Genome structure and function

Question 1

Structure and function of the human genome

Answer 1

• Material -> DNA
• Packages = chromatin and chromosomes
• Genome -> all DNA in cell (nuclear & mitochondrial DNA)
• Size = ~3 million bases, ~20,000 genes
• Exons: code for amino acids except for UTR (contain regulatory elements; important for control of protein synthesis)
• Introns: noncoding section of gene between exons
• Promoter regional: 5’ of gene; contains important regulatory elements for transcription

Question 2

MSAT

Describe the structure of chromosomes

What are the main function of centromeres and telomeres

Answer 2

• Chromosome structure: telomere at each end and centromere in the middle.
• Shorter arms (p arm) and long arm (q arm)

4 types:
* Metacentric (centromere in the middle)
* Sub-metacentric (centromere further up)
* Acrocentric (centromere even further up/ satellite p arms)
* Telocentric (centromere at the top) humans don’t have these

• Scientists look at size, banding pattern and centromere position to identify chromosomes
• Chromosomes usually exists as chromatin
  -> DNA double helix bounds to histones
  -> Octamer of histones form nucleosome

Function of centromeres
* Keep sister chromatids together
* Attach to microtubules during cell division
* Rich in heterochromatin
* Normally highly repetitive

Function of telomeres
* Protect the ends of chromosome (prevent them from being degrading)
* Telomerase repairs telomeres but is only active in certain cell types
* If telomerase is switched on in the wrong cells this can lead to cancer

Question 3

Define the genome, the exome and the epigenome

Answer 3

• Exome: the parts of the genome which code for protein i.e all the coding exons of all the genes in the genome
• Epigenome: chemical compounds that attach to DNA or histones and. can affect gene activity. E.g. DNA methylation, histone acetylation
  -> They affect gene activity by altering chromatin structure, recruit histone modifiers, repress transcription, genome-wide pattern established at fertilisation, important for differential gene expression (transcription) and responds to environmental cues (cellular and external)

Question 4

Demonstrate an understanding of differential gene expression

Answer 4

• Gene expression is controlled by DNA sequence and epigenome
• Same set of genes in every single cell of the body, same genome…

Differential gene expression
—> In time (temporal)
* Development (embryos V adults)
* In response to hormones, infection, other signals
—> Spatially
* Different tissues/cells express different genes (e.g. brain v liver)

Failure to regulate gene expression tightly may lead to different diseases
* e.g.. metabolism -> metabolic disease
* Cell shape/ motility -> metastasis
* Cell differentiation -> congenital disorders
* Cell proliferation -> cancer

Question 5

Difference between nuclear and mitochondrial genome?

Answer 5

Nuclear genome
* 22 pairs of autosomes
* 1 pair of sex chromosomes
* The actual DNA sequence
* Transcribed units = genes
* Protein coding
* RNA-only coding (so-called non-coding transcripts; make RNA but not protein)

Mitochondrial genome
* 16Kb
* D loop = contains promoters for light and heavy strands
* Contains 13 coding genes (coding for OXPHOS proteins – generation of energy)
* 24 noncoding genes (coding for RNA molecules)
* Only ova provide mitochondria -> maternal inheritance