Genomes And Gene structure

Question 1

Genomic organisation

Answer 1

• Prokaryotes
– Haploid
– Few genes have introns
– Most DNA encodes proteins or structural RNAs
– Generally 1 chromosome
– Often circular DNA
• Eukaryotes
– Diploid
– Most genes have introns
– Most DNA is not encoding proteins or structural RNA
– Can have many chromosomes
– Linear DNA

Question 2

Nucleosome structure

Answer 2

• The DNA and histone interact
• The double helix is wound around a core of proteins
– The histones
• Nucleosome histone plus DNA
– 147 bp of DNA
– 20-60 bp linker

Question 3

Histone complex

Answer 3

• The core particle is composed of 4 different proteins
• 8 subunits in total
– Quaternary level complex!
– Assemble to form core
• N-ter tails extend outwards
– Tails are important
– Get modified
– Impacts function

Question 4

Locking DNA in place

Answer 4

• DNA on core particle is latched down by binding H1
• These histone complexes then pack into higher order

Question 5

Gene structure

Answer 5

• Region of DNA where there is:
1. A start signal called a promoter region
2. An area of code, aka open reading frame
– Generally a contiguous region in prokaryotes
– Generally a disrupted region in eukaryotes
3. A stop signal called termination signal
• There may be other signals

Question 6

Simple gene architecture of a bacterial gene

Answer 6

• Promoter region upstream of the gene
– Before the 5’ end of gene
• RNA polymerase binds to promoter region
• RNA polymerase is the enzyme which makes RNA
• Termination signal downstream of gene
– After the gene at the 3’ end
• One gene in the mRNA is a monocistronic
message

Question 7

Organisation of an operon

Answer 7

• mRNA containing information from >1 gene is called a polycistronic message
• Bacteria often organize genes in groups called an operon
– The encoded proteins generally work together
• Single promoter for the area
• Single mRNA produced
• Often have control regions
– the operator, acts like an on/off switch and is sensitive to environmental stimuli

Question 8

Organisation of eukaryotic genes

Answer 8

• More complex that prokaryotic genes
• A single gene is composed of blocks called
– Exons: regions of DNA encoding protein
– Introns: regions of DNA not encoding protein
• Many control elements as well as promoter
• Termination sequence can be some distance from the gene
• The primary (first) transcript RNA includes the introns
• The introns are removed to yield the mRNA
• The mRNA has untranslated regions (UTR)