RNA TRANSCRIPTION

Synthesis of RNA using DNA as a template

- process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule

Enzyme: DNA-dependent RNA polymerase

- No primer needed, no proofreading activity - In eukaryotes, the polymerases synthesize specific RNA molecules:

 Pol I: rRNA

 Pol II: mRNA, lncRNA, miRNA, SnRNA

 Pol III: tRNA, 5S rRNA

Enhancers

are DNA sequences that stimulate transcription rate

Silencers

inhibit transcription

RNA polymerase

- synthesizes RNA molecule in the 5ʹ to 3ʹ direction

- unwinding of DNA

Primer

is not involved in transcription, unlike in replication

Translation

- process by which information in mRNAs is used to direct the synthesis of proteins - carried out in the cytoplasm (ribosomes) - involves

 mRNA (from nucleus)

 Ribosomes – rRNA

 tRNA

 Genetic coding – codons

Prokaryotic mRNA

have several coding regions (polycistronic).

Eukaryotic mRNA

codes for only one polypeptide chain (monocistronic)

Genetic Code

- Genetic information is encoded as a sequence of nonoverlapping base triplets, or codons a. 64 different codons

 61 specify amino acids :

AUG - Met

START codon:

UAA, UAG, UGA

 3 STOP signals:

Transfer RNA (tRNA)

- single RNA strand that is only about 80 nucleotides long - carries a specific amino acid on one end and has an anticodon on the other end

 Brings AAs to the ribosomes

 Anticodon: 3 RNA bases that matches the 3 bases of the codon on the mRNA molecule

- 4 base pair regions; 3 loops

TRANSLATION

I. INITIATION

II. ELONGATION

III. TERMINATION

INITIATION COMPLEX

 Ribosome (large and small)

 mRNA with instructions

 “Initiator” tRNA carrying the first amino acid (Met)

o Requirements for IC to form

 initiation factors

 GTP

Shine-Dalgarno Sequences

 Marks the start of each coding sequence,

tRNA

 (carrying Met) starts out in the P site (middle site)

 Matching tRNA lands to the A site - landing site for the next tRNA

Formation of Peptide Bond

 transfers Met from the 1st tRNA onto the amino acid of the second tRNA in the A site

 Met: N-terminus of the polypeptide

 Next AA: C-terminus

mRNA

pulled onward through the ribosome by exactly one codon

 Empty tRNA drifts out via the E site

 New codon in A site is exposed

 Cycle repeats → elongates

PROTEIN TITIN

: longest known polypeptide → up to 33,000 AAs

TERMINATION

o Stop codon

 polypeptide is released when (UAA, UAG, or UGA) in the mRNA enters the A site

DNA Polymerase

- editing mechanism that decreases the rate, but still exist

- most mutations are neutral: have no effect

Mutagens

- chemical or physical agents that interact with DNA to cause mutations

Substitution

changes a single base pair by replacing one base for another

Silent Mutations

 Do not affect the sequence of amino acids during translation

Nonsense Mutation

 translations stop prematurely

Missense Mutation

 change the amino acid specified by a codon

RNA SPLICING

- Removes introns and joins exons

- Ribozymes, spliceosomes

INTRONS

found in eukaryotes only

non-coding areas of DNA

removed in the nucleus before the mRNA moves to the cytoplasm

EXONS

in both prokaryotes and eukaryotes

coding areas of the DNA

mature mRNA contains it and moves to the cytoplasm from the nucleus

Insertion

: one or more bases are added to a DNA sequence

Deletion

one or more bases are removed from a DNA sequence

Frameshift mutations

: change the "reading frame" of the sequence