Transcript Protein Synthesis

Protein Synthesis
The Making of Proteins
Using Genetic Information
Central Dogma of Molecular Biology
Information flows in one direction –
from DNA to RNA to proteins.
Relationship between DNA, Genes, Proteins
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Genetic information passed from parent
to offspring is DNA
Each DNA molecule has hundreds of segments known as
_genes_.
Gene - section of DNA that provides the instructions for
making a _protein
 Protein molecules are polymers formed by joining
amino acids
 The type of protein made is determined by the
order of amino acids.
 1,000’s of different protein molecules are
made by the cell every minute
Protein Synthesis - process of using
DNA’s code to make proteins.
 Two-part process:
1. Transcription
2. Translation
Part 1
Transcription - DNA code used to make RNA
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Occurs in the cell nucleus.
DNA is too big to leave the nucleus. So its
“message” is rewritten in the language of RNA
molecules which are smaller and can leave the
nucleus
How Transcription Works
 Enzymes unwind DNA to expose bases in a specific
gene.
transcription complex
start site
nucleotides
How Transcription Works
• RNA polymerase attaches complementary RNA bases to the
DNA strand
• Free nucleotides base pair to the DNA strand
• RNA polymerase bonds the nucleotides together to form RNA
DNA
.
RNA polymerase
moves along the DNA
Transcription – rewriting DNA into RNA
• The newly made RNA strand detaches from the DNA after the
gene is transcribed.
• The small RNA strand exits the nucleus through pores in the
nuclear membrane
RNA
Transcription makes three types of RNA
• mRNA – Messenger RNA carries DNA’s protein
making code to ribosomes
• rRNA – Ribosomal RNA molecules needed to build
ribosomes the cell’s “protein factories”
• tRNA – Transfer RNA carries amino acids to
ribosome to make proteins as directed by the
mRNA’s code
8.4 Transcription
The transcription process is similar to replication.
• Transcription and replication both involve complex
enzymes and complementary base pairing.
• The two processes have different end results.
– Replication copies
all the DNA;
transcription copies
one
gene
growing RNA strands
a gene.
– Replication makes
one copy;
DNA
transcription can
make many copies.
8.4 Transcription
Translation – mRNA’s code is used to make proteins
(polypeptides)
8.4 Transcription
Amino acids are coded by mRNA base sequences.
• A codon is a sequence of three nucleotides on mRNA that
codes for an amino acid.
codon for
methionine (Met)
codon for
leucine (Leu)
8.4 Transcription
• The genetic code matches each codon to its amino acid or
function.
The genetic code matches each RNA codon with its amino acid or function.
– three stop
codons
– one start
codon,
codes for
methionine
8.4 Transcription
8.4 Transcription
 Site of protein synthesis – ribosomes in the
cytoplasm.
8.4 Transcription
Ribsomes are made from rRNA and proteins
8.4 Transcription
• Ribosomes consist of two subunits.
– The large subunit has three binding sites for tRNA.
– The small subunit binds to mRNA.
• Transfer RNA (tRNA) carries amino acids to the
ribsome.
• On the other end of tRNA there is a 3 nucleotide
sequence called an anticodon
• An anticodon is a set of three nucleotides that is
complementary to an mRNA codon.
8.4 Transcription
• For translation to begin, tRNA binds to a start codon and
signals the ribosome to assemble.
– A complementary tRNA molecule binds to the exposed
codon, bringing its amino acid close to the first amino
acid.
• tRNA’s anticodon matches with mRNA’s codon to deliver the
correct amino acid needed to make the protein.
• The Ribosome positions the tRNA molecules close enough so
that peptide bonds can form between the amino acids.
•
8.4 Transcription
Once the stop codon is reached, the ribosome
releases the protein and disassembles.
Central Dogma
KEY CONCEPT
Mutations are changes in DNA that can affect phenotype
What are Mutations?
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Mutations are changes to DNA that may be
harmful,
helpful or
have no effect.
Mutations can be caused by several factors.
• Errors during DNA Replication
• Mutagens ( environmental
factors that mutate genes):
- Radiation
- Chemicals in environment
• Errors during Meiosis
Rachel Carson
author of Silent Spring
2 Categories of Mutations:
1.Single gene – usually during DNA replication
affect one gene
2. Chromosomal – usually occur during meiosis
affect many genes
TRANSLOCATION
(think “transfer location”)
• The exchange of DNA segments between nonhomologous
chromosomes.
• DUPLICATION.
• Chromosomal mutations may occur during crossing over in
Prophase
I of Meiosis
.
– Gene duplication results from unequal crossing over.
• NONDISJUNCTION
• Homologous chromosomes don’t separate in
Anaphase I
• Sister chromatics don’t separate in
Anaphase II
• Gene Mutations
• Usually occur in replication
• Affect one gene and protein made from it
8.4 Transcription
• Frameshift Mutation
• A cat sat on my hat Ac ats ato nmy hat
• Adding or removing bases changes the codon reading
frame - this changes the amino acid sequence which
changes the protein made
• .
Ex. Deletion
Mutation (shifts the reading frame)
Point Mutation
• A point mutation substitutes one nucleotide
for another.
mutated
base
Types of Point Mutations
Nonsense point mutation
• Change codes for a STOP codon
• Full protein not made
• NO STOP
Missense Point Mutation
* Codes for wrong amino acid
* May or harmful or beneficial – depends…
Silent point mutation
Change in the DNA sequence that does not change the
amino acid sequence or the protein.
DNA changes but its expression (phenotype) does not.
Mutations affect the DNA
But they may or may not affect phenotype.
• Chromosomal mutations tend to have a big effect.
• Some gene mutations change phenotype.
– Protein not made
– May change protein shape or the active site.
blockage
no blockage
• Mutations in body cells do not affect
offspring.
• Only Mutations in sex cells affect offspring.
• Natural selection often removes mutant
alleles from a population when they are less
adaptive.
Transcription is similar to Replication
• SIMILARITIES
• Replication (making DNA) andTranscription (making RNA) both
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* Make a nucleic acid
* Occur in the nucleus (eukaryotes)
* Occur in all living organisms
* Require a number of enzymes to carry out process
* Involve complimentary base pariring
• DIFFERENCES
• The two processes have different end results.
• _ Replication produces DNA; Transcription produces RNA
– Replication copies all the DNA; transcription copies an individual gene on the DNA.
– Replication makes one copy; transcription can make many copies.