Flashcards in Genetics in Medicine 3 Deck (73)
What is the molecular difference between deoxyribose and ribose sugar and what consequences does this have for the structure?
Ribose sugar has a hydroxyl group on C2 which is missing in deoxyribose
This hydroxyl group gives potential for nucleophilic attack on nearby phosphodiester bonds meaning that RNA is transient (much more unstable) but DNA is much more stable
What direction is DNA written in and DNA and RNA synthesised?
written in 5' to 3' direction
When DNA sequence is written in shorthand the sequence of which strand in which direction is the one written out?
The sense strand in the 5 to 3 direction
In which stage of the cell cycle does DNA replication take place?
In G1 phase how much DNA do you have?
One paternal chromosome
One maternal chromosome
In G2 phase (after S phase) how much DNA do you have?
One paternal chromosome and one maternal chromosome each made up of 2 sister chromatids
How many strands of DNA is one individual chromosome made up of?
One strand of DNA
How many Mb of DNA do we have per haploid genome?
3000Mb per haploid genome
Which is the largest chromosome?
What percentage of DNA is non coding?
How many coding genes do we have approxiamtely?
The average gene size is 50-100kb but the average mRNA is ~ 2kb, why is this?
Because of splicing, introns in the gene are removed leaving just exons
What are the 2 main classes of sequences in the human genome?
Single copy sequences (non repetitive)
What are the 2 types of repetitive sequences on the human genome?
1) interspersed repeats
2) Satellite DNA
Alu repeats are an example of what kind of repeats?
What is meant by satellite DNA?
Large blocks of repetitive sequences - heterochromatin
What is the basic definition of a gene?
Functional units of DNA
What is meant by a gene being expressed?
All genes are transcribed at some point - ie copied into RNA
Some of those are then translated - turning the RNA into a protein, but not all as some are functional as mRNA eg. tRNA or rRNA - have short and long coding RNAs
What are the 3 main component parts of a gene?
3) Regulatory sequences eg. promoters, enhancers, locus control regions
By definition where does transcription begin and where does translation begin?
Transcription starts at the beginning of exon 1
Translation begins at the initiator site for translation
Dont get them mixed up
What is alternative splicing and what can it lead to?
Some exons can be removed during splicing leaving 2 or more possible recombinations for spliced mRNA
This leads to much greater genetic variation from just those 20,000 genes - you can end up with 2 different possible proteins from one gene
What is mutually exclusive exon choice in alternative splicing?
May be a piece of DNA with 2 exons that are identical and sometimes one may be incorporated and the other excluded and visa versa
What is meant by the term gene families?
When you look at how the genome has evolved it has evolved alot through duplication and divergence of ancestory genes - you thus end up with gene families of structurally related genes - these can end up close together on a genome or widely dispersed
What happens in the evolution of genes in terms of DNA duplication and divergence?
1) Start off with an ancestral gene
2) As a result of inaccuracies end up with gene duplication - these are initially tandomly arranged
3) As a result of further mutations you get divergence of these duplications
4) They may then aquire different functions
What are pseudogenes and why can they create problems clinically?
Genes that were once useful but as we've progressed through evolution are no longer useful so have been turned off
These can be very similar to functional genes so can interfere with medical diagnosis
What are processed genes and how do they form?
Intron-less copies of other genes which are usually remote from the parent gene
Reverse transcription of spliced mRNA and reintegration into the genome
Do processed genes remain functional?
They occasionally remain functional (eg. PGK2 testis specific) but most are non functional
Roughly how many coding, short non-coding, long non-coding and pseudogenes do we have?
Coding - ~20,000
Short non coding - ~9000
Long non-coding - ~13,000
Pseudogenes - ~14,000
What is meant by interspersed repeats?
Those scattered around the genome