4.1 - DNA, genes and chromosomes

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 or translation in your answer. (3)

Answer 2

• (Because) base/nucleotide sequence
• (In) triplet(s)
• (Determines) order/sequence of amino acid sequence/primary structure (in polypeptide)

Question 3

Define the term exon. (1)

Answer 3

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

Question 4

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

Answer 4

• Condensation (reaction)/loss of water
• (Between) phosphate and deoxyribose
• (Catalysed by) DNA polymerase

Question 5

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

Answer 5

• Histone

Question 6

Explain three differences between DNA in the nucleus of a plant cell and DNA in a prokaryotic cell. (3)

Answer 6

• (Associated with) histones/proteins v no histones/proteins
• Linear v circular
• No plasmids v plasmids
• Introns v no introns
• Long(er) v short(er)

Question 7

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

Answer 7

• DNA that does not code for protein/polypeptides
  / DNA that does not code for (sequences of) amino acids
  / DNA that does not code for tRNA/rRNA
• (Positioned) between genes

Question 8

Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus. (2)

Answer 8

In chloroplasts…
- DNA shorter
- Fewer genes
- DNA circular not linear
‘accept no chromosomes (in chloroplast) unlike nucleus’
- Not associated with protein/histones, unlike nuclear DNA
- Introns absent but present in nuclear DNA

Question 9

Give one other difference between the structure of a DNA nucleotide and the structure of an RNA nucleotide. (1)

Answer 9

• Deoxyribose in DNA and ribose in RNA

Question 10

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 10

• Triplets code for same amino acid
  ‘accept: DNA/code/triplets are degenerate’
• Occurs in introns /non-coding sequence

Question 11

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

Answer 11

• Nucleotide structure is identical
  ‘accept description of nucleotide as phosphate, deoxyribose and base’
• Nucleotides joined by phosphodiester bond
  / Deoxyribose joined to phosphate (in sugar, phosphate backbone)
• DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes
• 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 / bound to protein / histones prokaryotic DNA is not

Question 12

Suggest one way the structure of the chromosome could differ along its length to result in the stain binding more in some areas. (1)

Answer 12

• Differences in base sequences
• Differences in histones/interaction with histones
• Differences in condensation/ (super)coiling

Question 13

What is a homologous pair of chromosomes? (1)

Answer 13

• (Two chromosomes that) carry the same genes