Regulating Gene Expression and Protein Synthesis

Transcription Initiation

Promoter = DNA sequence that can be recognised by transcriptionn factors.

TATA Box = Thymine-Adenine- Thymine-Adenine - this sequence is present in promoters of over 25% of eukaryotic genes.

Transcription factors = proteins that bind to promoter region on DNA. Their names begin with TF.

TFIID is a multi-protein complex comprising

• TATA Binding protein (TBP) - saddle shaped protein

• TAFS (TBP associated factors

  

• Distorts the promoter DNA - introduces almost 90 degree bend

Initiation complex = TF bound to promotor. RNA becomes a part of this.

RNA polymerase helps to unzip the double-stranded DNA, exposing 2 strands, forming a transcription bubble.

The 2 strands:

• + strand/sense/coding strand - transcript (RNA sequence) will resemble its code.

• -/antisense/template strand - transcript will be complemenary to.

Anything after the promotor region and closer to the 5 prime end (downstream as DNA is transcribed 3’ to 5’ and RNA produced will be 5’ to 3’) is the coding region (what will actually be transcribed into the RNA).

There are factors upstream that allow control of level of transcription

Activator and repressor regions = close to promoter. Something that facilitates the formation of the initiation complex will bind there (like a protein).

Binds to Activator regions = Initiation complex more likely to form

Binds to Repressor region = less likelty to form

Enhancer region = DNA sequence that are specifically associated with gene promoters r can be a considerable distance from promoter.

Proteins can bind to this which facilitate the formation of initiation complex = increases likelihood of it and process of transcription beginning.

Transcriptional Activators

They are modular = made up of atleast 2 distinct domains

• DNA binding doman (DBD) - attaches to specific DNA sequences such as enhancers or promoters

• Activation domain - accelerates rate of transcription factors by interacting with initiation complex

Summary: Protiens and gene expression need to be regulated in a cell type specific manner. Transcription is regulated by a combination of regulatory DNA sequences and regulatory proteins (transcription factors).

Ribosome Assembly

Made of a large subunit (60s) and small subunit (40s) to make an 80s ribosome.

Made of rRNA and proteins necessary for protein synthesis.

60s = 5s, 28s and 5.8s rRNA + 49 proteins.

40s = 18s rRNA and 33 proteins.

tRNA

tRNA is the adaptor molecule for protein synthesis

Aminoacyl tRNA synthetases catalyse the esterification reaction between amino acid and tRNA.

3’ end is the longer end

The distince funcitonal regions of tRNA are the codon and anticodon.

Ribosome Function

The ribosome has an A-site (aminoacyl site) where aminoacyl-tRNA binds. Aminoacyl-tRNA carries a single amino acid attached to its 3'-end.

Peptidyl-tRNA holds the growing polypeptide chain and occupies the P-site (peptidyl site) of the ribosome.

During translation, the ribosome catalyzes the formation of a peptide bond between the amino acid attached to the aminoacyl-tRNA in the A-site and the polypeptide chain held by the peptidyl-tRNA in the P-site.

• After the peptide bond is formed, the ribosome shifts (translocates) along the mRNA by one codon.

• The now uncharged (deacylated) tRNA in the P-site moves to the E-site (exit site) and exits the ribosome.

• The aminoacyl-tRNA, which now carries the polypeptide chain, moves from the A-site to the P-site, leaving the A-site free to accept the next aminoacyl-tRNA.