Transcript Protein Synthesis

UNDERSTANDING
HEREDITY
PROTEIN SYNTHESIS
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Genes & Proteins
• Genes - sequences of nucleotide
bases
• Genes code for proteins
• Proteins - amino acids linked
together by peptide bonds
• 20 different amino acids exist
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Genes & Proteins
• Amino acid
chains are
called
polypeptides
• So: Protein =
polypeptide
Ribbon Structure of
Mouse Cholera
Antibody
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Starting with DNA
• DNA - found inside the nucleus
• Proteins are made in the
cytoplasm of cells by organelles
called ribosomes
• Ribosomes may be free in the
cytoplasm or attached to the
surface of rough ER
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Starting with DNA
• DNA ‘s code must be copied
and taken to the cytoplasm
• In the cytoplasm, this code
must be read so amino acids
can be assembled to make
proteins
• This process is called protein
synthesis
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RNA
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Roles of RNA & DNA
•DNA = COOKBOOK
•RNA = KITCHEN CREW
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Roles of RNA & DNA
• RNA is a disposable copy of a portion
of DNA
• It is used to construct a protein
• Comparison of RNA and DNA
RNA
DNA
Sugar
Ribose
Deoxyribose
Nitrogen Bases
C G A U(racil)
C G A T
Structure
Single Helix
Double Helix
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.
Three Types of RNA
• Messenger RNA - mRNA
• Ribosomal RNA - rRNA
• Transfer RNA
- tRNA
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Messenger RNA (recipe)
• Long chain of nucleotides
• Made in the nucleus
• Transcribes DNA &
leaves through nuclear
pores
• Carries information for a
specific protein to
ribosome
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Ribosomal RNA (head chef)
• rRNA is a single strand
100 to 3000 nucleotides
long
• Globular in shape
• Made in the nucleolus
of a cell
• Reads mRNA and
assembles proteins
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Transfer RNA (kitchen staff)
• Clover-leaf shape
• Single stranded molecule with
attachment site at one end for an
amino acid
• Found out in the cytoplasm
• Brings amino acid to ribosome
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The Genetic Code
• Codon - a set of three RNA bases
• Codon - designates a specific
amino acid
• An amino acid may have more
than one codon and there are 64
codons
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Transcription
and
Translation
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Protein Synthesis
• The production or synthesis of
polypeptide chains (proteins)
• Two phases:
Transcription & Translation
• mRNA must be processed before
it leaves the nucleus of eukaryotic
cells
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Transcription
• The process of mRNA copying the
sequence of one strand of DNA
• During transcription, RNA
polymerase binds to DNA and
separates the DNA strands
• RNA Polymerase then uses one
strand of DNA as a template to
assemble nucleotides into RNA
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• What would be the mRNA strand
for the following DNA sequence?
DNA 5’-GCGTATGAA-3’
mRNA 3’-CGCAUACUU-5’
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STEPS OF TRANSCRIPTON:
1. RNA polymerase binds to DNA
just before code for protein begins
2. Nucleotide bases are added to
make strand of mRNA
3. RNA polymerase detaches from
DNA and mRNA
4. Enzymes proofread the mRNA
sequence before it leaves nucleus
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mRNA Transcript
•mRNA leaves the nucleus through
its pores and goes to the ribosomes
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Translation
• Translation = the process of
decoding the mRNA into a
polypeptide chain
• Ribosomes read mRNA three
nucleotide bases or 1 codon at
a time
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STEPS OF TRANSLATION:
1. mRNA transcript start codon AUG
attaches to the ribosome
2. Amino acids are brought to the
ribosome by tRNA
3. tRNA plugs into the mRNA with an
anticodon
4. The amino acids are attached like
beads on a string
5. tRNA is released to cytoplasm;
mRNA is disassembled
6. Stop codon tells ribosome to release
protein
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Summary of Protein Synthesis
Transcription
DNA
code
Translation
mRNA tRNA
amino acid
copy
anticode assemble
protein
product
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Mutations are Mistakes
• Mistakes in gene code cause
mistakes in the protein
• These mistakes are called mutations
• In protein synthesis mutations
involving one base are called point
mutations
• Some mutations may do nothing to
the final protein others may do
serious damage
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