The Evolving Genome

Question 1

How large is the E.coli genome in Mbp and how many genes does it contain?

Answer 1

4.7 Mbp (megabase pairs)
4,500 genes

Question 2

How large is the human genome (in Mbp) and how many genes does it contain?

Answer 2

3000 megabase pairs
25,000 genes

Question 3

What important conclusion can be made when comparing genome size and genome density between different species?
e.g. compare E.coli to human

Answer 3

Genome size does not necessarily correlate to gene density (i.e. it is not linear)
e.g. a much smaller proportion/ percentage of the human genome codes for genes than E.coli

Question 4

Describe the relationship between genome size and genome density (when comparing different organisms)

Answer 4

NOT linear
genome size does not necessarily correlate to genome density

Question 5

Define ‘genome density’.

Answer 5

The ratio of the number of genes to the number of base pairs

Question 6

(Roughly) what percentage of the human genome encodes for genes?

Answer 6

less than 5%

Question 7

Describe the genome organisation (gene density and amount of introns) in single-celled prokaryotes

Answer 7

Densely packed with genes
Few (if any) introns

Question 8

Describe the genome organisation (density and number of introns) in lower eukaryotes such as budding yeast

Answer 8

Densely packed with genes
Few introns (approx. 5% of genes)

Question 9

Describe the genome organisation (gene density, presence of introns/repetitive DNA elements) in metazoa

Answer 9

MUCH less gene rich
Introns present
Repetitive DNA elements present

Question 10

Define ‘extragenic DNA’.

Answer 10

the DNA located between genes (and gene-related sequences) in the genome

Question 11

What is the difference between noncoding DNA and extragenic DNA?

Answer 11

noncoding DNA are gene-related sequences
whereas extragenic DNA are not gene-related

Question 12

What are the 3 main forms of noncoding DNA?

Answer 12

Pseudogenes
Gene fragments
Introns, leaders, trailers (usually regulatory)

Question 13

(Roughly) what percentage of the human genome composition makes up genes and what is ‘other stuff’?

Answer 13

Genes = 1.5-4.5%
other stuff = 98.5%

Question 14

(Generally) What key differences are there between gene organisation between prokaryotes and eukaryotes (consider the RNA)?

Answer 14

Pro = polycistronic RNA (has operons)
Euk = monocistronic RNA (‘one gene one promoter’)

Question 15

Define ‘operon’

Answer 15

a cluster of genes that are transcribed together to give a single mRNA molecules, which therefore encodes multiple proteins
often seen in prokaryotes due to polycistronic RNA (which directs expression to a set of related genes from one set of regulatory sequences)

Question 16

Are there exceptions to monocistronic eukaryotic RNA i.e. ‘one gene one promoter’? If so, what would expression look like?

Answer 16

yes, there are eukaryotic operons where genes are co-expressed
the promoter drives expression of the two genes by working in two opposite directions

Question 17

Roughly 1.5% of human DNA is made up of genes or gene-related sequences, but what makes up the majority of the remaining 98.5% of the human genome?

Answer 17

Mainly repetitive DNA

Question 18

What two broad categories make up Repetitive DNA elements?

Answer 18

Tandemly repeated DNA (satellite DNA)
Interspersed genome-wide repeats (majority)

Question 19

What are the names of the 3 types of tandemly repeated (repetitive) DNA elements? And how many nucleotides make them up?

Answer 19

Microsatellites (<10 nucleotides)
Minisatellites (10-60)
Macrosatellites (100-1000)

Question 20

What is the typical nucleotide length of the repeated section in tandemly repeated DNA?

Answer 20

2-6 nucleotide is common

Question 21

What is the typical range for number of times a nucleotide sequence is repeated in tandemly repeated DNA? And what is the COMMON number of times this section is repeated?

Answer 21

2-100s
usually 50 times

Question 22

Describe the mechanism of origin for short (di/tri) nucleotide tandemly repeated DNA.

Answer 22

Random changes in nonfunctional sequence …………………………………………………………

Question 23

Name the two main mechanisms of origin for larger repeat units in tandemly repeated DNA

Answer 23

duplication by unequal crossing over
replication errors (e.g. replication slippage at repetitive DNA regions)

Question 24

What is Microsatellite Instability (MSI)?

Answer 24

When the number of repeated DNA bases in a microsatellite is different from what it was when the microsatellite was inherited
(seen in some cancer cells)

Question 25

What disease is Microsatellite Instability associated with?

Answer 25

Colorectal Cancer

Question 26

Most tandemly repeated DNA has no function, name two placed where repetitive DNA is functional

Answer 26

Centromeres and Telomeres

Question 27

What is the name given to the highly repetitive, functional DNA located in and around the centromere?

Answer 27

Alphoid DNA

Question 28

What role does alphoid DNA play during mitosis and meiosis?

Answer 28

(Highly repetitive DNA) act as a site of interaction for spindle fibres that segregate chromosomes

Question 29

How long is the repetitive sequence found in telomeres? What is the sequence (the bases)?

Answer 29

6 nucleotides
TTAGGG

Question 30

Which enzyme synthesises the production of telomeres?

Answer 30

Telomerase

Question 31

(Simply) how do interspersed repeats differ from tandemly repeated DNA?

Answer 31

(clue in the name) they are not in tandem i.e. the repeats are not directly adjacent to each other much located at a distance

Question 32

What proportion of the human genome is made up of interspersed repeats?

Answer 32

About 50%

Question 33

What do interspersed repeats derive from?

Answer 33

Transposable elements

Question 34

What other names are given to Transposable elements?

Answer 34

“jumping genes”
transposons

Question 35

What are transposable elements?

Answer 35

Sequences of DNA that are mobile and can therfore move around to different regions of the genome