Transcript From Gene to Protein

From Gene to Protein
Chapter 17
Hon. Adv. Biology Notes
12/01/06
• Connection between Genes and Proteins
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Evidence that genes specify proteins
Overview: “Central Dogma of Molecular Genetics”
“The Code” – nucleotide triplets specify a.a.’s
Evolution of the code
• Synthesis and Processing of RNA
– A closer look: Transcription
– RNA processing occurs in eukaryotic cells
• Synthesis of Protein
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A closer look: Translation
Signal peptides – target specific cell destinations
Review: Roles of RNA in the cell
Review: Comparison of protein synthesis in prokaryotes and
eukaryotes
– Point mutations can affect protein structure and function
– Wrap-up: “So, What is a gene?”
Lecture Objectives
• Compare the structure of RNA with that of
DNA.
• Summarize the process of transcription.
• Relate the role of codons to the sequence of
amino acids that results after translation.
• Outline the major steps of translation.
• Discuss the evolutionary significance of the
genetic code.
Figure 17.1 Beadle and Tatum’s evidence for the one gene-one enzyme hypothesis
RNA vs. DNA
• there are 3 major differences between the 2
forms of nucleic acids
– Composition of sugar backbone (reflects name)
• RNA - ribonucleic acid (ribose)
• DNA - deoxyribonucleic acid (deoxyribose)
– number of strands: RNA = 1 and DNA = 2
– RNA does not have thymine as a nitrogen base,
instead uracil (U) is substituted in its place
(back)
Central Dogma of
Molecular Genetics
• Explains the flow of information from
gene to protein.
– Replication: flow of gene from one
generation to the next
– Transcription: flow of info from DNA to
RNA
– Translation: conversion of info from
nucleic acid language into protein
•Transfer of Information from
DNA to RNA
• Transcription: mRNA production, or the
synthesis of other types of RNA
• Short segment of DNA (gene) is transcribed
into mRNA for movement out of the
nucleus to the ribosome
• DNA is used as a template to make a
complementary piece of “messenger
RNA”. This requires the aid of the RNA
polymerase enzyme.
Translation: Protein Synthesis
• mRNA transcript arrives at the ribosome to
act as the genetic code for protein assembly
• proteins are composed of a unique sequence
of amino acids.
• mRNA carries the code for the order and
type of amino acids to be included in the
protein. The “genetic code” is the unique
order of triplet codons located on the
mRNA strand. (back)
The Genetic Code: ThreeNucleotide “Words”
• Genetic Code The genetic code
(codons) used by most organisms to
translate mRNA is nearly universal.
Sometimes called a triplet code.
Figure 17.3 The triplet code
Figure 17.4 The dictionary of the genetic code
What does a “Gene” really do?
• It is simply a blueprint in
the DNA that tells how to
build a protein.
• “WHO” we are
genetically is really just a
collection of “WHAT”
proteins are expressed in
our phenotype.
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Figure 17.5 A tobacco plant expressing a
firefly gene
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Evolution of the Genetic Code
• must have evolved very early in the history
of life…. Why?
• Answer: nearly universal, shared by
organisms from the simplest bacteria to
complex plants and animals
• Ex. CCG codes for the amino acid proline
in every organism ever studied (back)
The Synthesis/Processing of RNA
• The Stages of Transcription
– initiation
– elongation
– termination
• RNA processing occurs in eukaryotes
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alteration of mRNA ends
RNA splicing
ribozymes
introns (back)
Protein Synthesis (Translation)
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structure and function of tRNA
aminoacyl-tRNA synthetases
ribosomes
building a polypeptide
– initiation
– elongation
– termination
• signal peptides target protein destinations
Point Mutations
• types of point mutations
– base-pair substitutions
– insertions
– deletions
• Results of point mutations
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silent mutation
missense mutations
nonsense mutations
frameshift mutations (back)
Key Terms for Review!!