bio 111: chapter 15- transcription and translation

gene expression

how the information in the gene is used to make a functional product, genes code for polypeptide chains or for RNA molecules

central dogma

theory stating that genetic information moves in one direction from DNA to RNA to Protein

transcription

the synthesis of RNA under the direction of DNA, produces mRNA, catalyzed by RNA polymerase

translation

the actual synthesis of a polypeptide, which occurs under the direction of mRNA, occurs on ribosomes

in prokaryotes

transcription and translation occur simultaneously

in eukaryotes

RNA transcripts (pre-mRNA) are modified before becoming true or mature mRNA

mRNA

messenger RNA that codes for proteins

tRNA

transfer RNA that transfer amino acids during protein synthesis

rRNA

ribosomal RNA that is a part of the ribosome

codon

a sequence of nonoverlapping base triplets, either translated into an amino acid or serves as a translational stop signal

RNA polymerase

catalyzes DNA-directed synthesis of RNA, unwinds DNA, reads DNA, adds RNA nucleotides, no primer required, synthesis in 5' to 3' direction

RNA polymerase 1

transcribes rRNA genes

RNA polymerase 2

transcribes protein-coding genes

RNA polymerase 3

transcribes rRNA, tRNA, and small nuclear RNA genes

3 stages of transcription

initiation, elongation, termination

initiation in transcription

regulatory region has a binding site for RNA polymerase called promoter (a piece of DNA several base pairs upstream) that indicates where the RNA polymerase should start

promoters

region of DNA that indicates to an enzyme where to bind to make RNA, signal the initiation of RNA synthesis

transcription factors

proteins that help RNA polymerase recognize promoter sequences

TATA binding protein (TBP)

a general transcription factor that binds to the TATA box and assists in attracting other general transcription factors and RNA polymerase II to eukaryotic promoters, transcription initiation complex

elongation in transcription

RNA polymerase adds complimentary nucleotides (A, U, C, G) to make the mRNA

termination in transcription

differ in prokaryotes and eukaryotes

termination of transcription-prokaryotes

RNA polymerase transcribes to the terminator sequence, causing the polymerase to separate from the DNA and release the transcript

rho-dependent termination

rho protein interacts with RNA polymerase over a G-rich stretch of sequence and causes polymerase to release the transcript

rho-independent termination

RNA polymerase reads through a region of C-G nucleotides, RNA folds back on itself forming a hairpin and is released, does not need rho protein to terminate

termination of transcription-eukaryotes

RNA polymerase transcribes the polyadenylation signal sequence; the RNA transcript is released 10-35 nucleotides past this polyadenylation sequence

mRNA modification

occurs in eukaryotes, pre-mRNA is modified at both ends before leaving the nucleus

types of modifications

-5' ends receive a modified Guanine cap

-3' ends receive poly-A-tail (50-200 adenines added)

-5' and 3' UTR (untranslated region)

untranslated regions (UTR)

contain information on ribosome binding sites and other instructions including mRNA stability

RNA splicing

removing intervening, non-protein coding regions of mRNA

introns

intervening, non-protein coding regions

exons

protein-coding regions

splicesomes

enzyme that catalyzes the RNA splicing process

alternative RNA splicing

allows a single gene to be spliced multiple ways, resulting in different combinations of exons in the mature RNA

ribosomes

has two subunits and binding sites for mRNA and multiple tRNAs

tRNA

carry the amino acids to the ribosome, consists of a single RNA strand, roughly L-shaped, carries a specific amino acid attached to their 3' end, opposite end consists of anticodon that will hydrogen bond with the codon

aminoacyl-tRNA synthetase

specific enzyme that joins each amino acid to the correct tRNA

A site in ribosome

acceptor site for an aminoacyl tRNA, holds the tRNA that carries the next amino acid to be added to the chain

P site in ribosome

peptidyl, holds the tRNA carrying the growing polypeptide chain

E site in ribosome

exit site, where discharged tRNAs leave the ribosome

3 stages in translation

initiation, elongation, termination

initiation in translation

brings together mRNA, a tRNA with the first amino acid, and the two ribosomal subunits

elongation in translation

amino acids are added one by one to the preceding amino acid at the C-terminus, each addition involves proteins called elongation factors, codon recognition, peptide bond formation, translocation

termination in translation

occurs when a stop codon in the mRNA reaches the A site of the ribosome

polyribosomes

a number of ribosomes can translate a single mRNA molecule simultaneously in prokaryotes

protein targeting

proteins destined for the endomembrane system or for secretion must be transported into the ER, free ribosomes have signal peptides to which a signal-recognition particle (SRP) binds, enabling the translation ribosome to bind to the ER

mutations

changes in the genetic material of a cell, can affect protein structure and function

point mutations

changes in just one base pair of a gene in the DNA's template strand, leads to the production of an abnormal protein

base-pair substitutions

the replacement of one nucleotide and its partner with another pair of nucleotides, can be silent/missence/ or nonsense mutations

silent base-pair substitutions

no effect on the amino acid sequence

missence base-pair substitutions

specify a different amino acid

nonsense base-pair substitutions

creates a premature stop codon

base-pair insertions or deletions

additions or losses of nucleotide pairs in a gene, may produce frameshift mutations

nucleotide pair insertion

frameshift causing immediate nonsense

nucleotide pair deletion

frameshift causing extensive missense

3 nucleotide pair deletion

no frameshift, but one amino acid missing

spontaneous mutations

random change in the DNA due to errors in replication, recombination, or repair

induced mutations

mutagens which are physical or chemical agents that can cause mutations