Notes · BiologyUK · A-Levels

Genetic information, variation and relationships between organisms

Genes carried in DNA are expressed as proteins, and the way this information is copied, recombined and mutated generates the variation on which evolution acts. This chapter covers the genetic code and protein synthesis, meiosis and the sources of genetic variation, mutation and natural selection, the classification and phylogeny of species, and the quantitative measurement of biodiversity.

6 sections~18 min reading time3 competenciesLevel Standard 2 · Advanced 4

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Exam profile

AO1 · Describe the genetic code, protein synthesis, meiosis and the classification of speciesAO2 · Apply the genetic code and diversity calculations and interpret classification and sequence dataAO3 · Analyse variation and diversity data and evaluate quantitative investigations of biodiversity

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basic level

AS-Level covers DNA and the genetic code, transcription and translation, meiosis, mutation, classification and biodiversity.

higher level

The full A-Level expects the code applied to unfamiliar sequences, quantitative diversity work, and the interpretation of molecular evidence for relationships.

Depth

Text

Contents · 6 sections

Genetic information, variation and relationships between organisms

DNA, genes and the genetic code#

StandardAQA 7402 3.4.1DfE GCE Biology - DNA and the genetic code

The structure of a eukaryotic gene

Fig. 1Eukaryotic genes contain coding exons interrupted by non-coding introns.

A gene is a sequence of DNA bases that codes for the amino acid sequence of a polypeptide (or for a functional RNA). The sequence of bases is read in threes: each triplet of bases (a codon in mRNA) codes for one amino acid. The full set of an organism's DNA is its genome, and the full set of proteins it can make is its proteome; a gene occupies a fixed position, its locus, on a chromosome.

In eukaryotes a gene is not a continuous coding sequence. It contains coding regions called exons interrupted by non-coding regions called introns, and there are also regions of non-coding, often repetitive, DNA between genes. This matters because the intron sequences must be removed after transcription before a functional messenger RNA is made - a step (splicing) that prokaryotes, whose genes lack introns, do not need.

The genetic code has four defining features that are examined precisely. It is a triplet code (three bases per amino acid); it is non-overlapping (each base is read once, in one triplet only); it is degenerate (most amino acids are coded by more than one triplet, which cushions the effect of some mutations); and it is (almost) universal (the same triplets code for the same amino acids in nearly all organisms, which is powerful evidence for a common ancestor and underpins genetic engineering).

Because the code is a triplet code, a coding sequence of

n

bases specifies about

n/3

amino acids, and there are

4^3 = 64

possible triplets for the 20 amino acids - more than enough, which is why the code can be degenerate and can also include start and stop signals. Reasoning from the number of bases to the number of amino acids, or explaining why 64 triplets code for 20 amino acids, are common low-tariff application tasks.

4^3 = 64\ \text{possible triplets for 20 amino acids}

Why the code is degenerate

With 4 bases in triplets there are 64 combinations - far more than 20, so most amino acids have several codons.

Worked example

Bases to amino acids

A gene's coding sequence (exons only) is 900 bases long, of which the final triplet is a stop codon. How many amino acids does the polypeptide contain?

Number of triplets
$900 \div 3 = 300$ triplets.
Account for the stop codon
One triplet is a stop signal and does not code for an amino acid, so $300 - 1 = 299$ .
State the answer
The polypeptide contains 299 amino acids.
$\frac{900}{3} - 1 = 299$

Result: 299 amino acids.

Typical mistakes

Confusing exons (expressed, coding) with introns (removed) - a spelling-level error that loses marks.
Saying the code is degenerate because there are fewer amino acids than bases, rather than fewer amino acids than triplets.

Active revision

A section of mRNA that codes for a polypeptide is 450 nucleotides long, including a stop codon. Deduce the maximum number of amino acids in the polypeptide.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for the sciences (Department for Education) · AQA A-level Biology 7402 specification (AQA)

Transcription and translation#

AdvancedAQA 7402 3.4.2DfE GCE Biology - protein synthesis

From gene to polypeptide

Fig. 2Transcription in the nucleus and splicing, then translation at a ribosome in the cytoplasm.

Protein synthesis converts the base sequence of a gene into the amino acid sequence of a polypeptide in two stages. In transcription, in the nucleus, DNA helicase (here RNA polymerase unwinds the section) exposes the template strand; RNA polymerase then joins free RNA nucleotides that are complementary to the template, building a strand of pre-mRNA. Because the code is complementary, the pre-mRNA carries the same sequence as the coding (sense) strand, with uracil replacing thymine.

In eukaryotes the pre-mRNA is then spliced: the introns are removed and the exons joined to form mature mRNA, which leaves the nucleus through a nuclear pore. Splicing means that one gene can, through alternative splicing, give rise to more than one polypeptide, which is one reason the proteome is larger than the number of genes - a point often used to distinguish the genome from the proteome.

In translation, at a ribosome in the cytoplasm, the mRNA is read in codons. Transfer RNA (tRNA) molecules each carry a specific amino acid and have an anticodon complementary to a codon; as the ribosome moves along the mRNA, tRNAs bring the correct amino acids in turn, and peptide bonds form between adjacent amino acids (using ATP). The chain is assembled until a stop codon is reached, whereupon the polypeptide is released.

Two features make the whole process accurate and universal. Complementary base pairing at every step (DNA to pre-mRNA, then codon to anticodon) ensures the correct sequence is transferred, and the universality of the code means the same mRNA is translated into the same protein in almost any organism. This universality is what allows a human gene to be expressed in a bacterium in genetic engineering, a link forward to the gene-technology chapter.

The genetic code (selected codons)

Fig. 3Each mRNA codon specifies one amino acid; some signal start or stop.

Worked example

From DNA to mRNA to anticodon

The template strand of a gene reads TAC AAG CCT. Deduce (a) the mRNA sequence and (b) the tRNA anticodons.

Transcribe to mRNA
mRNA is complementary to the template, with U for T: TAC -> AUG, AAG -> UUC, CCT -> GGA. mRNA = AUG UUC GGA.
Find the anticodons
Each tRNA anticodon is complementary to its mRNA codon: AUG -> UAC, UUC -> AAG, GGA -> CCU.
State the answer
mRNA = AUG UUC GGA; anticodons = UAC, AAG, CCU.

Result: mRNA AUG UUC GGA; anticodons UAC, AAG, CCU.

Typical mistakes

Writing thymine instead of uracil in RNA, or muddling codon (on mRNA) with anticodon (on tRNA).
Saying splicing happens in prokaryotes - they have no introns to remove.

Active revision

A template DNA strand reads TAC GGA TTC. Write the mRNA codons transcribed from it and the anticodons of the tRNA molecules that would translate them.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for the sciences (Department for Education) · AQA A-level Biology 7402 specification (AQA)

Meiosis and genetic variation#

AdvancedAQA 7402 3.4.3DfE GCE Biology - meiosis

Meiosis produces four haploid cells

Fig. 4Two divisions halve the chromosome number and generate variation by crossing over and independent segregation.

Meiosis is the reduction division that produces gametes: from one diploid cell it makes four haploid cells, each with half the chromosome number, so that fertilisation restores the diploid number. It involves two divisions. In meiosis I the homologous chromosomes pair up and are separated, halving the chromosome number; in meiosis II the sister chromatids are separated (as in mitosis). The halving is essential - without it the chromosome number would double every generation.

Meiosis generates genetic variation between the gametes in two ways. The first is independent segregation (independent assortment): the homologous pairs line up randomly at the equator in meiosis I, so which chromosome of each pair goes to which pole is independent for every pair. With

n

pairs this alone gives

2^n

possible combinations of chromosomes - for humans, with 23 pairs, that is over eight million combinations from independent segregation alone.

The second source is crossing over. During meiosis I the homologous chromosomes come so close that lengths of their chromatids can be exchanged at points called chiasmata. This swaps alleles between the maternal and paternal chromatids, producing new combinations of alleles on a single chromosome (recombinants) that did not exist in either parent. Together with independent segregation, crossing over makes the number of genetically different gametes effectively unlimited.

The variation from meiosis is then multiplied by random fertilisation: any one of the vast number of possible male gametes can fuse with any one of the equally varied female gametes, so the number of possible offspring is

2^n \times 2^n

before crossing over is even considered. This is why sexually reproducing offspring differ from their parents and from one another, providing the raw material on which natural selection acts.

\text{combinations from independent segregation} = 2^n

Chromosome combinations

$n$ = number of homologous pairs; random fertilisation multiplies this to $2^n \times 2^n$ .

Worked example

Counting chromosome combinations

In humans the diploid number is 46. Calculate the number of possible chromosome combinations in a gamete from independent segregation, and state the number of combinations at fertilisation.

Find n
The number of homologous pairs is $46 \div 2 = 23$ .
Combinations per gamete
$2^{23} = 8\,388\,608$ possible chromosome combinations.
$2^{23} = 8\,388\,608$
At fertilisation
Random fusion of any male with any female gamete gives $2^{23} \times 2^{23} = 2^{46}$ combinations - about $7 \times 10^{13}$ , before crossing over.

Result: About 8.4 million per gamete; roughly $7 \times 10^{13}$ at fertilisation.

Typical mistakes

Saying meiosis produces two cells (that is mitosis) or that the daughter cells are genetically identical.
Confusing crossing over (exchange of alleles between homologues) with independent segregation (random alignment of pairs).

Active revision

A species has a diploid number of 12. Calculate the number of different chromosome combinations possible in its gametes from independent segregation alone.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for the sciences (Department for Education) · AQA A-level Biology 7402 specification (AQA)

Genetic diversity, mutation and natural selection#

AdvancedAQA 7402 3.4.3AQA 7402 3.4.4DfE GCE Biology - genetic diversity and selection

Directional and stabilising selection

Fig. 5Directional selection shifts the mean; stabilising selection narrows the range around it.

Genetic diversity is the total number of different alleles in a population, and it is the raw material of evolution. It arises ultimately from mutation - a random change in the DNA base sequence - supplemented by the new combinations produced by meiosis and random fertilisation. Greater genetic diversity gives a population a better chance of containing individuals able to survive an environmental change.

Natural selection acts on this variation. Because organisms produce more offspring than the environment can support, there is competition to survive; individuals whose alleles give them an advantage (are better adapted) are more likely to survive, reproduce and pass on those advantageous alleles. Over many generations the frequency of the advantageous allele increases in the population - evolution by natural selection. The adaptations selected may be anatomical, physiological or behavioural.

There are two patterns of selection to distinguish. Directional selection favours one extreme of a range, shifting the mean in that direction - for example antibiotic resistance in bacteria, where resistant individuals survive and the population becomes more resistant over time. Stabilising selection favours the intermediate phenotype and selects against both extremes, reducing variation around the mean - for example human birth mass, where very small and very large babies have historically had lower survival.

It is important to state natural selection precisely and to avoid the common Lamarckian error. Organisms do not change in order to adapt; rather, the variation already exists (from mutation), and the environment selects among it. A mutation for resistance is not caused by the antibiotic - it is already present by chance in a few individuals, and the antibiotic simply kills the non-resistant ones, allowing the resistant survivors to reproduce. Getting this causal direction right is essential.

Variation in a population

Fig. 6Continuous variation is usually normally distributed; selection acts on the extremes or the mean.

Worked example

Explaining antibiotic resistance

A bacterial population is repeatedly exposed to an antibiotic and becomes resistant. Explain this using natural selection.

Existing variation
By chance, a mutation makes a few bacteria resistant before the antibiotic is used; the mutation is not caused by the antibiotic.
Selection pressure
The antibiotic kills the non-resistant bacteria; the resistant ones survive.
Reproduction and inheritance
The survivors reproduce and pass on the resistance allele, so its frequency rises; over generations the population becomes resistant (directional selection).

Result: Pre-existing resistant mutants are selected by the antibiotic and pass on the allele, raising its frequency.

Typical mistakes

The Lamarckian error: saying organisms change or mutate in order to adapt, rather than that existing variation is selected.
Talking about individuals 'evolving' - evolution is a change in allele frequency in a population over generations.

Active revision

Explain, in terms of natural selection, how a population of bacteria can become resistant to an antibiotic over several generations.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for the sciences (Department for Education) · AQA A-level Biology 7402 specification (AQA)

Species, classification and phylogeny#

StandardAQA 7402 3.4.5DfE GCE Biology - classification

The three domains

Fig. 7The three-domain system, based largely on ribosomal-RNA evidence; Archaea are closer to Eukarya.

A species is a group of organisms able to breed together to produce fertile offspring. Each species is given a two-word Latin name by the binomial system - the genus (capitalised) followed by the species - for example Homo sapiens; this universal naming avoids the confusion of common names. Members of a species share a common gene pool and, in many animals, recognise potential mates through species-specific courtship behaviour.

Classification places species into a hierarchy of ever-larger groups with no overlap: domain, kingdom, phylum, class, order, family, genus and species. Each group at one level contains several groups at the level below, so the hierarchy is nested. Modern classification is phylogenetic - it reflects evolutionary relationships and ancestry - so members of a genus share a more recent common ancestor than members of the same family.

At the highest level, organisms are divided into three domains - Bacteria, Archaea and Eukarya - based on molecular differences, particularly in ribosomal RNA. The archaea, though superficially like bacteria, differ fundamentally in their molecular biology and are more closely related to the eukaryotes, which is why the three-domain system replaced the older five-kingdom scheme. This shows how classification is revised as new (especially molecular) evidence emerges.

Courtship behaviour has a role in maintaining species boundaries and is examined as an example of behavioural adaptation. It allows individuals to recognise members of their own species (so that mating is not wasted on other species that could not produce fertile offspring), to identify a mate that is sexually mature and receptive, and to form a pair bond - all of which increase the chance of successful reproduction and keep the gene pools of different species separate.

The taxonomic hierarchy

Fig. 8Each rank nests inside the one above; the human classification is shown.

Worked example

Interpreting classification

Species A and B share the same family but different genera; species A and C share the same genus. Which pair is more closely related, and why?

Recall the hierarchy
Genus is a lower (more specific) rank than family, so a shared genus means a more recent common ancestor than a shared family.
Compare the pairs
A and C share a genus; A and B share only a family.
Conclude
A and C are more closely related, because sharing a genus means they diverged more recently than A and B.

Result: A and C are more closely related (shared genus = more recent common ancestor).

Typical mistakes

Getting the order of the taxonomic ranks wrong, or writing the species name with a capital letter.
Saying archaea are just a type of bacteria - they form a separate domain, closer to eukaryotes.

Active revision

Explain why two organisms placed in the same genus are more closely related than two organisms placed only in the same class.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for the sciences (Department for Education) · AQA A-level Biology 7402 specification (AQA)

Biodiversity and quantitative investigation#

AdvancedAQA 7402 3.4.6AQA 7402 3.4.7DfE GCE Biology - biodiversity

Comparing the diversity of two communities

Fig. 9A species-rich, even community (meadow) has a higher index of diversity than a monoculture.

Biodiversity can be considered at several scales - the range of habitats, the number of species (species richness), and the genetic diversity within a species. Species richness alone (a simple count of species) ignores how evenly individuals are distributed, so a more informative measure is an index of diversity that accounts for both the number of species and the number of individuals in each.

The index of diversity is calculated as

d = \dfrac{N(N-1)}{\sum n(n-1)}

, where

N

is the total number of organisms of all species and

n

is the number of organisms of each individual species. A higher value indicates greater diversity. A community dominated by one species has a low index even if several species are present, whereas a community with individuals spread evenly across many species has a high index - which is why the index is preferred to a bare species count.

Human activity, especially agriculture, tends to reduce biodiversity. Practices such as clearing woodland, growing monocultures, removing hedgerows and using pesticides and herbicides all lower the index of diversity by removing habitats and species. There is therefore a tension between maximising food production and conserving biodiversity, which is assessed as an evaluation (AO3) question - weighing economic and conservation arguments.

Relationships between species and the diversity within a species can also be investigated at the molecular level, which is more objective than comparing observable features. Comparing the base sequences of DNA, the base sequences of mRNA, or the amino acid sequences of proteins reveals how similar two species are: the more similar the sequences, the more closely related the species and the more recently they shared a common ancestor. Immunological comparisons of proteins give similar information. These molecular methods have refined and sometimes overturned classifications based on appearance alone.

d = \dfrac{N(N-1)}{\sum n(n-1)}

Index of diversity

$N$ = total number of all organisms; $n$ = number of each species. A larger $d$ means greater diversity.

Worked example

Calculating an index of diversity

A quadrat sample contains three plant species with 10, 12 and 8 individuals respectively. Calculate the index of diversity $d$ .

Total number, N
$N = 10 + 12 + 8 = 30$ , so $N(N-1) = 30 \times 29 = 870$ .
Sum of n(n-1)
$\sum n(n-1) = (10 \times 9) + (12 \times 11) + (8 \times 7) = 90 + 132 + 56 = 278$ .
Divide
$d = \dfrac{870}{278}$ .
$d = \frac{870}{278} = 3.13$

Result: The index of diversity is 3.13 (3 s.f.).

Typical mistakes

Forgetting to sum $n(n-1)$ over every species in the denominator, or using $N$ where $n$ is required.
Treating species richness (a count) as if it fully described biodiversity, ignoring the evenness the index captures.

Active revision

A sample contains three species with 12, 8 and 5 individuals. Calculate the index of diversity, showing your working.

Active recall

Recall the key points — then reveal.

Sources: GCE AS and A level subject content for the sciences (Department for Education) · AQA A-level Biology 7402 specification (AQA)

References & sources

Sources

Department for Education

GCE AS and A level subject content for the sciences

AQA

AQA A-level Biology 7402 specification

Genetic information, variation and relationships between organisms

6 sections~18 min reading time3 competenciesLevel Standard 2 · Advanced 4

Genetic information, variation and relationships between organisms

Key points

Bases to amino acids

Key points

From DNA to mRNA to anticodon

Key points

Counting chromosome combinations

Key points

Explaining antibiotic resistance

Key points

Interpreting classification

Key points

Calculating an index of diversity

Sources

Genetic information, variation and relationships between organisms

Key points

Bases to amino acids

Key points

From DNA to mRNA to anticodon

Key points

Counting chromosome combinations

Key points

Explaining antibiotic resistance

Key points

Interpreting classification

Key points

Calculating an index of diversity

Sources