Storage and retrieval of genetic information Flashcards
DNA
The genetic material of an organism, forming a sequence that can be translated into all the proteins expressed.
Genotype
The complete set of genetic material of an organism, transferred from parents to offspring.
Phenotype
The observable characteristics or traits of an organism, produced by both its genotype and environment.
Structure of nucleic acids
Nucleic acids are comprised of a sugar-phosphate backbone linked by phosphodiester bonds, with each nucleotide containing a nitrogenous base.
Structure of DNA
- double stranded, helical structure
- uses thymine (T)
- no hydroxyl (OH) group
- stable
- high molecular weight
Structure of RNA
- single stranded
- uses uracil
- ribose has a hydroxyl (OH) group at C2
- unstable
- low molecular weight
Complementary base pairing
The process by which genetic information is encoded, allowing DNA to be replicated and RNA to be synthesised. 1 purine and 1 pyramidine are joined by hydrogen bonding (adenine+thymine= 2 H bonds, guanine+cytosine= 3 H bonds).
Non-coding DNA
Less than 5% of the human genome is made up of genes. Non-coding DNA includes gene regulatory sequences, introns and repeating elements.
Gene regulatory sequences
- ‘promoter’ and ‘enhancer’ elements play an important role in gene expression
- can be upstream (5’) or downstream (3’) of a gene
- typically several per gene
- can be ~450kb from a gene
Introns
- intervening segments found within genes, between coding DNA sequences (exons)
- removed from primary RNA by splicing machinery
Repeating elements
- represent ~25% of our DNA
- vary in size from 1.4-6kb
- most common repeats are LINE and SINE families
- likely derived from virus
- can be used to detect polymorphisms in DNA fingerprinting
Semi-conservative replication
One strand of replicated DNA is newly synthesised, while the other is an old copy.
Polarity of DNA
DNA has a 5’ end (carbon attached to triphosphate) and a 3’ end (carbon attached to hydroxyl).
Deoxynucleoside triphosphates
dNTPs are the building blocks for DNA synthesis, and are composed of a ribose sugar, phosphate group and nitrogenous base.
Joining dNTPs
1) Complementary dNTP hydrogen bonds to template strand.
2) Second dNTP lines up next to 3’ end of first dNTP.
3) DNA polymerase joins dNTPs via a covalent bond.
DNA polymerase
- proceeds in a 5’ to 3’ direction
- requires dNTPs
- must have a template strand and an RNA primer
- fast; adds 1000 bases per second
- has ‘proof reading’/editing activity; can undo incorrect pairing
RNA primer
Acts as a starting block for DNA polymerase by providing a free 3’ end to receive incoming dNTPs. Deposited by RNA primase.
Leading strand
DNA polymerase synthesises DNA continuously in a 5’ to 3’ direction, towards the replication fork.
Lagging strand
DNA polymerase binds at multiple sites, synthesising Okazaki fragments of DNA in a 5’ to 3’ direction, away from the replication fork. These are then joined by DNA ligase.
Replication bubbles
Replication of the whole chromosome occurs outwards from multiple origins of replication, forming replication bubbles.
DNA proof reading
During DNA replication, an error occurs every 100,000 bases. DNA polymerase adds a base, moves back to check it, excises it if it is wrong, then moves on.
Replication
The process by which the entire sequence of DNA is duplicated before cell division. Occurs in the nucleus.
Transcription
The process by which an RNA sequence is synthesised, as encoded for by a gene. Only one strand of DNA is copied. Occurs in the nucleus.
RNA polymerase
- proceeds in a 5’ to 3’ direction
- requires NTPs
- must have a DNA template
- adds 50 bases per second
Process of transcription
1) DNA strands separate to form a bubble
2) RNA polymerase binds to and ‘reads’ from template strand
3) RNA chain extends as the bubble moves in a 5’ to 3’ direction
4) Primary RNA transcript binds proteins involved in RNA processing (splicing–>spliceosome)
Translation
The process by which a strand of RNA is translated into a polypeptide chain to form a protein. Occurs at ribosomes in the cytoplasm or on the rough ER.
Transfer RNA
tRNA is an adaptor molecule with a cloverleaf structure, stabilised by hydrogen bonding between base pairs. It has distinct functional regions, including an anticodon and a region for attaching a specific amino acid.
Polysomes
More than one ribosome can attach to a single mRNA strand, resulting in multiple polypeptides being synthesised at once.
Ribosomes
Complexes composed of a large and small ribosomal subunit, which assemble strings of amino acids encoded by mRNA. Contain a peptidyl site, an aminoacyl site and an mRNA binding site.
Process of translation
1) Large and small ribosomal subunits assemble around the strand of mRNA.
2) First tRNA binds at the P site via its complementary anticodon.
3) Second tRNA binds at the A site.
4) Amino acids carried by tRNA are joined by covalent bonding.
5) Ribosome moves along one codon in a 5’ to 3’ direction.
6) Peptide chain extends until the ribosome reaches a STOP codon.
The genetic code
- universal; common between different organisms
- one codon=a sequence of 3 bases
- degenerate; more than one codon can code for the same amino acid
- read in a 5’ to 3’ direction
- 3 possible reading frames
- START codon=AUG–> methionine
- STOP codons= UAA, UGA, UAG
