The Genetic Code and

Transcription

Chapter 12

Honors Genetics

Ms. Susan

Chabot

TRANSCRIPTION

• Copy SAME

language

• DNA to RNA

• Nucleic Acid

to

Nucleic Acid

TRANSLATION

• Copy DIFFERENT

language

• RNA to Amino Acid

• Nucleic Acid

to

Amino Acid/Protein

The “Central Dogma” of Molecular Genetics

Transcription• DNA

Translation• mRNA

Protein Folding

• Protein

Characteristics of the Genetic Code

• mRNA is written in linear form using DNA as a

template for synthesis.

• Each “word” in the mRNA strand is composed of

a 3-letter sequence called a CODON.

• Each CODON specifies a SINGLE Amino Acid.

• There is 1 start codon for initiation of protein

synthesis and 3 stop codons for termination of

protein synthesis for specified protein.

• A given amino acid can have more than one

codon sequence.

There are Different RNAs with Distinct Functions

Transcription makes an mRNA copy of DNA.

This mRNA copy is complementary to the gene

sequence found on one strand of DNA.

DNA directs the synthesis of RNA in the nucleus.

Transcription is a Key Step in Gene Expression

RNA Review

RNA is a nucleic acid polymer that uses a slightly different

sugar than DNA and the base uracil in place of thymine.

RNA is Single-Stranded

Transcription in Eukaryotes

• Page 252 for more detail

#1: Transcription in eukaryotes occurs in the nucleus

under the direction of 3 different forms of RNA

polymerase.

#2: Eukaryotic arrangement of DNA must be

uncoiled around histones.

#3: In addition to PROMOTERS, ENHANCERS

assist in locating correct strand for replication to

begin (cis and trans acting factors).

#4: Processing or “capping” the 5’ and 3’ ends of the

mRNA transcript upon completion.

RNA Polymerase in Eukaryotes

FORM PRODUCT LOCATION

I rRNA

Ribosomal RNA

Nucleolus

II mRNA and snRNA

Messenger RNA

Single nucleotide RNA

Nucleoplasm

III 5s RNA and tRNA

Small ribosomal RNA

Transfer RNA

Nucleoplasm

Role of RNA Polymerase

• Enzyme capable of directing the synthesis of a

mRNA copy from a strand of DNA.

• NO PRIMER is required in synthesis of mRNA

as in complementary strands of DNA during

replication.

• Locates 3’ to 5’ directionality in DNA strand so

that mRNA can be constructed in 5’ to 3’

direction - - - THE LEADING STRAND!

Promoters• Step #1 in the production of a mRNA sequence is

TEMPLATE BINDING.

• Requires recognition of specific DNA sequences

called PROMOTERS.

• PROMOTERS are recognized by RNA polymerase.

• Once the promoter is recognized, the double helix

denatures in that region = TRANSCRIPTION

START SITE.

• Promoters govern the efficiency of mRNA

production, mutations in the promoter region result

in less transcription with dire consequences.

Promoter Sequences

• TATA box = sequences rich in A and T; TATAAT

• Roughly 30 nucleotide pairs upstream from the

start of transcription.

• Additional promoter elements regulate the

efficiency of transcription in response to cell needs:

– Enhancers increase transcription levels

– Silencers decrease transcription levels

Transcription

RNA polymerase acts here

A gene

Coding Regions of Eukaryotic Genes are

Interrupted by Intervening Sequences

• Discovered in 1977

• Discovery of DNA sequences not present in the

final mRNA transcript.

– Intervening sequences – INTRONS

– Expressed sequences – EXONS

• Splicing involves removing the INTRONS and

rejoining the EXONS into a final mRNA

transcript.

Eukaryotic Genes are Segmented

In some genes, more than 90% of the pre-mRNA is

destroyed, never to appear in the mRNA.

Introns are removed from the primary transcript and

exons are spliced together to make mRNA.

The bone marrow uses several

transcription factors to control the

type of hemoglobin produced by

the cells.

These factors determine whether

the cell makes embryonic, fetal, or

adult hemoglobin

LCR – Locus Control Region

directs the promoter to attach to

the fetal loci or adult loci for

transcription.

Transcription factors (purple and

aqua shapes) then direct the

production of mRNA.

Transcription Of Hemoglobin For Blood Cell Production

ASSIGNMENT

• Case Study page 258

• Questions p. 259-260

#9, 13, 16, 18

ONLINE TEXT

Case Study page 338

Questions p. 340-341

#8, 12, 15, 17

QUIZ Review

• 3 types of RNA

• Differences between DNA and RNA

• Function of RNA Polymerase II

• Location of Transcription and Translation

• Function of Promoters, Enhancers, and

Silencers

• Leading or Lagging? 3’ – 5’