Download Protein Synthesis: Transcription

Protein Synthesis:
Transcription
Objectives
What is the Central Dogma of biology?
What are the steps of transcription?
Recall

Why do we need proteins?
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Muscles, hair, nails cartilage, tendons,
ligaments
Enzymes
Hemoglobin
Antibodies
Central Dogma
What are proteins made of?

Amino Acids are the individual building blocks

Together to make one Protein
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There are 20 types of amino acids, some

There are infinite combinations of amino acids

These long chains are called polypeptide
chains
Protein Synthesis

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Protein synthesis is the process in
which a cell makes protein based on the
message contained within its DNA.
However:

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DNA is only found in the nucleus
Proteins are only made outside the nucleus –
in the cytoplasm.
Protein Synthesis
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How do the many different messages
within the DNA molecule get to the many
ribosomes outside the nucleus?
A molecular cousin of DNA – RNA – is
used to carry these messages.
Ribonucleic Acids - RNA

The job of RNA (ribonucleic acid) is to
carry messages from the DNA (in the
nucleus) to the ribosomes (in the
cytoplasm).
Differences between RNA and DNA

RNA

DNA
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Single Stranded

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Ribose sugar group
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Nitrogen Base Pairs
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A-U, C-G
Double-stranded
Deoxyribose sugar
group
Nitrogen Base Pairs
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A-T, C-G
Types of RNA
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mRNA: carries the message from the DNA
to the cytoplasm
tRNA: transports amino acids to the mRNA
to make the protein
rRNA: make up the ribosomes, which help
make the protein
Protein Synthesis Process

Occurs in TWO steps:
1. Transcription
– the genetic information from
a strand of DNA is copied into a strand of
mRNA
2. Translation
Steps of Transcription
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1. DNA unwinds
 Enzymes help split apart base pairs to open
the DNA double helix.
 Transcription complex: RNA polymerase and
proteins
 Uses one strand as a template
2. Bases line up
 Free nucleotides in the cell find their
complementary base pairs on the DNA
template strand
 What will be different??
Steps of Transcription
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3. RNA formed
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The growing RNA strand hands greely as it is
transcribed.
DNA strand must close back up
4. The completed RNA strand separates
from the DNA template
Steps of Transcription
http://www.stolaf.edu/people/giannini/flash
animat/molgenetics/transcription.swf
Try It!
What will the RNA Strand be?
TACGCATGGCATGCAA
Try It!
What will the RNA Strand be?
TACGCATGGCATGCA
AUGCGUACCGUACGU
Think, Pair, Share
1. What is the purpose of Protein Synthesis?
2. List 2 differences between DNA and RNA
3. Where in the cell does the process of transcription occur?
4. What enzyme completes the process of transcription?
5. What molecule is produced from transcription?
6. Where does this molecule go after it is made?
Introns and Exons

The DNA that is copied
contains both…

Introns: sequences of
nucleotides not involved in
coding for proteins.
AND
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Exons: DNA sequences
that code for proteins
Objectives
By the end of the day today you should:
1.
2.
Know the steps of translation
Identify the differences in transcription
and translation
Translation
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Now that the mRNA has been made, the
next step is for its’ message to be made
into a protein
Translation: the mRNA, with the help of the
ribosome, forms a chain of amino acids
(eventually forming a protein) based on the
information contained on the mRNA.

Occurs in the cytoplasm on the ribosome
Proteins
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Proteins are made of amino acids
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20 different amino acids total
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Adult humans can only make 12 out of the
24 amino acids
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How do we attain the other 8 amino acids?
Translation
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One Problem
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There are 20 amino acids
There are 4 RNA bases
How can just four nucleotides—A, U, C,
G—be translated into so many different
amino acids?
Language of RNA
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Just like the alphabet, 26 letters can make more than 26
words
 Letters of RNA are put together in different ways.
Codon: three nucleotide sequence which codes for the
insertion of a unique amino acid
 Language of the genetic code
 Multiple codons for the same amino acid
Anticodon: on one end of tRNA complementary to a specific
mRNA codon
 tRNA molecules carry different amino acids
Genetic Code
• Start Codon – signals the
start of translation (AUGMethionine)
• Stop codon – signal the
end of the amino acid
chain (3 total)
Translation
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Before translation begins, a ribosome will be assembled
from two ribosomal subunits (large and small). The
ribosome contains three attachment sites for tRNA
molecules.
 On the large subunit: E site, A site and P site
 On the small subunit: mRNA joins
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Step 1: mRNA attaches to the small ribosomal unit. The
large subunit attaches to the small subunit. The first codon
is aligned at the P site.
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Step 2: A tRNA carrying the amino acid
methionine attaches to the start codon,
AUG, on the mRNA. This happens at the P
site on the large ribosome
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Step 3: Attachment of the first amino acid
carrying tRNA to A binding site.
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Step 4: The ribosome forms a peptide
bond between the two amino acids
and breaks the bond between the first
tRNA and its amino acid.
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Step 5: The ribosome pulls the mRNA
the length of one codon. The tRNA and
polypeptide chain move to the P site. The
first tRNA is shifted to the E site.
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Step 6: tRNA is ejected from the E
binding site and returns to the cytoplasm.
The A site is now open for another tRNA.
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Step 7: This process continues to translate the
mRNA strand until it reaches a stop codon. The
protein is released and the large and small ribosome
disassemble.
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st. olaf transcription animation
whfreeman animation
http://www.stolaf.edu/people/giannini/flas
hanimat/molgenetics/translation.swf
Mutations
Objectives
What are different types of mutations
that can occur during protein
synthesis?
Mutations
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Mutations are changes in the DNA
sequence
1.
DNA damage from environmental agents:
1.
2.
2.
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UV Light
Chemicals
Mistakes that occur when DNA is copied
before cell division
Point Mutation: a change in a single
base pair of a gene
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No Mutation:
 DNA Sequence:
TACAAG
 mRNA Sequence: AUGUUC
 Amino Acid Chain: Met – Phen
Point Mutation:
 DNA Sequence:
TACATG
 mRNA Sequence: AUGUAC
 Amino Acid Chain: Met – Tyr
Other Types of Mutations
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Substitution
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change a codon to one that
encodes the same amino acid and
causes no change in the protein
produced.
change a codon to one that
encodes a different amino acid
and cause a small change in
the protein produced
Other Types of Mutations
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Insertions
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mutations in which extra base pairs are
inserted into a new place in the DNA
Other Types of Mutations
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Deletion
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mutations in which a section of DNA is lost,
or deleted.