Genetic code and Transcription (Week 3) Flashcards
What is the Central Dogma and who coined the idea of it?
- DNA to RNA to protein
- Francis Crick coined the idea
- viruses can preform reverse transcription
How do viruses use RNA?
- Reverse transcribed into dsDNA
- Moves to nucleus where it uses the host machinery for transcription
- Host machinery for translation
- New proteins assemble
- New virus particles released
How do retroviruses use reverse transcription?
- viruses have RNA genomes
- enzymes catalyze RNA genome into DNA then integrate the DNA into the host
What is important about coding sequences?
- triplet codons can be read in various combinations
- each codes for unique polypeptide chain (alternate splicing)
- can have long strands of non-coding DNA
What is important about genetic codes?
-can have some different codes for same codon (degenerate)
-Non-overlapping - each nucleotide is a part of one, and only one, triplet
-Nucleotide triplet in mRNA – Codon
-Unambiguous - each codon specifies a particular amino acid and only that amino acid
Important: AUG is start, UAA, UGA and UAG are stop
What are the different types of mutations?
-Missense mutation: one nucleotide changes, changes amino acid in protein
-Nonsense mutation: one nucleotide changes, causes premature stop codon
-Silent mutation: often in last nucleotide; one nucleotide changes but amino acid is still the same
-Frameshift mutation: adding or deleting a nucleotide in the sequence; can cause missense and nonsense mutations
Mutations in long DNA segments: insertion, duplication, inversion (reverse of pairs), translocation (switch spots; “crossing over”)
What is important about the reading frame?
- Open reading frame(ORF) - the part of areading framethat contains nostop codons.
- read in grouplets of 3 (triplets)
- code for amino acid or nothing (stop codon)
- if stops before the stop codon, get incomplete protein
What are the main effects of frameshift mutations?
- single mutant: like addition or deletion causes reading frame to shift
- if two deletion or two addition, reading shift off; if one addition and one deletion, doesn’t effect very much (one codon wrong)
- if three addition, cause a few to be wrong but most will be ok; same idea with three deletion
- frame shift least effected by triplet codon
What experiment proved that genetic code is non-overlapping?
- added codon to a DNA sequence, observed results; saw that codes didn’t overlap
- Crick and Brenner’s frameshift mutations would not have been observed if code were overlapping
Describe the transcription of mRNA to DNA
2 strands:Template strand and Non-template strand (coding strand)
-Utilizes the same base pairing rules as DNA replication, except uracil (U) in place of thymine
-Since mRNA molecules are synthesized in the
5’ 3’ direction (like DNA), 64 codons are also written in 5’ 3’ direction
-involves four stages; Binding, Initiation, Elongation and Termination
What is important factor in bacteria transcription?
- A large protein – a and b subunits and sigma factor
- Sigma factor promotes binding of RNA polymerase to specific DNA sequences called promoters
How does initiation of RNA synthesis begin?
- Initiation of RNA synthesis takes place once the DNA is unwound
- One of the DNA strands serves as a template for RNA synthesis, using incoming NTPs (nucleotide triphosphates) that are complementary to the template strand
- RNA polymerase catalyzes the formation of a phosphodiester bond between the NTPs
How does transcription occur in bacteria?
-RNA polymerase binds to promoter sigma (σ) factor -Initiation of synthesis -Elongation of RNA T-ermination of synthesis; termination signal - Use of GC loop or ‘rho factor’
What is the promoter sequence in bacteria?
- Six-nucleotide sequence at -10 (TATAAT) – called ‘-10 sequence’ or ‘pribnow box’
- Six nucleotide sequence at -35 (TTGACA) – called ‘-35 sequence’
- must see this before AUG start codon
What can be noted about Termination of Transcription in Bacterial Genes?
-the Hairpin loop filled with GC
