Organisation of the human genome Flashcards
Describe the structure of DNA
- Double helix
- 2 antiparallel complimentary strands
- Sugar, base and phosphate group
What are the 4 bases
Adenine, thymine, guanine, cytosine
Describe the mitochondrial genome
- Closed, circular DNA, densely packed
- 16 569 bp, 37 genes, 13 genes involved in respiratory chain
- 2 rRNAs, 22 tRNAs
- 13 polypeptides
- Genes do not have introns
What is the human karyotype?
Characteristic number and size of chromosomes
How is DNA packaged?
Using histones (nuclear-encoded genes)
Describe the nuclear genome
- 2x10^9 bp, >30 000 genes
- 23 pairs of chromosomes
- Genes spaced irregularly and code for proteins
- > 2m of linear DNA per cell
What is a LINE?
Long interspersed nuclear element
Describe LINEs
- 6 to 8 kn sequence
- Can copy themselves to other parts of the genome
- Encode proteins which are required for their integration into the genome
What are the 3 LINE families?
LINE1, LINE2, LINE3
What is a SINE?
Short interspersed nuclear element
Describe SINEs
Use LINE proteins to integrate into genome
What are the 2 SINE families
ALUs (active) and MIRs (not active)
Describe ALUs
- Occur only in primates ~ 1 every 3kb
- Classified into subfamilies based on sequence
- > 80M years old (molecular clocks)`
What are retrotransposons
- Sequences related to retroviruses
- Pol produces a reverse transcriptase which allows DNA to be incorporated into the genome
- Many truncated retroviral sequences 9lack env, or LTRs)
What are microsatellites?
- Short sequences (1-15bp) repeated in tandem many times
- Result in low complexity sequence
- Prone to expansion and contraction during replication due to polymerase slippage
- Not often found in coding sequences
What are the 5 major classes of non-coding RNA genes?
- tRNA (transfer)
- rRNA (ribosomal)
- snoRNA (small nucleolar)
- snRNA (small nuclear)
- Unknown function
What are pseudogenes?
- Sequences related to coding or non-coding sequences that have mutated such that function is lost
- Derived from genes by duplication or retrotransposition
- Whole genes
- Processed pseuodogenes
- Gene fragments
Describe coding genes
- 1 to 5% of the genome
- Produce proteins
- Single or multiple copy
- Evolved by duplication and divergence and found in clusters
- Some families can be grouped into superfamilies based on a common protein domain
How are coding genes identified?
By comparing mRNAs with genomic sequences
What is the general structure of a gene?
- Promoter at 5’ end
- 5’ UTR
- START codon (ATG)
- Introns and exons
- STOP codon (TAA, TAG, TGA)
- 3’ UTR
- Polyadenylation signal
What does alternative splicing produce?
Multiple protein isoforms
What is the content of a gene?
- Cell surface receptors
- Leader sequence
- Extracellular domains
- Stalk region
- Membrane anchoring sequence and/or transmembrane sequence
- Intracellular domain
