3.4.1 DNA, Genes and Chromosomes

Question 1

Gene

Answer 1

A base sequence of DNA that codes for:
* the amino acid sequence of a polypeptide
* or a functional RNA (including rRNA and tRNA)

Question 2

Locus

Answer 2

A fixed position on a particular DNA molecule which a gene occupies

Question 3

Triplet

Answer 3

A sequence of 3 DNA bases that codes for a specific amino acid.

Question 4

Features of the genetic code

Answer 4

• Universal
• Non-overlapping
• Degenerate

Question 5

Features of the genetic code: Degenerate

Answer 5

• Most amino acids are coded by more than 1 triplet
• There are 20 different amino acids and 4 bases( so 4^3 = 64 possible triplet codes)

Question 6

Features of the genetic code: Non-overlapping

Answer 6

Each base in the sequence is read only once

Question 6

Features of the genetic code: Universal

Answer 6

With a few minor exceptions each triplet code for the same amino acid in all organisms

Question 7

DNA in eukaryotes

Answer 7

A lot of the nuclear DNA does not occur for polypeptides

Question 8

Exons

Answer 8

Sequences of bases that code for amino acid sequences

Question 9

Introns

Answer 9

Non-coding multiple repeats of base sequences between genes

Question 10

DNA in prokaryotic cells

Answer 10

• e.g. bacteria
• Short, circular and not associated with proteins (histones)

Question 11

DNA in eukaryotic cells

Answer 11

• Long, linear and associated with proteins (histones)

Question 12

DNA in mitochondria and chloroplasts of eukaryotic cells

Answer 12

Similar to the DNA in prokaryotes (short, circular and not associated with protein)

Question 13

Chromosome Structure

Answer 13

• Only visible as distinct structures during cell division
• Otherwise widely dispersed throughout the nucleus

Question 14

Making a chromosome

Answer 14

1. DNA molecule (double helix) is wound around histones to fix it in position
2. DNA-histone complex is coiled.
3. This is further coiled before being packed into the chromosome

Question 15

Homologous pairs

Answer 15

1 chromatid comes from the maternal chromosome and 1 chromatid comes from the paternal chromosomes

2 chromosomes that carry the same genes

Question 16

Allele

Answer 16

• One of a number of alternative forms of a gene
• Each gene exists in two or more different forms

Question 17

Mutations and alleles

Answer 17

When the base sequence of a gene changes and a new allele is formed = mutation, a different polypeptide is formed