Biology 12

Transcription and Translation: A closer look

Joke of the day:

Review: Complete the following table

Transcription Translation

Location

Template(What is read)

Purpose

Outcome (End result)

Transcription

Transcription

Location Nucleus

Template(What is read)

DNA

PurposeTo 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

PurposeAmino 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

off the DNA molecule and the DNA double helix reforms.

Transcription

Transcription

Posttranscriptional modifications Only in eukaryotesBefore it can be used for translation the mRNA must be

modifiedA 5’ cap is added to the mRNA to protect it from digestion in

the cytoplasm and provide a starting point for translationAt 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.

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 cont’d

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