THE CENTRAL DOGMA OF BIOLOGY STATES

THAT DNA CODES FOR RNA, AND RNA CODES

FOR PROTEINS

When DNA codes for RNA, the process is called TRANSCRIPTION. When RNA codes for proteins, the process is called TRANSLATION.

THERE ARE 3 TYPES OF RNA, HOWEVER ONLY MRNA IS INVOLVED IN TRANSCRIPTION

Name mRNA rRNA tRNA

Function Carries genetic information from DNA during TRANSCRIPTION in the nucleus to direct protein synthesis

Associates with protein to form the ribosome

Transports amino acids to ribosomes during TRANSLATION.

Example

What is the purpose of Transcription?

Where does transcription take place?

LET’S WATCH TRANSCRIPTION BEFORE WE EXAMINE THE

STEPS! http://highered.mcgraw-hill.com/sites/007250

7470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html

TRANSCRIPTION STAGE 1: INITIATION

Transcription begins when RNA Polymerase attaches to a PROMOTER on the DNA strand

The promoter determines which of the two DNA strands will serve as the template strand.

The promoter indicates where RNA Polymerase can attach to the DNA strand

A common promoter on the DNA sequence is called the TATA Box. This is around 25 nucleotides from where transcription begins.

Once RNA Polymerase attaches to the promoter, it unwinds the DNA template strand.

RNA synthesis begins moving along the DNA template strand and RNA begins transcribing the DNA template strand. The new strand is created in the 5’ to 3’ direction.

What would RNA Polymerase transcribe from the following promoter sequence?

A C A G G C A T A T A

TRANSCRIPTION STAGE 2: ELONGATION RNA Polymerase

moves along the DNA strand and continues to unwind the helix.

Polymerase reads the strand and transcribes a complementary mRNA strand.

As polymerases passes over the strand the mRNA peels away and the DNA helix re-forms.

STAGE 3 TRANSCRIPTION : TERMINATION

Transcription proceeds until after the RNA polymerase transcribes a terminator sequence in DNA.

Shortly past the termination signal, the mRNA is cut from the Polymerase.

How might transcription take place if your body required a large

amount of mRNA to be made from a particular gene?

THE MRNA SEQUENCE IS MODIFIED BEFORE IT LEAVES THE NUCLEUS TO

ENTER THE CYTOPLASM

The 5’ end is capped with a modified guanine nucleotide.Reduces

degradation

The 3’ end received 50-250 adenine nucleotides . Reduces

degradation Aids the mRNA in

exiting the nucleus

A 5’ CAP is added

A POLY A tail is added

RNA IS SPLICED BEFORE IT LEAVES THE NUCLEUS.

Introns are non-coding regions in the mRNA sequence. They are removed before the mRNA exits the nucleus.

Exons are the coding regions in the mRNA molecule. They are connected to form a complete mRNA strand.

Think: Exons exit the nucleus

INTRONS EXONS