Topic 2.7: DNA Replication, Transcription & Translation Flashcards
DNA is semiconservative
1) One strand is from an original template molecule and one strand is newly synthesised
2) Each base only pairs with its complementary partner and ensures the sequence is conserved
Meselson-Stahl thesis
Supported the theory that DNA replication occurred via a semi-conservative process
Models for DNA Replication (3)
1) Conservative Model
2) Semi-Conservative Model
3) Dispersive Model
Dispersive Model
New molecules are made of segments of new and old DNA
Conservative Model
An entirely new molecule is synthesised from a DNA template (which remains unaltered)
Semi-Conservative Model
Each new molecule consists of one newly synthesised strand and one template strand
Meselson-Stahl experiment (2)
They incorporated radioactive nitrogen isotopes into
DNA
• Templates were prepared with heavier 15N
• New sequences were replicated with lighter 14N
Meselson-Stahl results (2)
The DNA was then separated via centrifugation to determine its composition of radioisotopes
• 1st division: DNA had 15N and 14N (i.e. mixed)
• 2nd division: DNA is mixed or has 14N only
DNA Replication Enzymes (2)
1) Helicase
2) DNA Polymerase III
Helicase (2)
1) Unwinds and separates the double stranded DNA
2) Breaks the hydrogen bonds between the base pairs
DNA Polymerase III
1) Free nucleotides line up opposite complementary partners
2) DNA Pol III covalently joins the free nucleotides together
DNA replication
Semi-conservative process whereby pre-existing strands act as templates for newly synthesised str
Polymerase Chain Reaction (PCR)
Artificial method of DNA replication that is used to rapidly copy a specific sequence.
PCR Steps in a Thermal Cycler
Denaturation (90°)
Annealing (55°)
Elongation (75°)
Denaturation
DNA is heated to separate the two strands
Annealing
Primers attach to ends of a target sequence
Elongation
A heat-tolerant polymerase copies strands (Taq polymerase)
30 Cycles in PCR = ? copies
2^30 copies of the target DNA sequence
Transcription
Synthesis of an RNA sequence from a DNA template
RNA Polymerase
1) Separates the DNA strands (breaks H bonds between base pairs)
2) Covalently joins free complementary RNA nucleotides together
Types of RNA
1) mRNA – Transcript used to make protein
2) tRNA – Transfers amino acid to ribosome
3) rRNA – Catalytic component of ribosome
Genetic code
Set of rules by which information encoded in mRNA sequences is converted into a polypeptide sequence
Codons
Triplets of bases which correspond to a particular amino acid
Order of codons
Codons determines the amino acid sequence for a protein
Start codon
AUG
STOP codons (3)
UAA
UAG
UGA
Features of the Genetic Code (2)
1) Universality – All organisms use the same genetic code
2) Degeneracy – Multiple codons may code for the same amino acid
Translation
Process of protein synthesis in which the genetic information encoded in mRNA is translated into a sequence of amino acids on a polypeptide chain
Steps of translation (Mr Cat App)
1) Messenger RNA (goes to…)
2) Ribosome (reads sequence in …)
3) Codons (recognised by …)
4) Anticodons (found on …)
5) Transfer RNA (which carries …)
6) Amino acids (which join via …)
7) Peptide bonds (to form …)
8) Polypeptides
Gene
Sequence of DNA that is transcribed into RNA
Human insulin in bacteria (4)
1) The gene responsible for insulin production is extracted from a human cell
2) It is spliced into a plasmid vector (for autonomous replication and expression) before being inserted into a bacterial cell
3) The transgenic bacteria (typically E. coli) are then selected and cultured in a fermentation tank (to increase bacterial numbers)
4) The bacteria now produce human insulin, which is harvested, purified and packaged for human use
