Biology 12 Transcription and Translation: A closer look

Joke of the day:

Review: Complete the following table TranscriptionTranslation Location Template (What is read) Purpose Outcome (End result)

Transcription LocationNucleus Template (What is read) DNA Purpose To change DNA into a form that can make a protein Outcome (End result) Messenger RNA (mRNA)

Translation LocationCytoplasm (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 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

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:   Introns, etc. 

Translation:  Review: It is the synthesis of a protein from mRNA  Read pages 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: 

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