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Gene to Protein
Chapter 17
Chapter 17
Central Dogma of Molecular Biology
Chapter 16
Chapter 17
Chapter 17
Overview of Central Dogma

Transcription: The process of building an mRNA strand from DNA.
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Translation: The process of building a polypeptide from an mRNA transcript.
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Ultimate goal: Use a sequence of DNA to get a protein.
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DNA  mRNA  Polypeptide
Transcription: DNA  mRNA
occurs in nucleus
Translation: mRNA  Polypeptide
occurs in cytoplasm or RER
Chapter 17
Transcription

DNA  RNA
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Much like replication with one major difference:
C goes with G
A goes with U (Not T!)
GATAGC TACAG TGGGAGT ATGCG T A TC TCC
CUAUCGAUGUCACCC UCAUACGCAUAGAGG
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The Genetic Code
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Codon: A three-nucleotide sequence
encoding one amino acid
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Eg – ACA (adenine cytosine adenine) codes for the
amino acid threonine
Near universality (slight differences in
mitochondrial DNA)
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Evolutionary significance
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The Genetic Code: A Tool
Chapter 17
Some things to be aware of…

Although mRNA doesn’t necessarily start with AUG (start codon), nothing gets
translated until the ribosome hits that point.
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Similarly, nothing gets translated after UAA, UGA, or UAG (stop codons).
3 Types of RNA
Chapter 17
1.
mRNA: The RNA transcript that exits the nucleus and is
used by the ribosome to translate a polypeptide.
2.
tRNA: The RNA located in the cytoplasm that brings amino
acids to the ribosome to be added to a growing
polypeptide.
3.
rRNA: The RNA located in the ribosome that helps the
ribosome to “read” the mRNA transcript.
Chromosome Regions
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1. Heterochromatin - highly condensed chromatin; areas that are not
transcribed.
2. Euchromatin - less condensed chromatin; areas of active transcription.
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Translation
Overview
• As mRNA goes through
ribosome, its codons are
matched up with
anticodons on tRNA.
• tRNA carries amino acids
that correspond with the
codons tRNA binds to in
mRNA.
• Amino acids are therefore
attached in a specific
order during translation.
Transfer RNA (tRNA)
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20 types of tRNA synthetases
join 20 different amino acids
to the correct tRNAs.
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There are ~45 different
tRNAs.

Wobble position allows
some tRNAs to bind to more
than one codon.
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Wobble in tRNA
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
Due to evolution of the genetic code, the third base can be less
discriminatory for the amino acid than the other two bases

This third position in the codon is referred to as the wobble
position. At this position Us and Cs may be read by a G in the
anticodon.
Polyribosomes

A single mRNA can make many
polypeptides simultaneously
because several ribosomes can
translate it at the same time
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Post Translation

polypeptide chains fold into their functional tertiary structure

tertiary proteins may join with other tertiary proteins becoming the subunits
of a protein with quaternary structure
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some amino acids may be chemically modified

enzymes may remove amino acids from the leading end or cleave a
polypeptide chain into two pieces
Rough ER Ribosomes
Chapter 17

translation always begins on ribosomes in the
cytoplasm

if the mRNA codes for a protein destined for cell
secretion or the endomembrane system, the
ribosome will attach to the rough ER

the cue to do so comes from a signal peptide
(~20 amino acid sequence) in the growing
polypeptide chain that is recognized by signalrecognition particle (SRP) in the cytoplasm
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Mutations: changes in DNA

Point mutations – changes in a single base-pair

substitution – replacement of one nucleotide & its partner with
another pair of nucleotides
can be “silent” – have no effect on the encoded protein
 are usually missense – change a single amino acid in the encoded
protein
 can be nonsense – change a codon to a stop codon causing
translation to terminate early (shortens encoded protein)
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Frameshift mutations
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insertion – addition of nucleotide pairs
deletion – loss of nucleotide pairs
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when insertions & deletions are not a multiple of 3, they
significantly alter the reading frame (change all codons
downstream of the mutation)
http://www.youtube.com/watch?v=eDbK0cxKKsk
Chapter 17
Mutations: changes in DNA

Point mutations vs. Frameshift mutations
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Point – a nucleotide is just wrong
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Frameshift – a nucleotide is added or deleted.
ACG TCC  Point Shift at the ‘G’  ACA TCC
ACG TCC  Frame shift, G deleted  ACT CC
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Mutagens
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physical & chemical agents that react with DNA in ways that cause mutations
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
examples: X-rays, UV rays, carcinogens
MUTAGENESIS- A process by which the genetic information of an organism is
changed in a stable manner, either in nature or experimentally by the use of
chemicals or radiation.