Replication, Transcription & Translation Flashcards
Describe Base Pairing in DNA replication
- ability of each strand to act as a template for producing a complementary strand
- copy of genes produced before passing them on
Define DNA polymerase
- catalyses the addition of nucleotides to the 3’ end of a growing strand of DNA using a parental DNA strand as a template
Describe DNA helicase
uses the energy of ATP hydrolysis to unwind the DNA double helix ahead of the replication fork
Describe the function of single-strand DNA binding protein
binds to single-stranded DNA exposed by DNA helicase, preventing base pairs from re-forming before the lagging strand can be replicated
Describe the function of DNA topoisomerase
produces transient nicks in the DNA backbone to relieve the tension built up by the unwinding of DNA ahead of the DNA helicase
Describe the function of sliding clamps
Keeps DNA polymerase attached to the template, allowing the enzyme to move along without falling off as it synthesises new DNA
Describe the function of clamp loaders
uses the energy of ATP hydrolysis to lock the sliding clamp onto DNA
Define Primase
synthesises RNA primers along the lagging-strand template
Describe the function of DNA Ligase
uses the energy ATP hydrolysis to join Okazaki fragments made on the lagging-strand template
Describe the Structure of the Replication Fork
- asymmetric structure
- DNA daughter strand that is synthesised continuously - known as the leading strand
- lagging strand - direction of nucleotide polymerisation is opposite to the overall direction of DNA chain growth
- synthesis of this strand is by a discontinuous ‘backstitching’ mechanism
Describe DNA replication
- double helix is unwind by helicase & topoisomerase
- 1 DNA polymerase produces the leading strand copy
- another binds to the lagging strand - this enzyme makes discontinuous segments = Okazaki Fragments
- DNA ligase joins them together
Describe Transcription
- process of synthesising an RNA transcript with the transfer of sequence information from a DNA template
- DNA molecule is transcribed into a complementary sequence of bases, mRNA, which is used for protein synthesis
Describe tRNA
- deciphers the 3 nucleotide condons of mRNA in protein synthesis
- a range of tRNAs exist which bind specifically to 1/20 amino acids based on the anti-codon
- so correct amino acid is brought in during protein synthesis
Describe rRNA
- Ribosomal RNA associates with a range of proteins to form the small & large sub-units of ribosomes
- these sub-units provide the framework by spatial & complementary structure to move along mRNA strand & to bind tRNA
What are the 3 stages of transcription ?
- Initiation
- Elongation
- Termination
Describe the first stage of transcription - initiation
- RNA polymerase &transcription factors bind to promoter DNA
- RNAP creates transcription bubble which separates 2 strands of DNA helix
- 6 general transcription factors = TFIIA, TFIIB, TFIID, TFIIE, TFIIF & TFIIH
Describe the 2nd stage of transcription - Elongation
- RNAP adds RNA nucleotides
- RNA sugar-phosphate backbone forms
- proofreading mechanism that can replace incorrectly incorporate bases
Describe the 3rd stage of transcription - Termination
- hydrogen bond of the RNA-DNA helix break, freeing the newly synthesised RNA strand
- RNA may be further processed
- the RNA may remain in the nucleus or exit cytoplasm through nuclear pore complex
Describe the structure of Ribosomes
Each ribosome has
- a binding site for mRNA
- 3 binding sites for tRNA –> A (aminoacyl) and P (peptide) sites & E (exit) site
Describe the initiation of translation
- translation is initiated by a special Met-tRNAi - 40S subunit with Met-tRNAi
- Eukaryotic Initiation Factors = eIF
Describe the termination of translation
- stop codon triggers release of the completed polypeptide chain - UAG, UAA, UGA
- Releasing factor = eRF
- eRF binds to A site in conjunction to GTP
- peptide group is transferred to water instead of aminoacyl-tRNA
- tRNA left in the P site is expelled with concomitant hydrolysis of GTP
Describe A & P sites
cavities on ribosomes where charged tRNA molecules bind during peptide synthesis
Describe E sites
Discharged tRNA’s leave the ribosome
