4.1 DNA, genes and chromosomes

Question 1

DNA molecules in eukaryotic cells are

Answer 1

Found in the nucleus, very long, linear and associated with proteins called histones.

Question 1

DNA molecules in prokaryotic cells are

Answer 1

short, circular and not associated with proteins.

Question 2

A chromosome is

Answer 2

a DNA molecule and its associated proteins.

Question 3

Apart form in prokaryotic cells, where else might you find DNA which is short, circular and not associated with protein?

Answer 3

The mitochondria and chloroplasts of eukaryotic cells.

Question 4

What is a gene, and where are genes found?

Answer 4

A gene is a base sequence of DNA that codes for:
* the amino acid sequence of a polypeptide
* a functional RNA (including ribosomal RNA and tRNAs).
A gene occupies a fixed position, called a locus, on a particular DNA molecule.

Question 5

The genetic code is universal, non-overlapping and degenerate. Define universal, non-overlapping and degenerate.

Answer 5

Universal: The same codon/triplet always codes for the same amino acid;

Non-overlapping: Each base is only part of one triplet/codon. Adjacent codons/triplets do not overlap.

Degenerate: More than one codon/triplet codes for each amino acid.

Question 6

Compare and contrast triplets and a codons.

Answer 6

Both a triplet and a codon are a sequence of three DNA bases that codes for a single, specific amino acid.

Triplets are found in DNA, whereas codons are found in mRNA.
Triplets have nucleotides with the nitrogenous base thymine, whereas codons have nucleotides with the nitrogenous base uracil.

Question 7

The nucleus and a chloroplast of a plant cell both contain DNA.

Give five ways in which the DNA in a chloroplast is different from DNA in the nucleus.

Answer 7

In chloroplasts:

1.   DNA shorter;

2.   Fewer genes;

3.   DNA circular not linear;

4.   Not associated with protein/histones, unlike nuclear DNA;

5.   Introns absent but present in nuclear DNA;