Transcript A closer look at Transcription and Translation

Transcription and
Translation:
A closer look
Biology 12
Joke of the day:
Review: Complete the following table
Transcription
Location
Template
(What is read)
Purpose
Outcome (End result)
Translation
Transcription
Transcription
Location
Nucleus
Template
(What is read)
DNA
Purpose
To change DNA into a form that
can make a protein
Outcome
(End result)
Messenger RNA
(mRNA)
Translation
Translation
Location
Cytoplasm (by ribosome)
Template
(What is read)
mRNA
Purpose
Amino acids assembled in particular order to make a
protein
Outcome
(End result)
Protein (polypeptide)
Transcription:
 Read pages 242-244 and make your own summary notes for
the four stages of transcription
 Make notes for EACH about:




Initiation (how it starts)
Elongation (how it is built)
Termination (how it ends)
Post-transcriptional modification
Transcription
Initiation:
 Adjacent (upstream) to the start of the gene is a promoter
region where the RNA polymerase enzyme binds to the DNA

The promoter region is rich in A and T and is a recognition site for
the RNA polymerase enzyme
 The DNA is unwound and the double helix is separated
Transcription
Elongation
 The RNA polymerase enzyme starts building the single stranded mRNA in the direction of 5’ to 3’ using one strand of
DNA as a template strand
Termination
 A terminator sequence at the end of the gene tells the
enzyme when to stop transcribing.
 When the RNA polymerase enzyme reaches the terminator
sequence it stops transcribing
 The mRNA is separated from the DNA , the polymerase falls
of f the DNA molecule and the DNA double helix reforms.
Transcription
Transcription
Posttranscriptional modifications
 Only in eukaryotes
 Before it can be used for translation the mRNA must be
modified
 A 5’ cap is added to the mRNA to protect it from digestion in
the cytoplasm and provide a starting point for translation
 At the 3’ end about 200 adenine ribonuclieotides are added
(the poly - A tail)
 This mRNA also contains regions which code for proteins
(exons) and regions which do not (introns). The introns must
be removed before the mRNA is ready.
 Spliceosomes remove the introns and stick the exons
together.
Introns and Exons:
 Exons: segments of DNA that code for part of a specifc protein
 Introns: non-coding region of a gene
Transcription Videos:
 http://www.youtube.com/watch?v=ztPkv7wc3yU
 Introns, etc.
 http://www.youtube.com/watch?v=aVgwr0QpYNE
Translation:
 Review: It is the synthesis of a protein from mRNA
 Read pages 250-253 and make your own summary notes the
process of translation
Start and Stop Codons
Start Codon
 Begins translation
 Signals the ribosome to start translation
 AUG (universal start codon)
 AUG also Codes for Met
 Sometimes GUG or UUG
Stop Codon
 Ends translation
 Signals to the ribosome to stop translation
 UGA, UAA, UAG
Translation
Stages:
 The ribosome units bind to the 5’ end of the mRNA strand
sandwiching the mRNA between them.
 Translation does not begin until the ribosome reaches the
start codon AUG which codes for the amino acid Met.
 A tRNA molecule approaches
carrying the Met (tRNA will have
the anticodon UAC)
 The UAC anticodon and the
codon AUG match up and
join together.
Translation cont’d
 The ribosome moves along the strand exposing the next mRNA
codon and binding the appropriate tRNA with amino acid.
 A peptide bond is formed between the two amino acids
beginning the polypeptide chain.
 The ribosome continues along the mRNA strand and the
“empty” tRNA is released to go a fetch another amino acid.
Translation con’d
 The process of elongation continues until the ribosome
reaches a stop codon
 There are no tRNA molecules to match these stop codons, so
translation stops.
 A release factor protein dismantles the ribosome -mRNA
complex and the completed polypeptide chain breaks away
from the mRNA.
 The protein is then folded and modified and delivered to the
area of the cell where it is needed.
Translation video:
 http://www.youtube.com/watch?v=Ikq9AcBcohA
Proteins Folding
Example:
 DNA template:
3’ TAC ACA CGG AAT GGG TAA AAA ACT 5’
 Complimentary DNA
 Read from DNA template (start reading at 3’)
 mRNA codon
 Read from DNA template (start reading at 3’)
 tRNA anticodon
 Read from mRNA
 Amino Acids (protein)
 Read from mRNA
To do:
 Page 241
 Questions: 5, 6, 9, 10, 11
 Page 249
 Questions: 3, 4, 8
 Page 254
 Questions: 1 (b, c), 3, 4, 6