Topic 4A: Genetic Information, Variation and Relationships Between Organisms

Question 1

Describe how mRNA is formed by transcription in eukaryotes. [5]

Answer 1

1. Hydrogen bonds (between DNA bases) break;
2. (Only) one DNA strand acts as a template;
3. (Free) RNA nucleotides align by complementary base pairing;
4. (In RNA) uracil is used instead of thymine;
5. RNA polymerase joins (adjacent RNA) nucleotides;
6. (By) phosphodiester bonds;
7. Pre-mRNA is spliced (to form mRNA)

Question 2

Describe how a polypeptide is formed by translation of mRNA. [6]

Answer 2

1. (mRNA attaches) to ribosome;
   [2. Ribosome finds start codon]
2. (tRNA) anticodons (bind to) complementary (mRNA) codons;
3. tRNA brings a specific amino acid;
4. Amino acids join by peptide bonds;
5. with the use of ATP;
6. tRNA released (after amino acid joined to polypeptide);
7. The ribosome moves along the mRNA codon by codon to form the polypeptide

Question 3

What is the role of RNA polymerase in transcription? [1]

Answer 3

1. Joins (adjacent) nucleotides using phosphodiester bonds to form a strand of mRNA

Question 4

Name the organelle involves in translation. [1]

Answer 4

1. Ribosome

Question 5

Give four structural differences between a molecule of mRNA and a molecule of tRNA. [4]

Answer 5

1. mRNA is linear chain, tRNA is cloverleaf;
2. mRNA does not have an amino acid binding site, tRNA does;
3. mRNA has more nucleotides;
4. (Different) mRNAs have different lengths, all tRNAs are similar length;
5. mRNA has codons, tRNA has an anticodon

Question 6

Describe how the production of mRNA in a eukaryote cell is different from the production of mRNA in a prokaryote cell. [2]

Answer 6

1. Pre-mRNA produced in eukaryote, mRNA produced in prokaryote;
2. Splicing only occurs in eukaryote;
3. Introns removed in eukaryote, not present in prokaryote

Question 7

Recombinant DNA technology can involve the transfer of fragments of human DNA into bacteria. The bacteria are then used to produce human proteins.

Give two reasons why bacteria are able to use human DNA to produce human proteins. [2]

Answer 7

1. The genetic code is universal;
2. (The mechanism of) transcription is universal;
3. (The mechanism of) translation is universal

Question 8

Compare and contrast the DNA in the nucleus of a plant cell with the DNA in a prokaryotic cell. [5]

Answer 8

Similarities:
1. Nucleotide structure is identical;
2. Nucleotides joined by phosphodiester bonds;

Differences: (plant DNA vs prokaryote DNA)
3. (Associated with) histones vs no histones;
4. Linear vs circular;
5. No plasmids vs plasmids;
6. Introns vs no introns;
7. Longer vs shorter

Question 9

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

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

b. People with the CCR5 mutation show a greater resistance to developing AIDS.
Explain why. [2]

Answer 9

a. 1. Change in DNA base sequence;
2. Change in primary structure;
3. Alters (position of) hydrogen/ionic/disulphide bonds;
4. Change in tertiary structure

b. 1. (Receptor) is not complementary (to attachment protein)
2. No destruction of (helper) T cells
OR
No replication of virus

Question 10

Describe how mRNA is produced from an exposed template strand of DNA.

Do not include DNA helicase or splicing in your answer. [3]

Answer 10

1. (Free RNA) nucleotides form complementary base pairs;
2. Phosphodiester bonds form;
3. By RNA polymerase

Question 11

Define the term exon. [1]

Answer 11

1. Base sequence coding for polypeptide

Question 12

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide.

Give two reasons why. [2]

Answer 12

1. Triplets code for same amino acid;
2. Occurs in introns

Question 13

What is the name given to any factor that can increase the rate of mutation? [1]

Answer 13

1. Mutagenic agent

Question 14

The genetic code is described as degenerate.

What is meant by this? [1]

Answer 14

1. More than one codon codes for a single amino acid

Question 15

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

Answer 15

1. Ribosomal RNA and protein

Question 16

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

Answer 16

1. mRNA binds to ribosome;
2. Holds two codons at a time;
3. (Allows) tRNA with anticodons to bind;
4. (Catalyses) formation of peptide bond (between amino acids);
5. Moves along (mRNA to the next codon)

Question 17

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

Explain why. [2]

Answer 17

1. Introns (in pre-mRNA);
2. Splicing

Question 18

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]

Answer 18

1. Pre-mRNA contains introns;
2. These regions are removed from pre-mRNA

Question 19

What is the proteome of a cell? [1]

Answer 19

1. The full range of different proteins that a cell is able to produce

Question 20

Starting with mRNA in the cytoplasm, describe how translation leads to the production of a polypeptide.

Do not include descriptions of transcription and splicing in your answer. [5]

Answer 20

1. mRNA attaches to a ribosome;
2. Ribosome finds the start codon;
3. tRNA brings specific amino acid;
4. Anticodon (on tRNA complementary) to codon (on mRNA);
5. Ribosome moves along to next codon;
6. (Process repeated and) amino acids joined by peptide bonds (to form polypeptide)

Question 21

Describe how the process of meiosis results in haploid cells.
Do not include descriptions of how genetic variation is produced in meiosis. [4]

Answer 21

1. DNA replication during interphase;
2. Two (nuclear) divisions;
3. Separation of homologous chromosomes (in first division);
4. Separation of sister chromatids (in second division)

Question 22

What is the biological importance of reducing the chromosome number when the cell divides by meiosis? [2]

Answer 22

1. Later fertilisation;
2. Restoring diploid

Question 23

A scientist crossed two strains of fungus. To cross these strains.

Describe and explain three ways in which the scientist would ensure he used aseptic techniques. [3]

Answer 23

1. Wash hands to kill microbes;
2. Burning Bunsen close by to create upward current of air;
3. Disinfect bench to kill microbes;
4. Flame instrument to kill microbes;
5. Lift lid slightly to prevent entry of microbes

Question 24

Give two differences between mitosis and meiosis. [2]

Answer 24

Mitosis vs meiosis
1. One division vs two divisions;
2. (Daughter) cells genetically identical vs daughter cells genetically different;
3. Two cells produced vs four cells produced;
4. Diploid to diploid vs diploid to haploid;
5. Separation of homologous chromosomes only in meiosis;
6. Crossing over only in meiosis

Question 25

Give one reason why butterfly eggs produced by meiosis are genetically different. [1]

Answer 25

1. Independent segregation
   OR
   Crossing over

Question 26

What is an allele? [1]

Answer 26

1. Version of a gene

Question 27

Explain how the chromosome number is halved during meiosis. [2]

Answer 27

1. Homologous chromosomes (pair);
2. One of each (pair) goes to each (daughter) cell

Question 28

Describe the process of crossing over and explain how it increases genetic diversity. [4]

Answer 28

1. Homologous pairs of chromosomes form a bivalent;
2. Chiasmata form;
3. (Equal) lengths of (non-sister) chromatids are exchanged;
4. Producing new combinations of alleles

Question 29

Apart from mutation, crossing over and independent segregation explain one other way genetic variation within a species is increased. [2]

Answer 29

1. Random fertilisation;
2. (Produces) new allele combinations