Transcript Ribosome - Mrs. J. Malito

From Gene To
Protein
Transcription
&
Translation
Review
The Central Dogma
of Molecular Biology
Cell
DNA
Transcription
Translation
mRNA
Ribosome
Polypeptide
(protein)
Transcription (DNA –> RNA)
•
3 steps:
1) Initiation 2) Elongation 3)Termination
Initiation
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DNA uncoils and separates the hydrogen
bonds between the nitrogen bases.
Only one side of DNA (the leading strand) is
transcribed.
RNA polymerase recognizes a specific base
sequence in the DNA called a promoter and
binds to it.
The promoter identifies the start of a gene,
which strand is to be copied, and the direction
that it is to be copied.
Uses initiation site and recognition sequences.
Transcription Initiation Complex (Factors)
Initiation
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Elongation
RNA polymerase moves along the DNA
template.
Complementary RNA nucleotides are added
to the 3’ end of the growing chain.
Base Pairing: Adenine on DNA directs the
bonding with the new pyrimidine Uracil
Base Pairing: Cytosine bonds with Guanine
New mRNA breaks away from the leading
DNA strand and the two original DNA
strands rejoin and wind back up.
Termination
Termination
• Transcription ends when RNA
polymerase reads a DNA sequence
called a terminator.
• Once the mRNA is made, it is almost
ready to carry the genetic
information from the nucleus to the
ribosomes for protein synthesis.
Transcription
Three types of RNA:
1) mRNA (messenger)
2) tRNA (transfer)
3) rRNA (ribosomal)
Functions of RNA
1) mRNA: directs the amino acid
sequence of polypeptides synthesized
at ribosomes
2) tRNA: carries amino acids to
proper locations within the growing
polypeptide chain
3) rRNA: combines with proteins to
form ribosomes that serve as the site
of protein synthesis
RNA Modifications
• After transcription, mRNA has
to be modified before leaving
the nucleus by:
1) covalent modifications of the
3’ and 5’ ends of pre-mRNA.
2) removal of intervening
sequences
Covalent Modifications
• A 5’ cap is added to protect the mRNA
from degradation and to help small
ribosomal subunits recognize the
attachment site on mRNA’s 5’ end.
• A poly-A tail is sequence of 30 – 200 A
nucleotides added to the 3’ end of mRNA
before it exits to:
• Prevent degradation
• Facilitate attachment
• Regulate protein synthesis
Removing Unnecessary
Sequences
• Sequences of nucleotides that code for
genes are interrupted by noncoding
segments of DNA called introns.
• The introns are excised from RNA
before it leaves the nucleus and
translation begins.
• Not all genes have introns.
• Exons – coding sequences of a genes that
are transcribed and expressed.
Removing Unnecessary
Sequences
RNA Splicing
•
Process that removes introns and
joins exons from pre-mRNA to create
mature mRNA that will move into the
cytoplasm from the nucleus.
•
Catalyzed by enzymes
Role of the Ribosomes
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Ribosomes coordinate the pairing of the codons
and the anticodons.
Ribosomes are made up of two subunits: one
large, one small.
Each subunit is made of proteins and rRNA.
There are 3 binding sites for mRNA:
1) P site – holds the tRNA that carries the
growing chain.
2) A site – holds the tRNA that carries the
amino acid to be added.
3) E site – exit site.
Ribosomes
Translation
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•
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The synthesis of a polypeptide
(protein), under the direction of
mRNA.
mRNA —> protein
3 Steps:
1) Initiation
2) Elongation
3) Termination
©1998 Timothy G. Standish
Translation – Initiation
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•
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Small ribosomal subunit binds to
mRNA and tRNA.
Large ribosomal subunit binds to the
small one to form a translation
complex.
Initiation factors (proteins) are
required.
©1998 Timothy G. Standish
Translation – Initiation
fMet
Large
subunit
E
• mRNA, a tRNA with the first
amino acid (met), and the two
ribosomal subunits come
together.
P
A
UAC
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
3’
Small mRNA
subunit
©1998 Timothy G. Standish
Translation – Elongation
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•
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Amino acids are added one by one to the
growing polypeptide chain.
Uses proteins called elongation factors.
3 Steps:
1) Codon recognition – mRNA codon hydrogen
bonds with the tRNA anticodon.
2) Peptide Bond formation – Amino acids are
joined together by peptide bonds to create
the polypeptide chain.
3) Translocation – the tRNA moves from the A
site to the P site and finally to the exit site,
from which it exits the ribosome.
©1998 Timothy G. Standish
Translation – Elongation
Polypeptide
Arg
Met
Phe
Leu
Ser
Aminoacyl tRNA
Gly
Ribosome
E
P
A
CCA
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
3’
mRNA
©1998 Timothy G. Standish
Translation - Elongation
Polypeptide
Met
Phe
Leu
Ser
Gly
Arg
Aminoacyl tRNA
Ribosome
E
P
A
CCA UCU
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
3’
mRNA
©1998 Timothy G. Standish
Translation - Elongation
Polypeptide
Met
Phe
Leu
Ser
Gly
Arg
Ribosome
E
P
A
CCA UCU
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
3’
mRNA
©1998 Timothy G. Standish
Translation - Elongation
Polypeptide
Met
Phe
Leu
Ala
Ser
Gly
Aminoacyl tRNA
Arg
Ribosome
E
P
A
CCA
UCU
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
3’
mRNA
©1998 Timothy G. Standish
Translation – Elongation
Polypeptide
Met
Phe
Leu
Ser
Gly
Arg
Ribosome
E
Ala
P
A
UCU CGA
5’GAG...CU-AUG--UUC--CUU--AGU--GGU--AGA--GCU--GUA--UGA-AT GCA...TAAAAAA
3’
mRNA
©1998 Timothy G. Standish
Translation - Termination
• A stop codon in the mRNA is
reached and translation stops.
• UAA, UAG, UGA
• A protein release factor binds to
the stop codon and the
polypeptide chain is released
from the ribosome.
©1998 Timothy G. Standish
Creation of Protein
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Polypeptides will then fold to
assume their specific
conformation.
Remember primary, secondary,
tertiary, and quaternary
structures!
©1998 Timothy G. Standish
Problem 1
Transcribe and translate the
following DNA sequence:
3’AATAGTACCGCAAATTTATCGCTT5’
5’UUAUCAUGGCGUUUAAAUAGCGAA3’
5’UUAUC,AUG,GCG,UUU,AAA,UAG,CGAA3’
Met--Ala--Phe--Lys--Stop
©1998 Timothy G. Standish