Genetics Flashcards
What are nucleic acids?
DNA and RNA
Polynucleotides made of nucleotide monomers
Each nucleotide has a nitrogenous base bonded to carbon-1 of a pentose sugar and phosphate group (which has a negative charge causing acidity) bonds to carbon-5 of pentose sugar
Difference between pentose sugar of DNA and RNA?
DNA: deoxyribose sugar, carbon-2 has a hydrogen bonded to it
RNA: ribose sugar, carbon-2 has a OH group bonded to it
Types of nitrogenous bases?
Purines: adenine and guanine
Pyrimidines:
DNA: thymine and cytosine
RNA: uracil and cytosine
Which bases can hydrogen bond?
Cytosine and guanine
DNA: adenine and thymine
RNA: adenine and uracil
How are polynucleotides formed?
Condensation reaction creating phosphodiester bond (strong covalent) between OH group of phosphate group and OH group of carbon-3
Each chain has a 5-prime end (where carbon 5 is attached to phosphate group) and a 3-prime end (where carbon-3 of the pentose sugar is nearest the end)
In DNA a second chain will run anti parallel (3-5 rather than 5-3) so nitrogenous bases are close to each other and hydrogen bonds will form between them (two bonds between a and t) (three bonds between g and c)
DNA curls up into a double helix to save space
Difference between RNA and DNA?
Different pentose sugar
DNA double strand, RNA single strand
DNA A and T, RNA A and U
Differences between purines and pyrimidines?
Purines are bigger
How does DNA replicate?
Called semi-conservative replication
1) helicase enzymes separate the DNA strands. The point at which two strands are separated is called replication fork
2) each strand is kept apart from single-strand bonding proteins
3) enzyme called DNA polymerase uses each patent strand as a template and adds the complementary base pairs in a 3 to 5 direction this creates the leading strand which grows continuously
4) the lagging strand is built away from the replication form in a 5 to 3 direction. It grows discontinuously in small sections called Okazaki fragments which are joined together by DNA ligase
5) helicase unwinds DNA little bit more and the previous steps happen again
6) there can be multiple replication bubbles
What is the origin of replication?
Begins at a specific nucleotide sequence
Enzymes involved in DNA replication?
Topoisomerase: untwists the DNA ahead of the replication forks to relieve the strain
Helicase: unwinds the double helix at replication forks, breaking hydrogen bonds between base pairs
Single-stranded binding proteins: keep the two parental strands separate
Primase: synthesises one short strand (10 nucleotides) called a RNA primer, in the 5 to 3 direction using the parental DNA strand as a template to ‘start of’ for DNA polymerase
DNA polymerase III: can only add to an existing chain in the 5 to 3 direction. Adds DNA nucleotides to the 3 end of RNA primer
DNA polymerase I: replaces RNA primer with DNA in both leading and lagging
DNA ligase: joins Okazaki fragments
What is transcription?
Making a RNA copy strand of gene required as a template
Transcription process?
At the promoter, a DNA sequence that determine which strand will be transcribed and in what direction. Transcription factors bind to promoter and enable RNA polymerase II to bind. This unwinds the double helix, separates the DNA strands and synthesises the pre-mRNA. The addition of nucleotides is called elongation.
DNA double helix reforms and pre-mRNA peels away
There is a polyadenylation signal (AAUAAA) which cause the transcript to be cut releasing the RNA polymerase and pre-mRNA
What is RNA processing?
Modification of the pre-mRNA to produce mRNA for translation
Step 1) end processing
Step 2) splicing
What is end processing?
At 5 prime end there’s a modified guanine added (5 prime cap)
At the 3 prime end there are 50-250 adenine added (poly-A tail)
These protect the transcript from enzymes which would degrade the transcript
They act as signals and to move the transcript through the nuclear pore for translation
The 5 prime cap binds the transcript in the cytoplasm to the ribosome
What is splicing?
Introns (non-protein coding) removed and exons (protein coding) joined together
Average mRNA is 95% shorter that pre-mRNA
Takes place in spliceosome (in the nucleus)
Small nuclear RNAs bind to the ends of introns by complementary base pairing
Small RNAs are ribozymes (RNA molecules that function as enzymes)
Alternative splicing (only joining some exons) can produce a variety of proteins
Introns contain regulatory sequences
