3.4.1 DNA, genes and chromosomes

Question 1

Define gene

Answer 1

• a section of DNA located on a fixed site called the Locus
• contains a specific sequence of bases for the formation of polypeptides and functional RNA (ribosomal and transfer)

Question 2

What are the three main features of the genetic code?

Answer 2

• universal - the same in almost all organisms
• non-overlapping - each triplet is read only once
• degenerate - all but two amino acids have more than one DNA triplet (as there are more possible triplet combos than there are amino acids)

Question 3

How many different triplets and amino acids are there?

Answer 3

64 triplets - there are 20 amino acids so some are coded for by more than one triplet

Question 4

Define stop code

Answer 4

don’t code for any amino acid and mark the end of a chain

Question 5

Define exon

Answer 5

a base sequence which codes for a polypeptide sequence of amino acids

Question 6

Define intron

Answer 6

non-coding sequences (only found in eukaryotes)
- removed during transcription and exons are joined (splicing)

Question 7

Define non-coding repeats

Answer 7

positioned outside of the gene and contain the same base sequence repeated multiple times

Question 8

Describe the main differences between prokaryotic and eukaryotic DNA

Answer 8

Prokaryotic VS Eukaryotic
* in cytoplasm - in nucleus
* circular - linear
* not associated with proteins - associate with histone proteins to form chromosomes
* shorter - longer
* no introns - introns

Question 9

Describe DNA structure in eukaryotic cells

Answer 9

• DNA is wound around histone proteins - these also support the DNA
• DNA-histone complex is coiled and coils fold to form loops
• loops pack to form a chomosome

Question 10

Describe DNA structure in prokaryotic cells

Answer 10

DNA loops condenses via supercoiling

Question 11

What is DNA like within chloroplasts and mitochondria?

Answer 11

similar to prokaryotic cells - short, circular and not associated with histone proteins

Question 12

Define homologous pair

Answer 12

two chromosomes which carry the same genes but not necessarily the same alleles of the genes (eg two chromosomes could have an allele for type A blood and an allele for type B blood)

Question 13

Define Allele

Answer 13

alternative forms of the same gene (one allele is gained from each parent)
- slightly different base sequences
- different amino acid sequence
- different polypeptide formed