Transcriptions, Post-transcriptional processing, Protein Synthesis, Post-translational modification

Transcription

Process of synthesizing RNA from a DNA template

RNA polymerase

Enzyme that synthesizes RNA during transcription

RNA Pol I

Synthesizes rRNA (28S, 18S, 5.8S)

RNA Pol II

Synthesizes mRNA and snRNA

RNA Pol III

Synthesizes tRNA and 5S rRNA

TATA box

Promoter sequence upstream of transcription start site in eukaryotes

General transcription factors (GTFs)

Proteins that help RNA Pol II bind promoter and initiate transcription

Chromatin

Complex of DNA and histone proteins in eukaryotes

Histones

Basic proteins that package DNA into chromatin

Exons

Expressed coding sequences retained in mature mRNA

Introns

Non-coding intervening sequences removed during RNA processing

Exon shuffling

Combination of exons from different genes to form functional genes

Transcription direction

RNA is synthesized in the 5’ → 3’ direction

Promoter

DNA sequence where transcription machinery binds

Post-transcriptional processing

Modification of pre-mRNA to form mature mRNA

Three major post-transcriptional modifications

5’ capping, polyadenylation, and splicing

5’ cap

7-methyl guanosine added to 5’ end of mRNA

Function of 5’ cap

Protects mRNA and assists export and ribosome recognition

Poly-A tail

Chain of adenines added to 3’ end of mRNA

Function of Poly-A tail

Increases mRNA stability and lifespan

Splicing

Removal of introns and joining of exons

Spliceosome

snRNP-containing complex that performs RNA splicing

snRNPs

Small nuclear ribonucleoproteins involved in splicing

Pre-mRNA

Primary RNA transcript before processing

Mature mRNA

Processed RNA ready for translation

Genetic code

Relationship between codons and amino acids

Codon

Triplet of nucleotides coding for an amino acid

Triplet code

Three nucleotides specify one amino acid

Non-overlapping code

Each nucleotide belongs to only one codon

Degenerate code

Multiple codons may code for the same amino acid

Universal code

Nearly all organisms use the same genetic code

Wobble hypothesis

Third base of codon can vary without changing amino acid

Total codons

64 codons exist in the genetic code

Coding codons

61 codons code for amino acids

Stop codons

3 codons terminate translation

Stop codons

UAA, UAG, and UGA

Start codon

AUG

AUG codes for

Methionine

Reading frame

Sequence in which codons are read during translation

Translation

Process of synthesizing protein from mRNA

Components of translation

mRNA, ribosomes, tRNA, amino acids, initiation and elongation factors

Ribosome

Site of protein synthesis

Eukaryotic ribosome size

80S

40S subunit

Small eukaryotic ribosomal subunit

60S subunit

Large eukaryotic ribosomal subunit

tRNA

Transfers amino acids to ribosome during translation

Anticodon

Three-base sequence on tRNA complementary to mRNA codon

Initiation

First stage of translation where ribosome assembles on mRNA

Elongation

Stage where amino acids are added to growing peptide chain

Termination

Stage where translation stops at stop codon

eIF-2

Initiation factor that binds GTP and Met-tRNAiMet

43S preinitiation complex

Complex of 40S subunit with initiation factors and Met-tRNA

Kozak sequence

Consensus sequence that helps identify start codon in eukaryotes

Kozak consensus sequence

ACCAUGG

Shine-Dalgarno sequence

Prokaryotic ribosome-binding site absent in eukaryotes

First amino acid in eukaryotes

Methionine

Protein synthesis direction

Protein synthesized from N-terminus to C-terminus

Release factor

Protein that recognizes stop codons and terminates translation

GTP

Required for initiation, elongation, and termination in translation

Post-translational modifications

Changes made to proteins after translation

Protein folding

Process by which protein attains functional 3D structure

Chaperonins

Proteins that assist proper protein folding

Hsp70

Chaperonin that binds unfolded hydrophobic protein regions

Proteolytic trimming

Cleavage of precursor proteins into active forms

Phosphorylation

Addition of phosphate group to protein

Glycosylation

Addition of carbohydrate group to protein

Hydroxylation

Addition of hydroxyl group to amino acids

Carboxylation

Addition of carboxyl group to amino acids

Gene expression regulation

Control of when and how genes are expressed

Importance of gene regulation

Allows specialization, adaptation, and energy conservation

Differential gene expression

Different cells express different genes

Enhancers

DNA regulatory sequences that increase transcription

Transcription factors

Proteins that regulate transcription initiation

Eukaryotic transcription regulation

Controlled by transcription factors, enhancers, and chromatin structure

Monocistronic mRNA

Eukaryotic mRNA encoding one protein

Polycistronic mRNA

Prokaryotic mRNA encoding multiple proteins

Operon

Cluster of genes regulated together in prokaryotes

Eukaryotes vs prokaryotes transcription

Separated in eukaryotes but simultaneous in prokaryotes

Central dogma

DNA → RNA → Protein

Reverse transcriptase

Enzyme that synthesizes DNA from RNA template

Retroviruses

RNA viruses that use reverse transcriptase

Reverse transcriptase clinical importance

Target for HIV therapy

RNA processing occurs in

Nucleus

Translation occurs in

Cytoplasm

Major regulator of gene expression

Initiation of transcription

Rifampicin

Antibiotic that inhibits bacterial RNA polymerase transcription

Diphtheria toxin

Inactivates EF-2 and blocks protein synthesis

EF-2

Eukaryotic elongation factor involved in translocation during translation