3.4.1 DNA, genes and chromosomes

Explain the similarities between DNA in eukaryotic cells with DNA in prokaryotic cells. (3 marks)

- DNA nucleotides have the same structure, with deoxyribose bonded to phosphate and a base.

- Nucleotides are joined by phosphodiester bonds and complementary bases are joined by hydrogen bonds.

- DNA in mitochondria and chloroplasts is similar to prokaryotic DNA which is short, circular, and not associated with proteins.

Explain the differences between DNA in eukaryotic cells with DNA in prokaryotic cells. (4 marks)

- Eukaryotic DNA is longer; prokaryotic DNA is shorter.

- Eukaryotic DNA is linear; prokaryotic DNA is circular.

- Eukaryotic DNA is associated with histone proteins; prokaryotic DNA is not.

- Eukaryotic DNA contains introns; prokaryotic DNA does not.

Define a chromosome. (2 marks)

- A long, linear DNA molecule with associated histone proteins.

- Found in the nucleus of eukaryotic cells.

Define a gene and a locus. (3 marks)

- A gene is a sequence of DNA bases that codes for the amino acid sequence of a polypeptide.

- Or a sequence coding for a functional RNA, such as rRNA or tRNA.

- A locus is a fixed position of a gene on a particular DNA molecule.

Describe the nature of the genetic code. (4 marks)

- Triplet code: a set of three DNA bases codes for a specific amino acid.

- Universal: the same base triplets code for the same amino acids in all organisms.

- Non-overlapping: each base is part of only one triplet and read separately.

- Degenerate: more than one triplet can code for the same amino acid.

Define non-coding base sequences and state where they are found. (2 marks)

- DNA sequences that do not code for amino acids or polypeptides.

- Found between genes (e.g. multiple repeats) and within genes (introns).

Define an introns and a exon. (2 marks)

- Exon is a base sequence in a gene that codes for amino acids in a polypeptide.

- Intron is a base sequence in a gene that does not code for amino acids, present in eukaryotic cells.