Chapter 6 | How Cells Read the Genome: From DNA to Protein

Ribosome function

Synthesis of polypeptides

Ribosome location

Free in cytosol or on rough endoplasmic reticulum

Cytosol

Aqueous portion of cytoplasm & dissolved substances

Cytoplasm

Everything inside the plasma membrane in prokaryotes; everything in between nuclear membrane & plasma membrane in eukaryotes

Large subunit

Catalyzes synthesis of polypeptide

Small subunit

Binds to mRNA for it to be read

Subunits together

• Aminoacyl tRNA binding site (A site)

• Peptidyl tRNA binding site (P site)

• Exit - peptide & tRNA leave (E site)

Ribosome components

rRNA (ribosomal) & proteins

Bacteria vs Eukaryotic Ribosome

70S & 80S

Homologous rRNA across subunits

Nucleus

• Assembly of ribosome

• DNA storage & replication

• Transcription of rRNA, mRNA, tRNA

Nucleolus

Synthesis of rRNA

Transcription

Synthesis of RNA

Gene

Sequence of DNA that is transcribed/codes for RNA

Regulatory regions

• Promoter region (+1)

• -35 region

• -10 region/TATA box

Consensus sequences

Transcription of sequence is dependent on closeness to consensus sequences

Operons

Genes that code for multiple polypeptides

σ-subunit

RNA polymerase recognizes & binds to regulatory regions

Transcription factors

General & specific

45S precursor rRNA

codes ¾ of rRNA by being cleaved

RNA Pol I

Synthesizes rRNA

RNA Pol II

Synthesizes mRNA & other RNAs

RNA Pol III

Synthesizes tRNA & others

Eukaryotic RNA polymerase

• Aided by elongation factors

• Unwinds DNA & forms transcription bubble

• Catalyzes synthesis 5’-3’

Processive enzyme

An enzyme that continually catalyzes reaction without releasing the substrate

Termination in Bacteria

Terminator sequence & hairpin loop pauses RNA polymerase at AU sequences

Rho proteins separate RNA polymerase from the DNA

Translation in bacteria

Transcription & translation occur simultaneously

RNA Processing

• 5’ cap

• Splicing

• Poly-A tail

5’ cap

Upside-down methyl guanosine that prevents degradation from Rnase

Splicing

Catalyzed by the SnRNP’s part of the spliceosome

Ribozyme

RNA acts as the enzyme instead of the protein

Poly-A tail

Synthesized by Poly-A polymerase, creating a consensus sequence that allows for the termination of transcription - delays degradation of the mRNA over time

mRNA export out of he nucleus

Is escorted & bound to proteins, kept linear & unfolded in order to be translated

tRNA processing

• Trimming, splicing, modification

• Base-pairing forms 2D structure; non covalent interactions form 3D structure

• Acceptor arm, anticodon arm, & variable arms

Acceptor arm

Binds to the amino acid

Anticodon arm

Reads mRNA

tRNA processing

• Cleavage

• Modification of uracil to pseudouracil & methylation of riboses

• Catalyzed by snoRP’s

• Folding

Ribosomal assembly

Processome catalyzes reaction; large & small subunit mature independently of one another

Escort out of the nucleus

• Escorted by proteins

• Cytosomal modification

• Different ribosomes translate different mRNA’s