Active Lecture PowerPoint ® Presentation for Essentials of Genetics Seventh Edition Klug, Cummings, Spencer, Palladino

Chapter 12 The Genetic Code and Transcription

Copyright © 2010 Pearson Education, Inc.

Outline

• Overview of gene expression • How is genetic information encoded?

• How is information transferred from DNA to RNA • Differences between Prokaryotes & Eukaryotes • Summary (animation)

Gene Expression

DNA

Transcription

mRNA Step 1

Translation

Step 2 Protein Gene Expression

Gene Expression

Step 1 Transcription Step 2 Translation

Gene Expression

• How is genetic information encoded?

• The Genetic code • How does the information transferred from DNA to RNA?

• Transcription

The Genetic Code

• Written in linear form • Uses ribonucleotide bases that compose mRNA molecules as “ letters ” • Sequence of RNA is derived from the complementary bases in the template strand of DNA

The Genetic Code

Figure 13-7

Copyright © 2006 Pearson Prentice Hall, Inc.

The Genetic Code

• In mRNA, triplet codons specify one amino acid • Code contains “start” and “stop” signals • Code is unambiguous, degenerate, commaless, nonoverlapping, and nearly universal

The Genetic Code

• The initial amino acid incorporated into all proteins is methionine or a modified form of methionine (fmet) •

AUG

is the only codon to encode for methionine • When AUG appears internally in mRNA, an unformylated methionine is inserted into the protein

The Genetic Code

• The degenerate code: 64 codons to specify the 20 amino acids • The triplet nature of the code was revealed by

frameshift mutations

DNA Problem 1: • Following is a sequence of a nontemplate of DNA strand 5’ ATGCGAATTAGTCCGCAT 3’ Assuming that transcription begins with the first nucleotide and ends with the last, write the sequence of the transcript (mRNA) in the conventional form

DNA Problem 2: • Using the genetic code, translate the transcript (mRNA sequence) in problem 1 into amino acid sequence nontemplate template 5’ ATGCGAATTAGTCCGCAT 3’ 3’ TACGCTTAATCAGGCGTA 5’ mRNA amino acid 5’ AUGCGAAUUAGUCCGCAU 3’ . . . . . . . . . . . . . . . . . .

Effect of Frame-shift mutations

Transcription

• RNA serves as the intermediate molecule between DNA and proteins • RNA is synthesized on a DNA template during transcription • Transcription selectively copies only certain parts of the genome. Many copies of the transcript of one gene region is made

RNA Polymerase Directs RNA Synthesis •

RNA polymerase

directs the synthesis of RNA using a DNA template • No primer is required for initiation. RNA polymerase can initiate transcription

de novo

• RNA polymerase uses ribonucleotides (rATP,rCTP, rGTP & rUTP)

Transcription in

E. coli

• RNA polymerase from

E. coli

subunits 2 a , b , b ', and s contains the • Transcription begins by RNA polymerase binding to template at the

promoter

• The s subunit is responsible for promoter recognition

Transcription in

E. coli

•

E. coli

promoters have two consensus sequences upstream of transcription initiation site: 1. TATAAT positioned at –10 1. TTGACA positioned at –35

Prokaryotic Promoters

Steps in Transcription

1. Initiation 2. Elongation 3. Termination

Transcription

Initaition • Transcription begins when RNA Polymerase binds to a region of gene known as a Promoter Elongation • Transcription proceeds in 5’ to 3’ direction Termination • Transcription stops when it reaches a region in the gene known as Terminator

RNA Polymerase & DNA binding

Transcription Initiation

Transcription Elongation in

E. Coli

• Once initiation completed with synthesis of first 8 –9 nucleotides, sigma (s) dissociates and elongation proceeds with the core enzyme • Core enzyme ( α2 β β’) elongates RNA chain by moving along the DNA template and adding ribonucleotides at the 3’end by forming phosphodiester bonds

Transcription Termination in

E. coli

• Transcription is terminated by signals within the DNA sequence at the end of the gene • Hairpin formatio n in RNA destabilizes the DNA/RNA hybrid and releases RNA transcript • In some cases, termination depends on the

rho (



)

termination factor

Transcription in Eukaryotes • Occurs in the nucleus • Is not coupled to translation • Requires chromatin remodeling

Table 13-7

Copyright © 2006 Pearson Prentice Hall, Inc.

Eukaryotic Promoters

•

TATA box (-35)

: a core promoter element; transcription factors bind to them and determines start site of transcription •

CAAT box (-80):

highly conserved DNA sequence found within promoter of many genes; recognized by transcription factors •

Enhancers

can be upstream, within, or downstream of the gene; can modulate transcription from a distance

Post-transcriptional Editing of Eukaryotic mRNA 1.

Addition of a 5’ cap 2.

Addition of 3’ poly A tail 3. Splice out introns

Introns in Various Eukaryotic Genes

Alternative Splicing

• Introns present in pre-mRNAs derived from the same gene can be spiced in more than one way • Yields group of mRNAs that, upon translation, results in a series of related proteins

Alternative Genome

• Read article on Alternative Genome

Simultaneous Transcription & Translation