Eukaryotic Genome and Chromosomes: Content

Question 1

Genome

Answer 1

Total DNA in the cell

Question 2

Nuclear genome

Answer 2

Linear chromosomes
Two copies

Question 3

Organellar genome

Answer 3

Such as:

Mitochondrial DNA (mtDNA)
- Circular DNA
- Multiple copies
- Endosymbiont hypothesis

Chloroplast DNA (in plants)

Question 4

Variation in Genome Size

Answer 4

• Eukaryotic genome size varies with an estimated 66,000-fold difference in genome size
• Can see the difference in microscopes

Question 5

How is genome size measured?

Answer 5

Genome size can be measured in terms of DNA weight (C-Value, pg) or DNA number (bases, bp)

pg = picogram = 10^-12g

• C-value = weight (pg) of one complete single set of chromosomes - assumes weight of 1 copy - (haploid genome)

Question 6

Converting from pg to bp

Answer 6

• 1 bp = 660 daltons
• 620 g of DNA = 1 mole = 6.02 x 10^23 bp (Avogadro’s number)

1 g of DNA = 6.022/660 x 10^23 bp = 9.4 x 10^20 bp

1 pg = 9.4 x 108 bp
1 pg = 9.4 x 102 Mb = 940 Mb
So 0.013 pg = 940 x 0.013 = 12.22 Mega base pair (million)

Question 6

How is DNA content (weight) determined?

Answer 6

• Extract DNA from known
  number of cells.
• Stain DNA with fluorescent dye.
• Use fluorescence microscopy or
  flow cytometry to determine
  fluorescence intensity of cells (more = more DNA)
• Estimate mean DNA content per
  cell over many cells

Question 8

C- value Paradox

Answer 8

There is no relationship between the C-value and organismal
complexity

Question 9

What is a Gene?

Answer 9

Genes:
- A segment of DNA that codes for a protein
- A segment of DNA that codes
for a functional RNA molecule.
- A segment of DNA that determines a specific phenotype

Not all DNA codes for genes
Not all genes code for proteins

Question 10

Why does genome size vary? - Summary

Answer 10

• Variation in gene structure
  (introns).
• Variation in gene density caused by repetitive DNA.
• Variation in ploidy

Question 11

Why does genome size vary?: Gene Structure and density

Answer 11

Introns = non-coding sections of genes
- Genes in eukaryotes are split by introns regions and separated by varying amounts of repetitive DNA.
- Prokaryote genes do not have introns
- Human genes may be huge e.g. Dystrophin is 2.3M

Question 12

Translation in Prokaryotes vs Eukaryotes - Introns

Answer 12

• Pro- : RNA is directly translated by ribosome
• Eu- : RNA processed to remove introns before translation

Question 13

Why does genome size vary?: the “Mobilome”

Answer 13

Some parts of the genome move around in the process called transposition – transposable elements (TE) or mobilome

Transposition can occur through:
* Copy and paste (Class I)
* Cut and paste (Class II)

Can insert itself into exon rendering dysfunctional/non-functional

Question 14

Why does genome size vary? (III): Polyploid

Answer 14

Polyploidy arise from the duplication of the genome of a species during meiosis or the hybridization of the genomes of two species.

• It contributes to increase in genome size for some species.
• More commons in plants than animals.
• Contributes to genome variation and adaptation

Question 15

Allopolyploid

Answer 15

Two distinct species bring their genomes together to form a new species

Question 16

Autopolyploid

Answer 16

• A species duplicates its own genome

Question 17

Size of Nuclear Genome

Answer 17

• 3,300Mb size
• 20,000 – 25,000 genes
• Haploid number n = 23
• 2% coding DNA (exons)

Question 18

Size of Mitochondrial DNA

Answer 18

• 16-18kb size
• 37 genes

Question 19

Breakdown ,in percentage, of human genome

Answer 19

• Nuclear genome (3,300Mb)
• Genic (38%)
  => Genes (exons) = 2%
  => Gene related (UTRs Introns
  Pseudo-genes Gene fragments) = 36%
• Non-genic (62%)
  =>Interspersed repeats (44%) transposable elements
  => Tandem repeats (18%) microsatellite