DNA Replication Flashcards
Origin Site
The origin of a replication bubble
Helicase
Unwinds the helix structure to allow access to the parent strand for replication
Single Strand Binding Proteins
Temporarily bind to the unwound template strands to keep them open for replication
Topoisomerase
Stabilizes the overtwist ahead of helicase
Primase
Lays down RNA primers at the beginning of Okazaki fragments (and one at the beginning of the leading strand) as a signal for poly III to start building there
DNA Polymerase III
- Reads the parent strand from 3 to 5
- Builds a new daughter strand from 5 to 3
- Cannot initiate synthesis on its own, so it relies on an RNA primer to tell it where to start
DNA Polymerase I
Replaces the RNA primers with the proper nucleotides
Ligase
Glues Okazaki fragments together
DNA Polymerase II
- Rereads the strand once it has been replicated and fixes any mistakes
Telomeres
- DNA strands get shorter every time they are replicated. This is because the last RNA primer is taken off the end of the strand, but Polymerase is unable to replace it since there is no 3’ end with a primer to kickstart it. This means the last few nucleotides on the end of the strand stay unpaired and are later removed because they are not useful
- Telomeres are long nucleotide sequences on the ends of DNA strands that do not code for anything. They help postpone the erosion of useful DNA
Telomerase
Catalyzes the lengthening of telomeres in germ cells
3 Stages of mRNA and Protein Synthesis
- Transcription
- RNA processing
- Translation
Transcription
- Catalyzed by RNA polymerase
- Builds strings of mRNA based off a DNA template strand
- Follows base pairing rules with thymine substituted for uracil
- 3 stages
Transcription Initiation
- Promoters signal for the initiation of RNA synthesis
- Transcription factors sense this signal and bind to the promoter site to help guide the RNA polymerase
- RNA polymerase attaches to the promotor site / transcription factors
- Promotors are long strands of adenine and thymine called the TATA box
Transcription Elongation
- RNA polymerase moves along the DNA strand
- It untwists the double helix, exposing 10-20 base pairs at a time
- Base pairs the DNA template strand with RNA nucleotides, which then fall away to create their own single pre mRNA strand
Transcription Termination
- After each stop codon is a poly-A sequence
- Proteins cut the mRNA free 10-35 nucleotides past the poly-A
- Polymerase falls away from the DNA strand
- This process is not well understood
RNA Processing
- 5’ end receives a modified nucleotide cap
- 3’ end gats a poly-A tail
- RNA splicing occurs, which cuts out introns and join together exons
Introns
Segments of RNA that do not code for any amino acids
Exons
Segments of RNA that code for amino acids
Spliceosomes
- snRNP (small nuclear ribonucleoproteins)
- These recognize splice sites and do the intron cutting
Translation
- Formation of a protein using mRNA strands as a template
Anti-codon
- Base pairs on the tRNA that bind with those on the mRNA strand
Aminoacyl-tRNA Synthetase
Binds amino acids to the correct tRNA strand
Binding sites on the Ribosome
- Add site - reads mRNA and calls in the matching tRNA
- Peptide site - adds on the amino acids
- Exit site
Translation Initiation
New mRNA is brought into the ribosome, as well as the first amino acid which matches with the start codon (Met)
Translation Elongation
Amino acids are added to the polypeptide
Translation Termination
- Ribosome reaches the mRNA’s stop codon
- It accepts the release protein
- Hydrolysis and release occur
Polyribosome
Many ribosomes translating a single mRNA strand at once
Post-translation Changes
- Some proteins may harm the cell if they are left in their regular state post-translation, like insulin when the cell is not ready for it
- In order to counter this, the cell changes the protein reversibly until it is ready to use it
Epigenetics
- Chemical messengers determine whether a certain gene is expressed
- Methylation suppresses small segments of genes
- Histones control longer portions
