Transcription

SBI 4UNovember 14th, 2012

In your notebook, respond to the following questions:

1. What is the central dogma?2. Where does translation occur in the

cell?3. Where does transcription occur in

the cell?4. In simple words, what is translation?

Transcription has 3 steps:

1. Initiation2. Elongation3. Termination

Initiation

Locates the correct spot on the original DNA template where transcription is to begin

Promoter sequence: a particular nucleotide sequence on the DNA molecule provides a binding site for RNA polymerase

Initiation

Strings of A’s and T’s RNA polymerase can only transcribe

in one direction RNA polymerase opens up a section

of the DNA to be transcribed

Elongation

Copies the correct number of nucleotides from the DNA template onto a particular type of RNA molecule called messenger RNA (mRNA)

Elongation

RNA polymerase works from 5’ to 3’ As RNA polymerase passes, DNA

helix re-forms and the mRNA strand separates from its template DNA strand

Template strand: strand used for transcription

Coding strand: strand that is not used for transcription identical to the template strand, except for ___________

Termination

Signals the right place to stop the copying process to make sure the mRNA molecule contains the complete set of instructions from the gene

Termination

Once transcription has been successfully initiated, the RNA polymerase continues along the DNA molecule until it encounters terminator sequences

After termination mRNA separates from the RNA polymerase

Prokaryotes vs. Eukaryotes

Posttranscriptional Modifications

Capping and Tailing:5’ cap added to the 5’ end of the

pre-mRNA modified G nucleotidePoly-A tail added to the 3’ end

long series of A nucleotides

Posttranscriptional Modifications

5’ cap and poly-A tail help protect mRNA from enzymes in the cytoplasm that may break it down. Also serve as signals. Poly-A tail also helps with transport of the mRNA

Posttranscriptional Modifications

mRNA Splicing:Exons: form part of the instructions

for protein synthesis Introns: intervening, non-coding

nucleotide sequences

Posttranscriptional Modifications

mRNA Splicing:Spliceosomes: Cut out the introns

and join the exons together mRNA molecule is formed and is now

ready to leave the nucleus

Homework

Page: 249 Questions 1, 2, 3, 5, 9