Gene Expression : Transcription and Translation

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Gene Expression Transcription and Translation
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How Are Different Types of Cells Created and
Maintained?
By differential gene expression.
The same genetic information is in all 100
trillion cells of any one person. Different cells
use the same blueprint in different ways.
How?
In essence, the control of gene expression occurs
by regulating the flow of information from DNA to
protein.
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The Central Dogma of Molecular Genetics
DNA
Trait
RNA
Protein
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Review

• DNA

• RNA

• Proteins

Made up of 4 different nucleotides
Made up of 4 different nucleotides
Made up of 20 different amino acids
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Gene Expression in Prokaryotes vs. Eukaryotes

• Transcription and translation are separated in
  time and space

• Transcription and translation may occur
  simultaneously

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The Genetic Code

• Problem How do only 4 different nucleotides code
  for the 20 amino acids that make up proteins?

• Solution Each group of 3 nucleotides codes for a
  different amino acid. These 3 nuclotide units are
  called codons.

DNA
RNA
Amino Acid
C
G
Valine
A
U
T
A
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Reading Frame

• The 3-nucleotide units (codons) must be read in
  the correct reading frame

• Start codons determine the reading frame

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The Genetic Code Dictionary

• There are multiple codons for each amino acid

• AUG is always the start codon

• UAA, UGA, and UAG are stop codons

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Transcription

• DNA is used to make a strand of RNA called the
  primary transcript (pre-mRNA)

• The pre-mRNA is further processed to create the
  finished mRNA

• mRNA exits the nucleus to be translated

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Transcription

• 3 main steps

1. Initiation
2. Elongation
3. Termination
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Transcription Initiation

• RNA polymerase binds to DNA at a region called
  the promoter

• RNA polymerase unwinds the DNA and adds
  nucleotides in the 5 ? 3 direction

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Transcription Elongation

• RNA polymerase moves along the DNA strand, adding
  60 nucleotides/sec

• DNA strands rejoin after polymerase passes by

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Transcription Termination

• Polymerase stops when it reaches a DNA sequence
  called the terminator

• The mRNA has been completely transcribed

• In eukaryotes, this is pre-mRNA and must be
  further processed

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mRNA Processing

• In eukaryotes, pre-mRNA must be further processed
  to mRNA before it leaves the nucleus

• Guanine is added to 5 end, forming the 5 cap

• 100s of adenines are added to 3 end, forming
  the poly-A tail

• Non-coding regions of RNA are spliced out

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• Intron (non-coding sequences) are cut out by
  spliceosomes. Leaving only Exons (Coding
  sequences) making up the mRNA that leaves the
  nucleus.
• Alternative splicing patterns means one gene can
  make more than one protein

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mRNA Splicing
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Translation

• The process in which mRNA is used to make proteins

• Occurs in the cytoplasm using ribosomes

• Requires tRNA bound to amino acid

• 3 steps initiation, elongation, termination

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tRNA

• A clover-shaped RNA molecule

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Structure of tRNA

• 3 end of tRNA binds to specific amino acids
• Anti-codon on tRNA complements mRNA codon

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tRNA Synthesis

• RNA is made in the nucleus

• Amino acids float free in the cytoplasm

• Aminoacyl-tRNA synthase joins each amino acid to
  the appropriate tRNA

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Ribosomes

• 2 subunits

• Composed of proteins and rRNA

• 3 tRNA binding sites

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Translation Initiation

• mRNA, tRNA and small ribosomal subunit bind with
  the P site at the start codon

• Large subunit binds using energy from GTP

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Translation Elongation

• mRNA is read 3 nucleotides at a time (Codons)
• tRNA brings corresponding amino acid into the A
  site of the ribosome

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• Ribosome catalyses dehydration synthesis reaction
  between amino acids in P site and A site
• Growing polypetpide now attached to tRNA in A
  site
• Ribosome moves forward one codon
• Free tRNA in P site exits out the back of
  ribosome
• tRNA (with polypeptide) moves into P site

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Translation Termination

• Elongation continues until reaching a stop codon

• Release factor binds and hydrolyzes the bond
  between the last tRNA and its a.a., freeing the
  new protein

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Polyribosomes

• Many ribosomes may simultaneously translate from
  a single mRNA

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Gene Expression Overview