Eukaryotic genomes & chromosomes

Question 1

What makes up Genetic material? Name the different types within a cell.

Answer 1

• Genetic material of organism consists of DNA.
• Total amount of DNA per cell = GENOME.
• Mitochondrial, nuclear and chloroplast DNA.
• Therefore, genome doesn’t just include nuclear DNA.

Question 2

What term describe the Genome size? How can you work it out? What does it correlate to?

Answer 2

• C-value = pictograms (pg) of DNA in a haploid genome/ weight of one complete single set of chromosomes.
• This can be worked out by finding Mr of 1 bp in daltons  find out how many bp in one g = Avogadro/Mr  find out how many bp in genome by: bp in 1g * C-value.
• C-value doesn’t correlate with no. of genes or organism complexity (paradox) due to amount of DNA repetitive sequences in genome.
• Can be worked out by extracting DNA from a known no. of cells – work out amount of DNA/cell: total DNA / total no. of cells.
• This should be repeated to produce a mean value.

Question 3

What is a gene?

Answer 3

• Contains both coding and non-coding regions.
• Centromere essential + functional but doesn’t code for anything.
• Euk genes are split by small gaps of non-coding DNA.
• Coding (genic) regions = exons
• Non-coding regions = introns.
• In Pro = ribosomes just sit behind RNAP as no/little splicing needed.
• In Euk = RNA must be fully spliced before translation.

Question 4

How much does Genome size vary? In what ways?

Answer 4

• Gene density:
o Yeast = 2kb/g. Drosophila = 13kb/g. Human varies = 64-155kb/g.
• Variation in size and intron no.:
o Yeast = 239 introns total. Humans = up to 100 per gene.
o Many TE’s (transposable elements) = repetitive sequences.
• Human:
o nuclear genome = 20-25,000 genes.
o Mitochondrial (mtDNA) = 37 genes around 16-18kb.

Question 5

How is the human nuclear genome organised? How much DNA is coding? How big is it? What does the rest of the DNA do?

Answer 5

• Size = 3,300Mb.
• No. of different linear DNA molecules: 23(XX) 24(XY).
• Total no. of DNA molecules/nucleus = 46
• Only around 2% of genome is coding DNA.
• Nuclear genome:
o 3,300 Mb  38% genic + 62% non-genic.
o 38% = 2% genes + 36% UTR/introns/fragments/pseudo.
o 62% = 44% interspersed repeats & TE’s + 18% tandem repeats.

Question 6

What words are used to describe the non-genic genome and how it is regulated?

Answer 6

• The 62% of non-genic genome repeats are dynamic:
o Autonomous: regulate themselves/move by themselves.
o Non-autonomous: regulated by another TE because they lack transposase or reverse transcriptase.

Question 7

What is the Structure of a Euk chromosome? What is tightly packed DNA called? How can you visualise DNA?

Answer 7

• Each chromosome: 1 linear, double stranded DNA molecule twice as much protein by weight than DNA.
• CHROMATIN = complex of DNA + chromosomal proteins.
• 2 major proteins of DNA in chromatin = histones & non-histones.
o Histone= spool that DNA winds around – allows compact.
o Non-histone= scaffold proteins (all protein after histones removed.)
• Heterochromatin = tightly packed chromatin that is generally inert.
• Many Ori, 1 centromere, 2 telomeres.
• All the genomic DNA in a single nucleus measures 2 metres and is packed into a 5-10m nucleus.
• Chromosomes only visualised during mitotic phase of cell cycle = 10% of time.

Question 8

How can you work out nuclear genome length?

Answer 8

• 1bp = 0.34nm
• 10Mb = 107 x 0.34 = 3.4mm
• human genome = 3300Mb
• 3300 x 106 x .34nm = 1.1 x 109 nm = 1m
• most human nuclei are diploid = 2m.