Genetic information, variation and relationships between organisms

Give two structural differences between a molecule of messenger RNA (mRNA) and a molecule of transfer RNA (tRNA)

• mRNA is linear whereas tRNA is clover-leaf shaped

• mRNA has codons whereas tRNA has anticodons

• mRNA does not have a amino acid bonding site, tRNA does

Describe how mRNA is produced in a plant cell. (5 marks)

• The DNA strands separate by breaking the hydrogen bonds

• One of the strands is used as a template to make mRNA

• Complementary base pairing so A → U, T → A , C → G, G → C

• RNA nucleotides joined by RNA polymerase

• Introns removed to form mRNA

Compare and contrast the DNA in eukaryotic cells with the DNA in prokaryotic cells. (5 marks)

Similarities:

• Nucleotide structure is identical

• Nucleotides joined by phosphodiester bond

Differences:

• Eukaryotic DNA is longer

• Eukaryotic DNA contain introns, prokaryotic DNA does not

• Eukaryotic DNA is linear, prokaryotic DNA is circular

• Eukaryotic DNA is associated with histones, prokaryotic DNA is not

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide. Give two reasons why. (2 marks)

• Triplets code for same amino acid

• Occurs in introns / non coding sequence

Explain how copying bases more than once may give rise to a difference in the protein. (2 marks)

It changes the base sequence of later triplets

Outline the similarities in, and the differences in between the structures of DNA and RNA. (6 marks)

Similarities:

• Polymers of nucleotides

• Joined by phosphodiester bonds

• Contain a pentose sugar, base and a phosphate

Differences:

• Deoxyribose v ribose

• Thymine v uracil

• Long v short

Haemoglobins are chemically similar structures found in many different species.

Differences in the primary structure of haemoglobin molecules can provide evidence of phylogenetic (evolutionary) relationships between species.

Explain how. (5 marks)

• Mutations change nucleotide sequence

• Causing change in amino acid sequence

• Mutations build up over time

• More mutations between distantly related species

• Distantly related species have an earlier common ancestor

Some birds feed on animals found in mud in estuaries. The drawing shows the heads of three species of these birds and their prey.
Use the information in the drawing to explain how interspecific competition between the birds is reduced.

• there is less competition between birds with beaks of different lengths

• birds have different size beaks to reach the different types of prey they feed upon.

Crossing over greatly increases genetic diversity in this species of moss. Describe the process of crossing over and explain how it increases genetic diversity. (4 marks)

• Homologous pairs of chromosomes associate

• Chiasma form

• Lengths of alleles are exchanged

• Producing new combinations of alleles

Define the term exon (1 mark)

DNA bases that code for a specific polypeptide

Describe how a gene is a code for the production of a polypeptide. Do not include information about transcription or translation in your answer. (3 marks)

• Because the base/nucleotide sequence

  • is in triplets

• Determines the order of amino acid sequence in polypeptide

Scientists investigated the genetic diversity between several species of sweet potato. They studied non-coding multiple repeats of base sequences.

Define 'non-coding base sequences' and describe where the non-coding multiple repeats are positioned in the genome. (2 marks)

• Non-coding base sequences are DNA that do not code for proteins

• They are positioned between genes.

What is a codon? (2 marks)

• Three bases on mRNA

• That code for an amino acid

Describe how mRNA is formed by transcription in eukaryotes. (5 marks)

• Hydrogen bonds between DNA bases

• Only one DNA strand acts as a template

• Free RNA nucleotides align by complementary base pairing

• In RNA Uracil base pairs with adenine on DNA

• RNA polymerase joins adjacent RNA nucleotides

  • By phosphodiester bonds

• Pre - mRNA is spliced

Describe how a polypeptide is formed by translation of mRNA. (6 marks)

• mRNA attaches to ribosomes

• tRNA anticodons bind to complementary mRNA codons

• tRNA brings a specific amino acid

• Amino acids join by peptide bonds

• Amino acids join together with the use of ATP

• tRNA released after amino acid joined to polypeptide

• The ribosome moves along the mRNA to form the polypeptide

Name the type of mutation that changed the number of chromosomes in S. townsendii to produce S. anglica. Explain your answer. (3 marks)

• Non disjunction

• In meiosis

• Chromosomes not separated

Genetic variation within a species is increased during meiosis by crossing over and the independent segregation of homologous chromosomes. Apart from mutation, explain one other way genetic variation within a species is increased. (2 marks)

• Random fusion of gametes

• Produces new allele combinations

The offspring produced from farmed trout are sterile. Suggest and explain why. (2 marks)

• The offspring produced from farmed trout have extra copies of chromosomes.

• Their homologous chromosomes do not pair.

Scientists set up three fish tanks, each containing a separate population. Each year the scientists removed all the fish from each tank and determined the mean mass of the fish removed. They then put back 10% of each population in the following way.

Tank A – put back only the largest fish.

Tank B – put back fish at random.

Tank C – put back only the smallest fish.

During each year the fish were left to grow and reproduce.

What type of selection were the scientists modelling in this investigation by putting back only the largest or only the smallest fish in Tank A and Tank C? Give a reason why (2 marks)

• Directional

• One extreme selected

The diagram shows two different ways of classifying the same three species of snake.

• Classification X is based on the frequency of observable characteristics

• Classification Y is based on other comparisons of genetic characteristics. All three species of snake belong to the Python family.

State three comparisons of genetic diversity that the scientists used in order to generate Classification Y. (3 marks)

• The base sequence of DNA

• The base sequence of mRNA

• The amino acid sequence of proteins

Some populations of animals that have never been hunted show very low levels of genetic diversity. Other than hunting, suggest two reasons why populations might show very low levels of genetic diversity.

• Population might have been very small

• Population might have started with small number of individuals

• Inbreeding

Explain why it is more useful to calculate an index of diversity than to record species richness. (2 marks)

• Measures abundance of each species

• May be many of some species

Bees are flying insects that feed on nectar made in flowers. There are many different species of bee. Scientists investigated how biodiversity of bees varied in three different habitats during a year. They collected bees from eight sites of each habitat four times per year for three years.

Suggest and explain three ways in which the scientists could have improved the method used for data collection in this investigation. (3 marks)

• Collect at more times of the year so more points on graph/better line (of best fit) on graph

• Counted number of individuals in each species so that they could calculate index of diversity

• Collected from more sites/more years to increase accuracy of data

Describe how you would investigate the effect of an invasion by a non-native species of plant (a biotic environmental factor) over many years on the abundance of a native species of plant in a community. (4 marks)

• Set up grid system with coordinates

• Place large number of quadrats at coordinates selected at random

• Count number of / estimate percentage cover of native plant in quadrats

• Repeat at same time each year (for many years)

The index of diversity of the insects was higher in the hedge than in the barley field. Suggest why. (3 marks)

• More plant species

• More food sources / variety of food

• More habitats / niches;