Features of the triplet codons

1. there is a specific a.a. for each condon.

2. several different codons code for the same a.a. (redundant or degenerate)

3.AUG start codon-Methionine

4. UAG, UAA, UGA stop codon (cause peptidyltransferase to add water and releast peptidyl from t-RNA)

5. Code is not overlapping, the first tRNA binds to mRNA sets the reading frame.

Point mutation: is change of a single nucleotide

1. Missense Mutation: subtitution with a different nucleotide

2. Nonsense Mutation: subtitution results in a premature stop codon

3. Frame Shift Mutation: insertion or deletion causes a shift in the reading frame.

What is the direction in which the mRNA is read during translation?

1. Charging Amino Acids

2. Initiation

3. Steps in Elongation

4. Termination

5. Posttranslational protein Processing

charging Amino Acids: linking amino acids to their specific tRNA using aminoacyl-tRNA synthetase.

Initiation: Initiator tRNA (methionine) binds to the small subunit of ribozyme that then binds to the 5' end of the mRNA (signaled by the cap). The small subunit moves along the mRNA until they find the start codon AUG. The charged initiator tRNA is set in the p-site.

step 1: Initator tRNA binds to P site. next charged tRNA binds to A site.

step 2: uncoupling of peptide from tRNA in the P site and joined to the free amino group of the amino acid on the tRNA in the A-site. This is catalyzed by the RNA-based peptidyltransferase (part of 23s ribosomal RNA)

step 3 and 4: Large subunit translocate (shifts) so that the peptidyl-tRNA is in the P-site, and he uncharged tRNA is in the E-site. (catalyzed by energy relased from GTP hydrolysis)

Histone Modifications:

Histone acetylation: control gene expression. Histone acetylases (HATs) catalytically add acetyl groups to some lysines of exposed histone tails.Histone acetylation is associated with transcription ACTIVATION.

Histone deacetylases: remove acetyl groups and lead to transcription repression.

Histone Methylation: is done by different methyltransferases. The main methylated base in DNA is 5-methyl-cytosine. methylation of DNA usually DECREASE gene expression

Chromatin remodeling reversibly changes local chromatin structure and allows proteins access to the DNA.

ATP driven remodeling complexes have many subunit include ATPase. ATP hydrolysis loosens the nucleosomal DNA. many rounds of nucleosome sliding decondenses the chromatin allowing proteins to bind to DNA.

enhancers, when bound to transcription regulators proteins greatly increase the rate of transcription.

usually locate upstreatm from the promoter and can turn genes on or off.

these regulator bind to regulatory DNA sequences. they also interact with other proteins to activate transcription.

regulatory DNA sequences and regulators act as switches to turn on transcription.

regulators usually have binding motif that insert itself into DNA major groove so that helix is not disrupted. some example: zinc finger, leucine zipper

transcription regulator (activator) bound to the DNA making contact with the transcription initiation complex at the promoter. Looping of the DNA allows the regulatory sequences with their associated transcription regulators to physically interact with the transcription initiation complex.

study of inherited changes in phenotype or gene expression caused by mechanisms that dont involve the underluing DNA sequences.

epigenetic change may be inherited in sperm or egg . Change to DNA that do not alter sequence but chemically modify it which may have implication for fuction and transcription rates.