Download Transcription and Translation

Chapter 7
III. Protein Synthesis
Overview
Chromosome #1 Has
over 3000 genes
• DNA contains genes
• Genes are specific
sequences of nitrogenous
bases.
• There are many genes on
one chromosome
• Genes direct the synthesis
of proteins.
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Proteins
• Proteins have many
functions
– Structure (collagen in
bones)
– Storage (Egg whites)
– Transport (cells)
– Enzymes (chemical
reactions)
– Defense (immune
system)
Proteins
• A monomer of a protein
is called an amino acid
• Amino acids link
together through
dehydration synthesis
• Many amino acids joined
together form a protein
(polypeptide)
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Proteins
• DNA codes for 20
amino acids
• We must go from
the language of
DNA to the
language of
proteins.
RNA
•
•
•
•
RNA = Ribonucleic Acid 5'
Monomers are nucleotides
Has both a 5’ and 3’ side
Base pairing rules differ
from DNA.
– Adenine binds to Uracil
– Guanine binds to Cytosine
3'
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DNA vs. RNA
DNA
•One type of DNA
RNA
•Three types of RNA
–mRNA, rRNA, tRNA
•Double stranded
•Single stranded
•Deoxyribose sugar
•Ribose sugar
•Has the nitrogenous
bases: Adenine,
Guanine, Cytosine and
Thymine
•Has the nitrogenous
bases: Adenine,
Guanine, Cytosine and
Uracil
Protein Synthesis
• DNA is
transcribed into
mRNA and then
translated into a
protein
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Protein Synthesis - Transcription
• Transcription  The
base sequence of DNA is
copied onto a strand of
messenger RNA (mRNA)
• DNA cannot leave the
nucleus.
• mRNA carries the
information out of the
nucleus to the
ribosomes.
Protein Synthesis Transcription
• One DNA strand called the
template strand is used to
create mRNA.
• mRNA is synthesized by
RNA polymerase.
• RNA polymerase begins
transcription of the DNA at
the promoter site.
• Transcription continues until
RNA polymerase encounters
a terminator sequence.
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Protein Synthesis - Transcription
• Synthesis of the mRNA
occurs in the 5' to
3'direction
• The base pairing rules are
the same as DNA except
Uracil replaces Thymine.
Protein Synthesis - Transcription
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Protein Synthesis - Transcription
• The newly formed mRNA is called pre-mRNA and must
be modified
• Introns (Intervening sequences) – noncoding segments
of mRNA.
• Exons (Expressed) – Coding segements of mRNA
• The introns are removed leaving only the exons.
Protein Synthesis - Transcription
• Pre-mRNA becomes mRNA after the introns are
removed.
• Why mRNA is modified.
– Removing Introns enables a single gene to
code for more than one polypeptide.
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Protein Synthesis - Transcription
• The mRNA leaves the
nucleus through the
nuclear pores.
The Genetic Code
• Each combination of three nucleotides is
called a codon.
• Each codon specifies an amino acid.
• Codons are read in the 5' to 3' direction
along the mRNA.
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The Genetic Code
• Three base codons are
enough to code for all 20
amino acids.
• The genetic code is
redundant  Different
codons can code for the
same amino acid
– Limits the number of
transcription errors.
– Protects our genes from
mutations.
• The genetic code is not
ambiguous  Each codon
specifies only one amino
acid.
Protein Synthesis - Translation
• Translation  The cell
interprets the mRNA
and builds a protein.
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Protein Synthesis - Translation
• tRNA (transfer RNA) –
transfers amino acids to
a ribosome where the
tRNA
amino acids are joined
together to form a
Anti-codon
polypeptide.
Amino Acid
– One side attaches the
amino acid.
– The other side is called the
anticodon and attaches to
correct part of the mRNA
– Each tRNA carries a
specific amino acid
Protein Synthesis - Translation
• rRNA (Ribosomal RNA)
Brings the tRNA together
with the mRNA and joins
the amino acids to form a
polypeptide.
– Made up of a large subunit
and a small subunit.
– Three binding sites:
• P site – holds the tRNA
carrying the growing
polypeptide.
• A site – Holds the tRNA
carrying the next amino
acid.
• E site – tRNA exits from
the ribosome at this site.
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Protein Synthesis - Translation
• The mRNA is read
codon by codon in the
5’ to 3’ direction.
• The tRNA brings in
each amino acid and
the ribosome builds
the growing
polypeptide.
Protein Synthesis - Translation
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Protein Synthesis - Translation
Interpreting the Genetic Code
• This table can be used
to translate a codon
• Example
– CAG = Gln
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Interpreting the Genetic Code
• Translation begins at the
start codon and ends at a
stop codon on the mRNA.
• The start codon also
codes for the amino acid
Met
• The stop codon does not
code for an amino acid
• The mRNA is read codon
by codon from 5’ to 3’
Interpreting the Genetic Code
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Free and Bound Ribosomes
• Multiple ribosomes can translate a single
mRNA.
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Free and Bound Ribosomes
• Free ribosomes –
Makes proteins that
remain in the
cytoplasm.
• Bound ribosomes
(RER) – makes
proteins that leave the
cell (insulin,
hormones, enzymes)
Mutations
• A mutation is a change in the DNA
• Mutagens
– Alter an organism’s DNA.
– Chemicals, radiation such as x-rays/UV light, or
mistakes during DNA replication.
– Many mutagens can lead to cancer.
• The sequence of codons is important in
order to create the correct protein!
– Example  “The car was won” Start at the wrong
point and you get “Hec arw asw on”
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Mutations
•
•
Point mutation - A single nucleotide base change.
Base-pair substitution - Replacement of one
nucleotide pair with another.
–
–
–
–
No effect if it is the same amino acid  Redundancy!
Missense – codes for a different amino acid
Nonsense – codes for a stop codon
Can be harmful to the organism if a useless or less active
protein is created.
Mutations
• Base-pair insertions or
deletions
– Addition or loss of
nucleotide pairs.
– Frameshift Mutation:
alters the reading frame.
– Can cause extensive
missense and eventually
nonsense.
– Protein will most likely be
non functional.
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