Question

Uluruunu us RJ15 1. Draw or describe the process of eukaryotic transcription and translation, using the following terms as needed (not all terms will be used): sigma factor, RNA polymerase, DNA polymerase, origin of replication, ribosome, start codon, transcriptional start site, stop codon, nucleus, -10 and -35 sequences, TATA box, TBP, inducer, transcriptional stop site, Shine-Delgrano sequence, Kozak sequence, RNA splicing. 2. Draw or describe the process of prokaryotic/eubacterial transcription and translation, using as many of the terms above as you need.

Answer 1

Ans: Eukaryotic transcription: There are three types of RNA polymerase enzymes named I, II, III are present in eukaryotes.

RNA polymerase I for preribosomal RNA

RNA polymerase II for mRNA

RNA polymerase III for tRNA and 5s RNA

Eukaryotic transcription can be divided into 3 steps: initiation, elongation, termination

Initiation:

1. Eukaryotic RNA polymerase requires transcription factors to initiate the transcription. In my next explanation TF stands for a transcription factor.
2. TF II D binds to the TATA box that is the AT-rich sequence present at the promotor
3. Now TBP (TATA-binding protein) will bind to the TATA box this will cause bending of DNA that will help other transcription factor binding.
4. TFII-A and TF II- B then binds to either side of the TBP.
5. Now RNA polymerase will bind to the promotor with the help of TFII F and thus pre-initiation complex is formed.
6. This is followed by the binding of TFII E and TFII H. TFII H has ATPase and Kinase activity that unwinds the DNA strand to create a preinitiation complex.
7. The transcription start site is the start point of transcription designated as +1.
8. Now RNA polymerase leaves the promotor and elongation will start.

Elongation:

1. Now 2 elongation factors TF Eb and TFII S binds to RNA polymerase and mRNA synthesis begins in 5' to 3' direction.

Termination:

1. Once RNA Polymerase will reach to the end of the gene termination will occur at the transcription stop site.
2. Here RNA polymerase will bind with 2 proteins

(a) cst F (cleavage stimulation factor) that cuts the mRNA.

  (b) CPSF (cleavage and polyadenylation specificity factor) that adds polyA tail at the 3' end of mRNA with the help of PolyA polymerase.

The mRNA thus formed is the pre-mRNA. RNA splicing is done to remove the introns (non-coding regions) and join the exons (coding regions). Now, this mRNA comes out from the nucleus into the cytoplasm.

Eukaryotic translation: Translation requires a ribosome that is made up of rRNA and protein. Eukaryotic ribosome is divided into 2 subunits known as the 60s (large subunit) and 40s (small subunit) subunits.

mRNA brings the message from the nucleus hence called messenger m-RNA and tRNA brings amino acids as per the information present on mRNA.

Steps of translation: Initiation:

1. The small subunit of ribosome binds with the methionyl tRNA which then binds to the 5'cap of the mRNA.
2. Now, this complex scans mRNA for the start codon that is AUG. In eukaryotes, the AUG codon is present in the Kozak sequence. Various initiation factors help in this process.
3. Once the start codon is scanned large subunit of ribosome binds to this complex and now initiation factors will be released.

Elongation:

1. There are 3 sites present on the ribosome:

A site that is the entry site for the incoming tRNA with the bounded amino acid.

P site will have the growing polypeptide chain.

E site also referred to as an exit site empty tRNA will leave the ribosome from this site.

2. mRNA has a codon that binds with complementary anticodon present on the tRNA. In this way tRNA with amino acid enters the A site and the peptide bond is formed between 2 adjacent amino acids.

3. empty tRNA will leave from exit site and ribosome moves one triplet ahead.

Termination: termination will occur at the stop codon which is present on the mRNA.

polypeptide chain will be released.