4 Genetic information, variation and relationships between organisms

Question 1

Name the fixed position occupied by a gene on a DNA molecule. (1)

Answer 1

Locus/Loci

Question 2

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

Answer 2

The gene’s base sequence is read in triplets, which determines the order of the amino acid sequence in the polypeptide.

Question 3

Define the term exon. (1)

Answer 3

The base sequence coding for a sequence of amino acids.

Question 4

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule. (2)

Answer 4

There is a condensation reaction between the phosphate group and the deoxyribose sugar. This reaction is catalysed by DNA polymerase.

Question 5

Name the protein associated with DNA in a chromosome. (1)

Answer 5

Histone.

Question 6

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

Answer 6

Non-coding sections are DNA which do not code for an amino acid sequence.
They are positioned between genes.

Question 7

Give three ways in which the DNA in a chloroplast is different from the DNA in a nucleus. (3)

Answer 7

1. In chloroplasts, DNA is shorter whereas in the nucleus, DNA is longer.
2. In chloroplasts, the DNA is circular whereas in the nucleus it is linear.
3. In chloroplasts, the DNA is not associated with histones whereas in nuclear DNA it is.

Question 8

Other than the uracil base in place of thymine, give one other difference between the structure of a DNA nucleotide and the structure of an RNA nucleotide. (1)

Answer 8

DNA has deoxyribose sugar and RNA has ribose sugar.

Question 9

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 9

1. Triplets code for the same amino acid (degenerate)
2. They can occur in the non-coding sequence.

Question 10

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

Answer 10

Nucleotide structure are identical in both.
Their nucleotides are joined by phosphodiester bonds.

Eukaryotic DNA is longer than prokaryotic DNA.
Eukaryotic DNA is linear, prokaryotic DNA is circular.
Eukaryotic DNA is associated with histones, prokaryotic DNA is not.

Question 11

What is a homologous pair of chromosomes? (1)

Answer 11

Two chromosomes that carry the same genes.

Question 12

What is the bond between complementary base pairs in a DNA molecule? (1)

Answer 12

Hydrogen bond.

Question 13

What is the bond between adjacent nucleotides in a DNA strand? (1)

Answer 13

Phosphodiester bond.

Question 14

Describe two differences between the structure of a tRNA molecule and an mRNA molecule. (2)

Answer 14

1. tRNA is a ‘clover leaf’ shape, mRNA is linear.
2. tRNA has an amino acid binding site, mRNA is not.

Question 15

Describe and explain a difference in the structure of mRNA used in translation and pre-mRNA molecules. (2)

Answer 15

mRNA has no introns, only exons. pre-mRNA has both introns and exons. This is due to splicing.

Question 16

Describe how one amino acid is added to a polypeptide that is being formed at a ribosome during translation. (3)

Answer 16

tRNA brings a specific amino acid to the ribosome. The anticodon on the tRNA binds to the codon on mRNA.The amino acids join by a condensation reaction using ATP.

Question 17

Describe how mRNA is produced from an exposed template strand of DNA. (3)

Answer 17

Free RNA nucleotides join form complementary base pairs, phosphodiester bonds form by the action of RNA polymerase.

Question 18

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

Answer 18

Hydrogen bonds between the DNA bases break. Only one DNA strand acts as a template where free RNA nucleotides align by complementary base pairing. Uracil pairs with adenine on the DNA strand. RNA polymerase joins the adjacent nucleotides by phosphodiester bonds between them.

Question 19

What is the proteome of a cell? (1)

Answer 19

The proteome is the full range of proteins that a cell is able to produce.

Question 20

Give two structural differences between mRNA and tRNA. (2)

Answer 20

mRNA does not have hydrogen bonds, tRNA does.

mRNA does not have an amino acid binding site, tRNA does.

Question 21

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 21

mRNA associates with a ribosome. The ribosome moves to the start codon. tRNA brings the specific amino acid. The anticodon on the tRNA is complementary to the codon on the mRNA. The ribosome moves along to the next codon. The process is repeated and the amino acids are joined by peptide bonds.

Question 22

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

Answer 22

The DNA strands separate by the hydrogen bonds being broken. Only one of the DNA strands are used. Complementary base pairing joins adenine with guanine, thymine with adenine, cytosine with guanine and guanine with cytosine. The RNA nucleotides are joined by RNA Polymerase. Pre-mRNA is formed which is then spliced to form mRNA.

Question 23

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

Answer 23

RNA

Proteins

Question 24

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

Answer 24

mRNA binds to the ribosome. The codons allow tRNA with anticodons to bind, which catalyses the formation of peptide bonds between amino acids. The ribosome moves along the mRNA to the next codon.

Question 25

In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA. Explain why. (2)

Answer 25

pre-mRNA has introns. mRNA doesn’t have these introns because they have been spliced out.

Question 26

Define the term mutagenic agent. (1)

Answer 26

A factor that increases the rate of mutations.

Question 27

Apart from mutation, explain one other way in which genetic variation within a species is increased. (2)

Answer 27

Random fertilisation produces new allele combinations.

Question 28

Define ‘ gene mutation ‘ and explain how a gene mutation can have :
- no effect on an individual
- a positive effect on an individual
(4)

Answer 28

A gene mutation is a change in the base sequence of DNA. A mutation causes a formation of a new allele, which doesn’t have an effect because the genetic code is degenerate.
The new allele is is recessive so doesn’t influence the phenotype, which has a positive effect because it results in a change in the polypeptide that positively changes the named protein.

Question 29

Give two differences between meiosis and mitosis. (2)

Answer 29

Mitosis has one division, meiosis has two divisions.

The daughter cells in mitosis are genetically identical, however in meiosis they are genetically different.

Question 30

Explain how the chromosome number is halved during meiosis. (2)

Answer 30

Homologous chromosomes pair up. One of each pair goes to each daughter cell.

Question 31

Describe the process of crossing over and explain how it increases genetic diversity. (4)

Answer 31

Homologous chromosomes associate, forming a chiasmata. Equal lengths of the chromatids are exchanged, which produces new combinations of alleles.

Question 32

What is the term used to describe the method of naming organisms by their species name and genus name? (1)

Answer 32

Binomial

Question 33

Other than hunting, suggest two reasons why populations might show very low levels of genetic diversity. (2)

Answer 33

Population may be very small.

Population might have started with very few individuals.

Question 34

Suggest and explain one advantage and one disadvantage to the farmer of replanting hedges on her farmland. (2)

Answer 34

Increase of predators to pests so there is less damage to crops.

Reduced land area for crop growth.

Question 35

What is meant by ‘species richness’? (1)

Answer 35

A measure of the number of different species in a community.

Question 36

What is the difference between species richness and index of diversity? (1)

Answer 36

Species richness only measures a number of species, not the number of individuals.

Question 37

What is meant by genetic diversity? (1)

Answer 37

The number of different alleles of each gene.