Molecular Genetics Flashcards
Purines
Double Ringed
A, G
Pyrimidines
Single Ring
C, T
DNA Structure
Turns clockwise, complete turns every 10 nucleotides
5’ ends with open phosphate
3’ ends with open OH
C numbered clockwise (1= right of oxygen)
-N base bonds to carbon one with glycosyl bond
-C 5 and C 3 bond with phosphodiester bond
Virus Structure
- Single molecule of DNA/ RNA surrounded by protein coat
- Can be single or double stranded DNA or single stranded RNA
- DNA forms loops
DNA in Prokaryotes
Circular DNA packed in one area of cell (nucleoid)
-Loop of DNA twisted into supercoiled structure
Have small circular DNA called plasmids- carry genes such as antibiotic resistance
Levels of Chromosome packaging
1) DNA wrapped around histones (proteins)-> nucleosome
2) Nucleosomes fold back and form chromatin
3) DNA compresses when dividing, chromatin forms folded loops that attach to protein scaffold
4) Protein scaffold folds into chromosomes
Conservative Model
DNA stays intact, new double strand made using old strand as template
Results in 1 molecule of new DNA, one old
Semi Conservative Model
DNA splits into strands, each strand paired with matching nucleotides
Results in 2 strands with half old, half new
Meselson and Stahl proved this in 1957 using N isotopes to track DNA replication
Dispersive Model
DNA breaks into small fragments and reassembles with mixture of old and new on both sides
DNA Replication- Initiation
DNA Helicase unwinds helix by breaking H bonds starting at the origin
SSB (Single Stranded Binding Proteins) keep strands apart
DNA Gyrase releases tension from unwinding by cutting and reforming bonds
DNA Replication- Elongation
2 Helicase working in opposite direction, creates 2 replication forks
Primase places RNA primer of a few nucleotides
DNA polymerase III attaches new nucleotides (can only work in 5-3 direction- leading strand, built towards replication fork)
Other side replicated in discontinuous fashion (lagging strand)
Lagging Strand
-Built away from replication fork
RNA primers placed by primase
Okasaki fragments of DNA built in 5-3 direction from one RNA primer to the other using polymerase III
-DNA polymerase I codes over RNA primer and replaces it with DNA (leaves a gap in backbone), also proofreads
DNA ligase joins okizaki fragments by joining phosphodietster bonds
DNA Replication- Termination
-2 molecules made
Primers removed by DNA polymerase I
-Ligase fills in gap b/w Okazaki fragments
-Exonuclease (enzyme) cuts out wrong nucleotides and adds correct ones when mistakes occur
Central Dogma of Protein Synthesis
- DNA = plan
- Copy of DNA (RNA)= action
Protein Synthesis- Transcription- Initiation
1) Initiation- RNA polymerase binds to DNA and opens double helix
- RNA binds upstream” of gene at region called promoter (TATA box).
Protein Synthesis- Transcription- Elongation
2) Elongation- RNA polymerase builds mRNA in 5’-3’ direction
- No primer required
- template strand= transcribed side
- coding/nonsense strand- side not copied
- mRNA complimentary to template, identical to coding
Protein Synthesis- Transcription- Termination
- RNA polymerase recognizes end when it comes across terminator sequence and releases template strand of DNA
- RNA breaks away
- RNA polymerase now free to bind to another gene
- Several RNA polyermase can be transcribing a gene at the same time
Protein Synthesis- Transcription- mRNA modification
1) Capping and Tailing
- 5’ cap added (7 methyl guanosine- modified guanine nucleotide triphosphate)
- Poly-A tail added (200-300 adenine ribonucleotides added to 3’ end by poly A polymerase enzyme
2) mRNA splicing
- introns (non coding regions) removed by spliceosomes
Protein Synthesis- Translation- Initiation
- mRNA binds to rRNA on small ribosomal subunit
- tRNA with anticodon UAC binds to mRNA/rRNA romplex (carried methionine)
- aminoacyl-tRNA synthetase charges tRNA by adding a.a.
- Binds to large subunit