4.1: DNA, Genes, and Chromosomes

Name the fixed position occupied by a gene on a DNA molecule [1 mark]

Locus/loci

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]

1. Because base/nucleotide sequence

2. In triplets

3. Determines order/sequence of amino acid sequence/primary structure in polypeptide

Define the term exon [1 mark]

Base/nucleotide/triplet sequence coding for polypeptide/sequence of amino acids/primary structure

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule [2 marks]

1. Condensation reaction/loss of water

2. Between phosphate and deoxyribose

3. Catalysed by DNA polymerase

Name the proteins associated with DNA in a chromosome [1 mark]

Histone

List three differences between DNA in the nucleus of a plant cell and DNA in a prokaryotic cell [3 marks]

1. Eukaryotic DNA in a plant cell is associated with histones/proteins whereas prokaryotic DNA is not associated with histones

2. Eukaryotic DNA is linear wheras prokaryotic DNA is circular

3. Eukaryotic DNA does not have plasmids whereas prokaryotic DNA does have plasmids

4. Eukaryotic DNA contains introns located between genes whereas prokaryotic DNA does not contain introns

5. Eukaryotic DNA is longer due to it containing both introns and exons whereas prokaryotic DNA is shorter since it only contains exons

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide [2 marks]

1. Triplets code for the same amino acids (accept DNA/code/triplets are degenerate)

2. Occurs in introns/non-coding sequence (reject codons because the questions states within genes)

What is a homologous pair of chromosomes [1 mark]

Two chromosomes that carry the same genes

Name the structures labelled W and X in the diagram [1 mark]

W = amino acid binding site

X = anticodon

Give three differences between the structure of a DNA nucleotide and the structure of an RNA nucleotide [3 marks]

1. Deoxyribose in DNA and ribose in RNA

2. DNA is double stranded whereas RNA is single stranded

3. In DNA, adenine is complimentary to thymine whereas in RNA, adenine is complimentary to uracil