SC335Unit five

Prof. Christine S. Snyder, Ph.D.

CSnyder@kaplan.edu

Discussion board (DB)• One of the energy-harvesting steps of the citric acid cycle results

in the production of GTP. What is the structure of GTP?

• Except for the functional groups attached to the rings, the nitrogenous bases are largely flat, hydrophobic molecules. Explain why the arrangement of the purines and pyrimidines found in DNA molecules is very stable.

• Why is DNA-polymerase is said to be template-directed? Why is DNA replication a self-correcting process?

What is occurring in the replication fork?

Detailed View of the Replication ForkLagging strand DNA synthesis is more easily visualized hereDNA polymerase III reads: Discontinuously In the opposite direction

Transcription

• Transcription is catalyzed by RNA polymerase• Produces a copy of only 1 DNA strand • Process of transcription has 3 stages:

- Initiation binds RNA polymerase to the promoter region at the beginning of the gene

- Chain elongation then occurs forming a 3'-5' phosphodiester bond, generating a complementary copy

- Termination is the final step of transcription when the RNA polymerase releases the newly formed RNA molecule

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Other transcription factors RNA polymerase II

TATA box

Transcriptionfactor

EukaryoticDN A

What is the function of each of the classes of RNA molecules?

Classes of RNA Molecules

• Messenger RNA (mRNA)- mRNA directs the amino acid sequence of proteins - A complimentary copy of a gene - It has the codon for an amino acid in a protein

• Ribosomal RNA (rRNA)- Structural and functional component of the ribosome - Forms ribosomes by reacting with proteins- 3 types in prokaryotes- 4 types in eukaryotes

• Transfer RNA (tRNA)- Transfers amino acids to the site of protein synthesis

•Which amino acids are encoded by six codons?

Genetic Code Details

• All 64 codons have meaning- 61 code for amino acids

- Three code for the “stop” signal

• Multiple codes for an amino acid tend to have two bases in common

- CUU, CUC, CUA, CUG code for leucine

- Makes the code mutation resistant

• Codons are written in a 5' 3' sequence

The Genetic Code

20.6 Protein Synthesis

• Protein synthesis is called translation- Carried out on ribosomes, complexes of

- rRNA - Proteins

• Protein synthesis occurs in multiple places on one mRNA at a time

- mRNA plus the multiple ribosomes are called a polysome

• tRNA - Binds a specific amino acid aided by aminoacyl tRNA synthetase- Recognizes the appropriate codon on the mRNA

Schematic of the Translation Process

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Primary RNA transcript

Poly-A tail

Mature mRNACut intron

5´ cap

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1. RNA polymerase II in the nucleus copies one strand of the DNA to produce the primary transcript.

2. The primary transcript is processed by addition of a 5´ methyl-G cap, cleavage and polyadenylation of the 3´ end, and removal of introns. The mature mRNA is then exported through nuclear pores to the cytoplasm.

3. The 5´ cap of the mRNA associates with the small subunit of the ribosome. The initiator tRNA and large subunit are added to form an initiation complex.

4. The ribosome cycle begins with the growing peptide attached to the tRNA in the P site. The next charged tRNA binds to the A site with its anticodon complementary to the codon in the mRNA in this site.

5. Peptide bonds form between the amino terminus of the next amino acid and the carboxyl terminus of the growing peptide. This transfers the growing peptide to the tRNA in the A site, leaving the tRNA in the P site empty.

Primary RNA transcript

RNA polymerase II

Primary RNA transcript

Poly-A tail

Mature mRNACut intron

5´ cap

Largesubunit

mRNA

Smallsubunit Cytoplasm

tRNA arrivesin A site Amino acids

mRNA

A siteP site

E site

Lengtheningpolypeptide chain

EmptytRNA

Cytoplasm

RNA polymerase II

Primary RNA transcript5´

3´

5´

5´

3´

3´

5´

3´

5´ cap

Questions?

Image credit: Microsoft Clip Art

References

Raven, Johnson,Mason,Losos,and Singer,Biology, 9th editionMcGraw Hill.

Denniston, Topping, Caret, Introduction to Molecular Genetics,6th Edition McGraw Hill.