Protein Synthesis - Transcription and Translation

DNA replication and DNA transcription

DNA replication → occurs before cell division

DNA transcription → formation of RNA for protein synthesis

Protein synthesis

When a cell makes a protein, the gene is “expressed”

• nearly all the body’s cells can make all human proteins, but each type of cell makes only the kinds of protein it needs

DNA and protein synthesis

DNA directs protein synthesis → one gene codes for 1 polypeptide

Ribosome and translation

Translation → polypeptide synthesis

Transcription

Synthesis of mRNA from DNA → messenger RNA

Translation

Synthesis of polypeptide → via RNA

Ribosome in cytosol

Transcription and translation in Prokaryotes

Prokaryotes → bacteria

Transcription and translation occur together

Transcription and translation in eukaryotes

Transcription in nucleus

Pre-mRNA modified before becoming true mRNA

RNA vs DNA

RNA → single stranded, ribose sugar, bases (C,G,A,U)

DNA → double helix, deoxyribose sugar, bases (C,G,A,T)

RNA transcription

Uses RNA polymerase

• pries DNA strands apart

• Reads.1 DNA strand → adds nucleotides to RNA

• Follows base-pairing rules → uracil substitutes for thymine

Stages of Synthesis of an RNA transcript

Initiation

Elongation

Termination

Initiation of synthesis RNA transcript

Promoters signal initiation of RNA synthesis

Transcription factors → help eukaryotic RNA polymerase recognize promoter sequences

Elongation of synthesis RNA transcript

RNA polymerase moves along the DNA

• untwists double helix

• Exposes ~ 10-20 DNA bases at a time

• Matches RNA nucleotides with DNA template

Termination of synthesis RNA transcript (Eukaryotes vs prokaryotes)

Prokaryotes → terminator sequence (polymerase detaches)

Eukaryotes:

• polymerase transcribes polyadenylation sequence (AAUAAA) in pre-mRNA and beyond

• Proteins cut mRNA free (10-35 nucleotides past poly-A)

• Polymerase falls away from DNA (not understood)

RNA processing (pre-mRNA)

5’ end receives a modified nucleotide cap

The 3’ end gets a poly-A tail:

• helps to export mRNA to cytosol

• Protects mRNA from degradation

• Helps ribosomes attach in cytosol

RNA processing (RNA splicing)

Removes introns and joins exons

• introns → non-coding

• Exons → expressed

Ribozymes

• catalytic RNA molecules

• Function as enzymes

• Can splice RNA

Introns

Allow for alternative RNA splicing

Increased number of different proteins possible

Proteins characteristics

• made from amino acids

• Modular structure

• Each exon codes for different domain

Messenger RNA

MRNA carries message as a series of codons

Codons

Triplet sequence of bases along mRNA

Codon in mRNA → either codes for an amino acid or as stop signal

Pairs with anti-codon on T-RNA

Transfer RNA (t-RNA’s)

Shuttle amino acids to building polypeptide

Each tRNA is specific for an amino acid

Anti-codon → binds to mRNA codon

RNA strand (~80 nucleotides)

~L-shaped

Ribosomes

Facilitate coupling of tRNA anticodons with mRNA codons during protein synthesis

Made of proteins and ribosomal RNA or rRNA

Ribosome binding sites

Has 3 binding sites for tRNA:

• the P site

• The A site

• The E site

Initiation phase of translation

At start codon

Brings together:

• mRNA

• Initiator tRNA (with first amino acids -Met)

• Two subunits of a ribosome together

Elongation phase of translation

Amino acids are added one by one to polypeptide

Termination phase of translation

Ribosome reaches a stop codon in the mRNA

Polyribosome

Many ribosomes can translate 1 mRNA at once

Possible changes after translation

• enzyme may be cleaved (e.g. insulin)

• Sugars or lipids may be attached

• Removal of lead amino acids

Mutations characteristics

Wobble position → can contribute to mutations

One wrong nucleotide—one wrong amino acid—dysfunctional protein

Substitutions, insertions, or deletions

Produce nonsense or mutation

Epigenetics

Chemical mechanisms that control the expression of genes

Methylation → repressions

Histones → control longer sections