain
Kingdom
Phylum
Class
Order
Family
Genus
Species

Question 16

Binomial naming system

Answer 16

The binomial system is a universal system for naming species that identifies each organism with two names. The first name is the genus it belongs to; this should be written starting with a capital letter. The second is the species it belongs to, in lower case. Names should be written in italics.

Question 17

Courtship behaviour

Answer 17

Courtship behaviour is important for the following reasons:
Recognise members of own species
Identify a mate capable of breeding

Courtship behaviour is important for species recognition as members of different species can be differentiated from one another through their different behaviour.

Question 18

Species richness

Answer 18

Species richness is a measure of the number of different species in a community.

Question 19

Index of diversity

Answer 19

d = N(N-1) / Σn(n-1)

Question 20

Agriculture and species diversity

Answer 20

Impacts of agriculture on species diversity:
Removal of hedgerows
Creating monocultures
Draining wetland
Filling in ponds
Over-grazing of land
Use of pesticides and non-organic fertilisers

Conservation techniques that could be used:
Maintain existing hedgerows
Plant hedges instead of using fences
Use crop rotation that includes a nitrogen-fixing crop instead of fertilisers, and genetically modified crop instead of pesticides
Maintain existing ponds, make new ones where possible

Question 21

Comparing genetic diversity

Answer 21

Comparison of observable characteristics - most of these are coded for by <1 gene (they are polygenic) so this method has mostly been replacing by directly observing DNA.
Comparison of DNA base sequences - this is more useful as the information can be very quickly processed by computers to see how closely related two species are.
Comparison of mRNA base sequences - this is also useful and is very similar to comparing DNA.
Comparison of amino acid sequences in proteins - this can be used to see how closely related each species are by writing down a number of amino acids in order and counting the number of similarities/differences.

Question 22

Quantitative investigations

Answer 22

Sampling - random sampling using large samples and analysing the data carefully make sure that sampling bias is avoided and that the data is representative.
Calculations - analysis of data includes calculating mean values and the standard deviation. If two means +/- their standard deviations overlap, the values aren’t significantly different.
s = √( Σ(x-x̅ )² / (n-1) )