Download 13 Transcription and translation

Gene Expression
Transcription/Translation
Protein Synthesis
1. Describe how genetic information is transcribed into sequences of bases in
RNA molecules and is finally translated into sequences of amino acids in proteins
2. Explain how restriction enzymes cut DNA molecules into smaller fragments
and how ligases reassemble them
3. perform simulations to demonstrate the replication of DNA and the
transcription and translation of its information
Amino Acids
● Frederick Sanger 1952: - proteins consist of sequence of molecules called amino
acids
- specific sequence of amino acids determines chemical
properties of each protein
- proteins produced by cell determine structure, function &
development of cell
● Sequence of base pairs in DNA makes up genetic code of
organism
● Genetic code determines how amino acids strung together
(how proteins are made)
Genetic Code
● In a gene: each set of 3 bases is a CODON
● Genetic code always interpreted in terms of mRNA codon
rather than original DNA sequence
● Each mRNA codon codes for a specific amino acid
● Only 20 amino acids found in proteins, depend on
combination of bases in codon
- 4 bases, 3 in codon = 64 possible different combinations
for the 20 different amino acids
- therefore some amino acids have more than one codon
● Start and stop codons initiate or terminate protein synthesis
Ex)
AUGCUUAAAGCCUGA
Gene Expression
● Way information in a gene is converted into a specific trait
●
●
●
●
through production of a polypeptide (protein)
- products of all genes = polypeptides
RNA utilized to convert genes into proteins
- messenger RNA (mRNA)
- transfer RNA (tRNA)
- ribosomal RNA (rRNA)
2 stages of gene expression: transcription and translation
Transcription: genetic information converted from DNA
sequence into mRNA, carries information from nucleus to
cytoplasm
Translation: genetic information from mRNA used to
synthesize polypeptide chains of specific amino acids
Gene Expression
● Central Dogma: 2 step process of transferring genetic
information from DNA to RNA (transcription), then
from RNA to protein (translation)
Protein Synthesis Rap
Transcription Overview
● DNA sequence copied (transcribed) into the sequence of a
single stranded mRNA molecule
● THREE PROCESSES:
1. Initiation: RNA polymerase binds to DNA at specific
site near beginning of gene
2. Elongation: RNA polymerase uses DNA as template to
build mRNA molecule
3. Termination: RNA polymerase passes the end of gene
and stops
● mRNA then released from template strand
● Carried through nuclear pores, into cytoplasm of cell
Transcription – Initiation
● Only one strand of the double-stranded DNA is transcribed
for each gene
- template strand: one that is transcribed
- either strand can serve as the template strand for different
genes
● RNA polymerase binds to segment of DNA
- opens double helix
- binds in front of gene to be transcribed, region called the
promoter
● Promoter sequence: contains A & T bases, recognition site for
RNA polymerase
Transcription - Elongation
● DNA strand to be transcribed =
template
● RNA polymerase moves along
template strand of DNA and
begins building mRNA in 5’ to
3’ direction
● Promoter not transcribed
● mRNA strand complementary
to DNA template strand
EXCEPT thymine is replaced
with URACIL
Transcription - Termination
● RNA polymerase reaches end of gene
● Termination sequence: RNA polymerase recognizes it as
end
● Transcription stops, mRNA disconnects from DNA
template strand
● RNA polymerase free to bind to another promoter region
- transcribe another gene
● mRNA free and diffuses out of nucleus into cytoplasm
Review
Transcription
Translation Overview
● mRNA now in the cytoplasm needs to
be translated - puts amino acids together according
to genetic information and creates
proteins
● Ribosomes: site of translation
- composed of 2 subunits: clamp
together around mRNA, moves along
mRNA
● THREE PROCESSES:
1. Initiation
2. Elongation
3. Termination
Translation - Initiation
● Occurs when ribosome recognizes specific sequence on mRNA
●
●
●
●
– binds to it
Ribosome moves along mRNA in 5’ to 3’ direction
- adds amino acids to polypeptide chain once it reads a codon
Must start reading in correct spot on mRNA
- START codon (AUG)
- ensures ribosome translates code using reading frame of
mRNA molecule
- results in correct sequence of amino acids
Transfer RNA (tRNA) carries amino acids back to ribosome
structure of tRNA contains an ANTICODON
- complementary to codon of mRNA
- tells tRNA which amino acid to bind to
- amino acid binds to opposite end
- tRNA delivers amino acid to ribosome
tRNA Structure
Translation - Elongation
● Start codon recognized by ribosome
●
●
●
●
●
●
- codes for methionine
2 sites for tRNA to attach
- A (aminoacyl)
- P (peptidyl)
tRNA with anticodon complementary to start codon enters P
site
Next tRNA carrying required amino acid enters the A site
- peptide bond formed between both amino acids
Ribosome shifts over one codon so that SECOND tRNA is
now in P site
Released first tRNA from P site and allowed THIRD tRNA to
enter empty A site
- like a tickertape!
Released tRNA’s recycled back into cytoplasm
Translation - Termination
● Ribosome reaches one of THREE STOP CODONS
●
●
●
●
●
- UGA, UAG, UAA
Do not code for an amino acid, no corresponding tRNA’s
Protein release factor recognizes that ribosome has stalled
and helps release polypeptide chain from ribosome
TERMINATION.
Translation
Transcribe & Translate!