Topic 4 AQA A Level

Explain the role of independent segregation in meiosis/2

1. To provide genetic variation

2. Allows different combinations of maternal and paternal chromosomes / alleles

Explain how the chromosome number is halved during meiosis/2

1. Homologous chromosomes (pair);

2. One of each (pair) goes to each (daughter) cell / to opposite poles

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

1. Homologous pairs of chromosomes associate / form a bivalent;

2. Chiasmata form;

3. Equal lengths of non-sister chromatids are exchanged

4. Producing new combinations of alleles

Define each of the following terms.i) species ii) species richness / 2

1. Species = (A group of) organisms that are able to produce fertile offspring;

2. Species richness = the number of (different) species in a community;

Give the two types of molecule from which a ribosome is made./1

One of RNA / ribonucleic acid(s) / nucleotide(s)/nucleic acid(s) / rRNA / ribosomal RNA / ribosomal ribonucleic acid

AND one of protein(s) / polypeptide(s) / amino acid(s) / peptide(s) / ribosomal protein;

Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer./3

1. mRNA binds to ribosome;

2. Idea of two codons / binding sites;

3. (Allows) tRNA with anticodons to bind / associate;

4. (Catalyses) formation of peptide bond between amino acids (held by tRNA molecules);

5. Moves along (mRNA to the next codon) / translocation described;

In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA./2

1. Introns (in pre-mRNA);

2. Removal of sections of (pre-mRNA) / splicing;

Introns removed' scores 2 marks.

State three comparisons of genetic diversity that scientists use in order to generate classification based on genetic diversity/3

1. The (base) sequence of DNA

2. The (base) sequence of mRNA;

3. The amino acid sequence (of proteins);

The genetic code is degenerate and non-overlapping.

Explain the meaning of: i) degenerate ii) non overlapping

1. Degenerate: more than one (base) triplet for each amino acid;

2. Non-overlapping: each base is part of only one triplet.

In a eukaryotic cell, transcription results in a molecule of pre-mRNA that is modified to produce mRNA. In a prokaryotic cell transcription produces mRNA directly.Explain this difference/ 2

1. DNA of eukaryotic cell has non-coding regions / introns within gene

Allow converse: (But) a prokaryotic cell does not have non-coding regions / introns in DNA;

OR

pre-mRNA contains non-coding regions / introns;

2. (After transcription / during modification) these regions are removed from (pre-)mRNA;

Ignore references to 'cells need / bacteria do not need'

Give two differences between the structure of mRNA and the structure of tRNA./2

1. mRNA longer

OR

Has more nucleotides than tRNA;

2. mRNA is a straight molecule but tRNA is a folded molecule / clover-leaf shaped molecule;

3. mRNA contains no paired bases / hydrogen bonds but tRNA has some paired bases / hydrogen bonds.

DNA is a polymer of nucleotides. Each nucleotide contains an organic base. Explain how the organic bases help to stabilise the structure of DNA/2

1. Hydrogen bonds between the base pairs holds two strands together

2. Many hydrogen bonds provides strength

Name two ways in which meiosis produces genetic variation./2

1. Independent segregation of homologous chromosomes

2. Crossing over / formation of chiasmata.

Explain how a gene mutation could result in a new protein./3

1. Change in sequence of bases

2. As a result of a deletion or substitution;

3. Change in amino acid sequence

4. Change in tertiary structure of protein

Contrast the structures of DNA and mRNA molecules to give three differences./3

Contrast requires both parts of the statement

1. DNA double stranded and mRNA single-stranded;

2.DNA very long and mRNA short;

Accept ‛RNA shorter' or ‛DNA bigger/longer'

3. Thymine in DNA and uracil in RNA

4.Deoxyribose in DNA and ribose in RNA;

5.DNA has base pairing and mRNA doesn't/ DNA has hydrogen bonding and mRNA doesn't

6.DNA has introns/non-coding sequences and mRNA doesn't

HIV attaches to a specific protein receptor on helper T cells. A low percentage of people have a mutation of the CCR5 gene which codes for this protein receptor. This mutation results in a non-functional protein receptor.

Explain how this mutation can result in the production of a non-functional protein receptor./4

1. Change in DNA base

2. Change in amino acid sequence

3. Alters position of hydrogen/ionic/disulfide bonds

4. Change in tertiary structure of receptor

Explain what is meant by a phylogenetic group/1

Grouped according to evolutionary relationships / common ancestry

Explain what is meant by a species/2

1. Able to reproduce

2. To produce fertile offspring

What is meant by species diversity?/1

Number of species in a community

Give two pieces of information needed to calculate an index of diversity for a community./2

1. Number of organisms of each species

2. Total number of organisms of all species / Total number of species

There are nine subspecies of giraffe. These subspecies evolved when populations of giraffe were separated for long time periods. Each subspecies has distinct coloured skin markings. Some biologists have suggested that up to six of these subspecies should be classified as different species.

Explain how different subspecies of giraffe may have evolved from a common ancestor. Use information from the passage in your answer./5

1. No interbreeding / geographical isolation

2. Mutation linked to different markings

3. Survival linked to different) markings

4. Adapted organisms breed

5. Change/increase in allele frequency

Why is the genetic code described as being universal?/1

In all organisms the same triplet codes for the same amino acid

Name the process that removes base sequences from pre-mRNA to form mRNA./1

Splicing

Homologous chromosomes carry the same genes but they are not genetically identical.Explain why./2

1. Holds chromatids together

2. Attaches chromatids to spindle;

3. Allows chromatids to be separated / move to opposite poles

Other than independent segregation, give one way in which meiosis allows the production of genetically different cells./1

Crossing over / alleles exchanged between chromosomes or chromatids / chiasmata formation / genetic recombination;

The genetic code is described as being degenerate. What does this mean?/1

An amino acid can be coded for by more than one triplet

What is a codon?/2

1. Triplet on mRNA

2. That code for an amino acid

What is the role of RNA polymerase during transcription?/1

To join nucleotides together to form mRNA

mRNA can be converted to cDNA. Name the enzyme used in this process./1

Reverse transcriptase;

Explain how a change in the base sequence of the mRNA could lead to a change in the tertiary structure of a protein/1

Different primary structure / amino acid sequence coded for

Explain how a mutation may lead to the formation of non-functioning enzymes/3

1. Change in amino acid /sequence of amino acids

2. Change in hydrogen / ionic / disulphide bonds alters tertiary structure / active site of enzyme

3. Substrate not complementary to active site, so no enzyme- substrate complexes form

Frequent treatment with a specific antibiotic can result in resistant strains of bacteria. Explain how./2

1. Mutation present

2. Resistance gene / allele

3. Resistant bacteria survive and reproduce;

Explain what is meant by genetic diversity./1

Difference in DNA / base sequence / difference in alleles / genes / gene pool;

Apart from genetic factors what other type of factor causes variation within a species?/1

Environmental

The number of species present is one way to measure biodiversity. Explain why an index of diversity may be a more useful measure of biodiversity./2

"Also measures number of individuals in a species / different proportions of species; (must reference species) Some species may be present in low / high numbers;"

Give one way in which the structure of an mRNA molecule is different from the structure of a tRNA molecule./1

"Does not contain hydrogen bonds / base pairs / contains codons / does not contain anticodon / straight / not folded / no amino acid binding site / longer;"

Explain the difference between pre-mRNA and mRNA./1

(pre-mRNA) contains introns / mRNA contains only exons;

What name is used for the non-coding sections of a gene?/1

Introns

What is an allele?/1

(Different) form / type / version of a gene / different base sequence of a gene

Comparing the base sequence of a gene provides more information than comparing the amino acid sequence for which the gene codes. Explain why./2

"Reference to base triplet / triplet code / more bases than amino acids / longer base sequence than amino acid sequence; Introns / non-coding DNA; / same amino acid may be coded for / DNA code is degenerate;

The number of patients infected with the bacterium MRSA has increased in some hospitals. Scientists have suggested ways to reduce the transmission of MRSA in hospitals. Suggest two ways to reduce the transmission of MRSA in hospitals./2

Isolation

Screening of patients / doctors

Sterilisation of wards / equipment

Explain how a change in a sequence of DNA bases could result in a non-functional enzyme./3

Change in sequence of amino acids

Change in hydrogen / ionic / disulfide bonds leads to change in tertiary structure

Substrate cannot bind / no enzyme-substrate complexes form

Meiosis results in cells that have the haploid number of chromosomes and show genetic variation. Explain how./6

1. Homologous chromosomes pair up / bivalents form;

2. Crossing over / chiasmata form;

3. Produces new combination of alleles;

4. Chromosomes separate;

5. At random;

6. Produces varying combinations of chromosomes / genes / alleles (not twice) ;

7. Chromatids separated at meiosis II / later;

A student investigated the process of meiosis by observing cells on a microscope slide. The cells on the slide had been stained. Explain why a stain was used/1

to make chromosomes / chromatids / DNA / genetic material visible;

What is meant by a gene?/2

a length of DNA; that codes for a single protein / polypeptide

In which phase of the cell cycle does DNA replication take place?/1

Interphase / S-phase

Describe the role of the spindle during mitosis./2

Attachment of centromeres / chromosomes / chromatids; Separation of centromeres / chromatids / chromosomes;

Meiosis also occurs during the life cycle of organisms. What is the importance of meiosis?/2

Halves chromosome number / haploid;

Diploid / full number restored at fertilisation

Give two differences between the structure of mRNA and the structure of tRNA./2

1 mRNA has no base-pairing, tRNA has base-pairing / 2 mRNA linear, tRNA cloverleaf shape;

3 mRNA has no binding site for amino acids, tRNA has

4 mRNA different for each gene / many kinds, only few / 20 / 64 kinds of tRNA

5 accept mRNA longer / larger / more nucleotides than tRN

Tetracycline prevents bacterial growth by preventing protein synthesis. Give two other ways in which antibiotics can prevent bacterial growth./2

disrupts cell wall / prevents cell wall synthesis;

stops DNA replication;

Deforestation reduces the diversity index of an area cleared in this way. Explain why./2

deforestation removes many habitats / niches fewer species / fewer types of organisms;

Describe how crossing over occurs during meiosis I/2

" chromosomes in each (homologous) pair twist around each other; chromatids break and rejoin to chromatid on sister chromosome;"

Explain how resistance to an antibiotic could become widespread in a bacterial population following a gene mutation conferring resistance in just one bacterium./5

" 1. frequent use of antibiotic creates selection pressure / antibiotic kills bacteria; 2. bacteria with mutation / resistance have (selective) advantage over others / described; 3. (survive to) reproduce more than other types pass on advantageous allele / mutated allele in greater numbers; 4. frequency of (advantageous) allele increases in subsequent generations; 5. frequency of resistant types increases in subsequent generations;"

Give the stages / main event which takes place in the cell cycle i) metaphase - event is... ii) ...... - chromosomes coil and shorten iii) ...... - Daughter chromosomes move to poles of the cell iv) S phase - event is.... V) ..... - nuclear envelope reforms /5

Chromosomes attach to equator / middle of cell / spindle;

Prophase;

Anaphase;

DNA replication / synthesis / chromosome copying / duplication;

Telophase;

Explain one way in which the behaviour of chromosomes during meiosis produces genetic variation in gametes./2

Two linked points:

Crossing over / exchange of material (between chromatids);

Different combinations of alleles / linkage groups changed / broken;

OR

Independent assortment / alignment of (homologous) chromosomes;

Different combinations of (maternal and paternal) chromosomes / alleles;

What is the role of RNA polymerase in transcription?/1

join / attach nucleotides, to form a strand / along backbone / phosphodiester bonds;

Name the organelle involved in translation./1

ribosome / RER

Apart from increasing genetic variation, explain why meiosis is important in organisms which reproduce sexually./2

haploid cells produced / halves chromosome number;

fertilisation / fusion of gametes, diploid number restored;

chromosome number constant at each generation;

In tissues that produce gametes, there is a greater proportion of cells undergoing meiosis in male tissue than in female tissue. Suggest one advantage of this./1

(in males) more gametes produced / rapid gamete production / more lost;

Describe one way by which different species may be distinguished./1

"do not interbreed to produce fertile offspring / different DNA / different niches;"

Give two factors, other than cost, that should be considered when selecting an antibiotic to treat a bacterial disease./2

side effects / allergic reactions / low toxicity to cells;

interaction with other drugs / effective in conditions of use / reasonably stable;

should only act on the problem bacteria / narrow spectrum;

how much resistance the bacteria have built up;

Describe the role of tRNA in the process of translation./3

anticodon complementary to codon / reads message on mRNA;

specific amino acid;

carried / transferred (to ribosome);

correct sequence of amino acids along polypeptide;

Explain what is meant by an allele./1

different form of a gene;

Explain how DNA replicates./4

hydrogen bonds broken;

semi-conservative replication / both strands used (as templates);

nucleotides line up complementary / specific base pairing / A and T / C and G;

DNA polymerase;

Explain why a mutation involving the deletion of a base may have a greater effect than one involving substitution of one base for another./3

deletion causes frame shift / alters base sequence (from point of mutation);

changes many amino acids / sequence of amino acids (from this point);

substitution alters one codon / triplet / one amino acid altered / code

degenerate / same amino acid coded for;

Explain the principles which biologists use to classify organisms into groups./3

large groups are divided into smaller groups;

(not just 'hierarchical')

members of a group have features in common based on anatomy

/ fossils / embryology / DNA / specific aspect of cell biology

/ homologous structures;

reflects evolutionary history;

Explain why meiosis is important in sexual reproduction, apart from producing gametes that are genetically different./2

produces haploid cells / chromosome number halved;

fertilisation maintains the diploid / chromosome number (in next generation)

Give two processes, other than crossing over, which result in genetic variation. Explain how each process contributes to genetic variation./4

mutation - so different allele formed

random fusion of gametes - so new combination of alleles

independent assortment of chromosomes - so new combination of alleles

During meiosis, one chromosome from each homologous pair goes to each of the cells produced. Explain why this is important./2

to get haploid / n / half number of chromosomes (in cells);

so that each cell gets one copy of each chromosome / gene / full

set of genes / so that fertilisation produces diploid / constant

chromosome number; results in independent assortment;

Explain how crossing over can contribute to genetic variation./3

sections of chromatids exchanged;

sections have different alleles;

new combinations of (linked) alleles;

Explain the importance of meiosis in the life cycles of organisms which reproduce sexually./3

meiosis halves the chromosome number / from diploid to haploid / produces haploid / n cells;

when gametes fuse / at fertilisation, the diploid number is restored / this keeps the chromosome number constant / correct from one

generation to the next / after sexual reproduction;

introduces genetic variation / independent assortment / crossing over;

Starting with mRNA, describe how the process of translation leads to the production of a polypeptide./4

mRNA attaches to ribosome;

codon on mRNA;

binds to an anti-codon on tRNA;

each tRNA brings a specific amino acid;

sequence of codons / bases on mRNA determines order of amino acids;

formation of peptide bonds / amino acids joined by condensation

reactions;

Describe the part played by RNA in protein synthesis./3

Carries coded information about the sequence of amino acids;

Copied from DNA/gene;

Code is in sequence of bases / triplet / three bases / a codon codes for one amino acid;

Moves out of nucleus/goes into cytoplasm;

To ribosomes;

Two differences between DNA and RNA/ 2

DNA is double stranded, RNA is single stranded;

DNA has thymine present, RNA has Uracil present;

Accept T and U

DNA is larger/heavier/longer, RNA is smaller/lighter/shorter;

DNA has a deoxyribose sugar, RNA has a ribose sugar;

DNA stays in the nucleus, RNA leaves the nucleus;

Two differences between the structure of DNA and RNA./2

DNA Large molecule; RNA Smaller

Double stranded; Single stranded

Contains Thymine (T) ; Contains Uracil (U)

Contains deoxyribose; Contains ribose

Explain how a gene codes for a protein./2

Base sequence (on DNA/in gene);

Determines sequence of amino acids;

By determining base sequence on (messenger) RNA;

Code is a triplet code/three base code for an amino acid;

What are homologous chromosomes?/2

Pairs of chromosomes/two chromosomes;

With genes for same features / with same genes;

At same loci / in same sequence;