Transcription & Translation. Part of Protein Synthesis

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Catherine Lang
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1 Transcription & Translation Part of Protein Synthesis

2 Three processes Initiation Transcription Elongation Termination

3 Initiation The RNA polymerase binds to the DNA molecule upstream of the gene at the promoter site Promoter is a characteristic base-pair sequence normally rich in A and T. Promoter sites differ from species to species and between prokaryotes and eukaryotes RNA polymerase only recognizes the promoter region and therefore can only bind upstream of the gene-coding region RNA requires assistance Prokaryotes: sigma factors Eukaryotes: transcription factors

4 Elongation RNA polymerase bind to the promoter and opens the double helix (this is done by DNA helicase in replication) Builds the single-stranded mrna in 5 to 3 direction Doesn t require a primer to begin Elongation occurs as soon as RNA polymerase gets to beginning of gene-coding region Uses the template strand of DNA to synthesize the mrna. The strand not used for transcription is the coding strand mrna will be identical to the coding strand and complementary to the template strand

5 Termination Once the end of the gene is reached, the RNA polymerase comes across the terminator sequence The mrna detaches from the DNA template strand RNA polymerase is then free to bind to another promoter region and begin transcribing another gene In prokaryotes, protein Rho is involved in the termination process

6 Posttranscriptional Modifications In eukaryotes, modifications need to be made to the primary transcript of the mrna Capping and tailing: A Guanine cap (a 7-methyl guanosine) is added to the 5 end Protects mrna from digestion by enzymes as it leaves the nucleus into the cytoplasm Aids in initiation of translation String of 200+ adenine ribonucleotides are added to the 3 end by enzyme poly-a polymerase. This is called the poly-a tail Protects mrna from degradation Facilitates transport of mrna to cytoplasm for translation

Posttranscriptional Modifications Introns are non-coding regions of eukaryotes and could lead to improper protein folding (and render the protein nonfunctional)

7 Posttranscriptional Modifications Introns are non-coding regions of eukaryotes and could lead to improper protein folding (and render the protein nonfunctional) if translated Spliceosomes remove the introns in a process called RNA splicing and join the remaining exons so all the coding regions are continuous in the mrna

8 After Posttranscriptional Modifications The mrna molecule is now ready to leave the nucleus Molecule is now called the mrna transcript No quality control enzyme to ensure the transcription is correct therefore more errors are made compared to during replication. However, a gene is transcribed repeatedly to produce so an error is not as detrimental. An incorrect protein is susceptible to degradation

9 Video of Transcription Transcription Video

10 Translation Three processes Initiation Elongation Termination

11 Initiation The ribosomes bind to the mrna by recognizing the 5 cap. This is the initiation process of translation mrna transcript provides the specification for the amino acid sequences from the gene In eukaryotes, the ribosome consists of 2 subunits: large subunit and small subunit. These clamp together with the mrna between

12 Elongation Ribosome moves along the mrna in 5 to 3 direction adding new amino acids to the polypeptide chain each time it reads a codon Ribosomes synthesize different proteins by associating with different mrnas and reading the codons Reading frame is the phase in which the triplets of nucleotides in the mrna are read

13 Elongation The ribosome cannot synthesize the amino acids therefore they are delivered by trna One arm of the trna has a sequence of 3 bases (the anticodon) which recognize the codon of the mrna as complementary

14 Elongation The opposite arm of the trna, called the acceptor site, carries the corresponding amino acid. This makes the trna called an aminoacyltrna. The enzymes responsible for adding the appropriate amino acids are aminoacyl-trna synthetases. There are 20+ of these enzymes, each specific for a particular amino acid and trna

15 Elongation Every trna can only carry 1 amino acid specific to the sequence of the anticodon Because more than 1 codon can code for an amino acid, if only the 3 rd base is different, the trna can still bind. This can correct for some errors in the transcription process E.g. UAU and UAC both code for tyrosine. If the trna s anticodon is AUA it can still bind to the codon UAC

16 Elongation The start codon AUG ensures the correct reading frame is used. Since AUG codes for Met, every protein initially starts with the amino acid Met The ribosome has 2 sites for trna: the A (acceptor) site and the P (peptide) site the trna that carries Met enters the P site. The next trna enters the A site. The amino acids bind together with peptide bonds as the ribosome shifts over each codon. The 3 rd trna enters the A site while the 2 nd amino acid moves to the P site.

17 Elongation As elongation occurs, the polypeptide chain trains in the cytoplasm The trnas that have been released (exited from the P site) are recycled by aminoacyl-trna synthetase which adds an amino acid to them again

18 Termination Once the ribosome reaches a stop codon, for which there are no amino acids, a protein called the release factor aids in the release of the polypeptide chain from the ribosome The two sub-units of the ribosome fall off the mrna and translation ceases

19 Video of Translation Translation Video

20 Homework Transcription Chapter 5.3 Pg 249 Questions 3, 4, 5, 8, 9, 12 Translation Chapter 5.4 Pg 254 Questions 3 & 4

DNA Replication & Protein Synthesis. This isn t a baaaaaaaddd chapter!!!

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