DNA Flashcards
Genes are carried on
Chromosomes
What are chromosomes made of
DNA and Proteins
Genes are made of
DNA
What directions to strands run
In antiparallel
Run from 5’ to 3’
- This means that each DNA strand has a complementary sequence to the other strand
- This is important for copying DNA
What is the structure of the double helix
It has two grooves
A major and a Minor
How are nucleotides held together
Hydrogen bonds
double or triple
These form base pairs
How does the genetic code work
Not fully understood- seemingly random when first discovered
Now seeing general patterns
In genes you have coding and non coding regions
Coding regions code for a transcript of a protein
Non coding regions - depends on where they lie in relation to the genes
DNA Replication
Goes from 5’ to 3’ ( only ever in this direction)
Dangerous–> incorrect base pairing can cause mutations (leads to diseases such as cancers)
Templates
Complementary DNA sequences
How does DNA replicate
DNA begins at the replication origin
Indicator proteins open up the helix at the replication origin.
Starts in lots of places along the strand and works in two directions outwards–> More efficient
How many base pairs are there per second in humans
100 pb/ second
DNA Replication steps
DNA polymerase makes a new DNA from the template
Catalyses addition to the 3’ end of the chain
This forms a phosphodiester bond from a phosphoanhydride bond
Pyrophosphate is released and further hydrolysed into 2 inorganic phosphates
DNA polymerase stays bound to the chain after each round of catalysis
DNA moves along the chain and is kept in place by another special protein
How, if replication only proceeds from 5’ to 3’, does DNA replication work for both strands
DNA polymerase can only catalyse DNA synthesis in one direction
Therefor the replication form is asymmetrical and this problem is overcome with backstitching
The growing 5’ end is made discontinuously in short stretches and these short sequences are then stitched together.
Continuous strand -> Leading
Discontinuous strand -> Lagging
What happens to errors during replication
DNA Polymerase is able to self correct mistakes
Errors occur at 1 in every 10^7 base pairs
–> This can form less stable base pairs which causes DNA mutation
DNA Polymerase can proofread - checks if the correct nucleotide is inserted and if so it continues –> If not it cleaves it out and tries again
The correcting system corrects 99% of mistakes
DNA repair mistakes are called mismatched nucleotides
A complec of mismatch repair proteins recognises the mismatch–> excises the new strand and replaces it
Still do not understand how the old and new pairs are recognised–> thought that maybe the new strands are preferentially nicked
How many and what are the functions of DNA polymerase
2 Enzyme functions
5’ to 3’ polymerase
3’ to 5’ nuclease
How does back stitching work
Starts with a primer - these are short lengths of RNA which bind and kick start the next stage of the process.
DNA polymerase adds to the RNA primer to start a new DNA fragment.
Once this is fully formed the old RNA primer is erased and replaced by DNA
How long are back stitches
About 100 nucleotides
Why does DNA grow from 5’ to 3’
Growth from 3’ to 5’ is not energetically favourable
What is responsible for the synthesis of the RNA primers
Primase
What enzymes work on the lagging strand
Nuclease
Repair polymerase
Ligase
What is at the head of the replication machine
Helicase
Proteins at the replication fork form a protein machine
Replicative enzymes are thought to be present in a large multienzyme complex which enables both strands to be synthesised at the same time
What this replicative complex looks like is not entirely known.
What do mutations cause and what causes them
Diversity of organisms is due to genetic change
Genetic change is frequently detrimental
For species survival organisms must be genetically stable
DNA changes are the cause of mutations –> e.g. sickle-cell anaemia
Sickle cell anaemia
Mutation of Glutanic acid –> a valine in the beta globin protein
This was caused by a single nucleotide change
This is a single amino acid mutation
The protein folds in a different way so does not bind to oxygen as well
Diseases
The same protein will be impacted
But the mutation for each individual will be slightly different –> the amino acids changed will be different
What has happened to cancer rates in recent years
30% of deaths in Europe and the USA
Older you are the more likely it is in general.
Mutations
Outside of synthesis
Energy has to go into the DNA
One type of mutation is a depurination –> you loose a base, you still have the sugar phosphate back bone but one of the bases vanishes. –> This means when you come to reproduce the DNA the logical sequence is lost–> This is a frame shift mutation
Ultraviolet light forms thymine dimers –> if left unrepaired these changes can be potentially fatal
DNA Repair
Involves 3 steps
Damaged DNA is recognised and removed by a nuclease forming a gap.
A repair DNA polymerase synthesises new DNA using the old strand as a template
The remaining gap is closed using DNA ligase
