The Evolving Genome Flashcards
How large is the E.coli genome in Mbp and how many genes does it contain?
4.7 Mbp (megabase pairs)
4,500 genes
How large is the human genome (in Mbp) and how many genes does it contain?
3000 megabase pairs
25,000 genes
What important conclusion can be made when comparing genome size and genome density between different species?
e.g. compare E.coli to human
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
Describe the relationship between genome size and genome density (when comparing different organisms)
NOT linear
genome size does not necessarily correlate to genome density
Define ‘genome density’.
The ratio of the number of genes to the number of base pairs
(Roughly) what percentage of the human genome encodes for genes?
less than 5%
Describe the genome organisation (gene density and amount of introns) in single-celled prokaryotes
Densely packed with genes
Few (if any) introns
Describe the genome organisation (density and number of introns) in lower eukaryotes such as budding yeast
Densely packed with genes
Few introns (approx. 5% of genes)
Describe the genome organisation (gene density, presence of introns/repetitive DNA elements) in metazoa
MUCH less gene rich
Introns present
Repetitive DNA elements present
Define ‘extragenic DNA’.
the DNA located between genes (and gene-related sequences) in the genome
What is the difference between noncoding DNA and extragenic DNA?
noncoding DNA are gene-related sequences
whereas extragenic DNA are not gene-related
What are the 3 main forms of noncoding DNA?
Pseudogenes
Gene fragments
Introns, leaders, trailers (usually regulatory)
(Roughly) what percentage of the human genome composition makes up genes and what is ‘other stuff’?
Genes = 1.5-4.5%
other stuff = 98.5%
(Generally) What key differences are there between gene organisation between prokaryotes and eukaryotes (consider the RNA)?
Pro = polycistronic RNA (has operons)
Euk = monocistronic RNA (‘one gene one promoter’)
Define ‘operon’
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)
Are there exceptions to monocistronic eukaryotic RNA i.e. ‘one gene one promoter’? If so, what would expression look like?
yes, there are eukaryotic operons where genes are co-expressed
the promoter drives expression of the two genes by working in two opposite directions
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?
Mainly repetitive DNA
What two broad categories make up Repetitive DNA elements?
Tandemly repeated DNA (satellite DNA)
Interspersed genome-wide repeats (majority)
What are the names of the 3 types of tandemly repeated (repetitive) DNA elements? And how many nucleotides make them up?
Microsatellites (<10 nucleotides)
Minisatellites (10-60)
Macrosatellites (100-1000)
What is the typical nucleotide length of the repeated section in tandemly repeated DNA?
2-6 nucleotide is common
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?
2-100s
usually 50 times
Describe the mechanism of origin for short (di/tri) nucleotide tandemly repeated DNA.
Random changes in nonfunctional sequence …………………………………………………………
Name the two main mechanisms of origin for larger repeat units in tandemly repeated DNA
duplication by unequal crossing over
replication errors (e.g. replication slippage at repetitive DNA regions)
What is Microsatellite Instability (MSI)?
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)
What disease is Microsatellite Instability associated with?
Colorectal Cancer
Most tandemly repeated DNA has no function, name two placed where repetitive DNA is functional
Centromeres and Telomeres
What is the name given to the highly repetitive, functional DNA located in and around the centromere?
Alphoid DNA
What role does alphoid DNA play during mitosis and meiosis?
(Highly repetitive DNA) act as a site of interaction for spindle fibres that segregate chromosomes
How long is the repetitive sequence found in telomeres? What is the sequence (the bases)?
6 nucleotides
TTAGGG
Which enzyme synthesises the production of telomeres?
Telomerase
(Simply) how do interspersed repeats differ from tandemly repeated DNA?
(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
What proportion of the human genome is made up of interspersed repeats?
About 50%
What do interspersed repeats derive from?
Transposable elements
What other names are given to Transposable elements?
“jumping genes”
transposons
What are transposable elements?
Sequences of DNA that are mobile and can therfore move around to different regions of the genome
