Genes, chromosomes and the genetic code

Question 1

What is the difference between prokaryotic, eukaryotic and mitochondrial genomes (10)

Answer 1

Prokaryotes:
1. Single circular molecule
2. Chromosome is stored in the nucleoid
3. Non‐essential genes are stored outside of chromosomes – in plasmids
4. Genome is very compact – contains very little noncoding DNA sequences.

Whereas

Eukaryotes:
1. A number of linear chromosomes
2. 2 copies of each chromosome
3. One set of chromosomes from each parent
4. Contain mitochondrial genome, which is:
1. Proportionally much smaller than DNA found in nucleus
2. Maternal inheritance

Question 2

What is the concept of the genome and base pairs as a measurement of genome size (4)

Answer 2

• The genome is measured in size of DNA measured as base pairs (bp)
• Humans have over 3 billion bp which is approximately 2m of DNA per each cell.
• Width of double spiral = 2nm
• length of 10 base pairs = 3.4nm

Question 3

What is the concept of protein-coding and intergenic regions of the genome

Answer 3

Exons are the parts which code for proteins and introns the intergenic regions which are the non-coding parts.

Question 4

How is DNA supercoiled to form chromosomes (7)

Answer 4

1. chromatin is a double-stranded helical structure of DNA
2. DNA is complexed with histones to form nucleosomes consisting of 8 histone proteins with DNA wrapped around it 1.65 times
3. chromatosome is formed from a nucleosome plus an H1 histone
4. Nucleosomes fold up to produce a 30nm fibre
5. The fibre forms loops averaging 300nm in length
6. the fibres are compressed and folded to produce a 250nm wide fibre
7. Tight coiling of the 250nm fibre produces the chromatid of a chromosome

Question 5

What is the concept of epigenetics (2)

Answer 5

• Epigenetics is the study of the changes in gene expression (phenotype) that is not due to change in genotype.
• Change can be influenced by environment, age, lifestyle, disease state etc.. but can also be caused by chemical changes such as methylation of DNA, gene silencing by non-coding RNA (ncRNA) and histone modification

Question 6

What is the concept of the genetic code (6)

Answer 6

• 3 nucleotide sequence = CODON
• 1 CODON = 1 amino acid
• 4 nucleotides (A, U, C and G) in a 3 nucleotide code = 64 (43) combinations
• 20 essential amino acids

Genetic code – is DEGENERATE (multiple codons that mean the same amino acids)

is UNAMBIGUOUS (any codon will only mean one amino acid).

The reading frame can be shifted with nucleotide insertions or deletions - mutations.

Question 7

What are the 4 main codons

Answer 7

AUG - MET (START CODON)

**UAA - STOP CODON

UAG - STOP CODON

UGA - STOP CODON**