The Genetic Code and Transcription

Chapter 12Honors GeneticsMs. Susan Chabot

Molecular Genetics - From DNA to 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 ending protein synthesis for a specific protein.

• A given amino acid can have more than one codon sequence.

The “Central Dogma” of Molecular Genetics

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 rRNARibosomal RNA

Nucleolus

II mRNA and snRNAMessenger RNASingle nucleotide RNA

Nucleoplasm

III 5s RNA and tRNASmall ribosomal RNATransfer RNA

Nucleoplasm

Role of RNA Polymerase

• Enzyme capable of directing the synthesis of a mRNA copy from a strand of DNA.

• Substrate nucleotides contain RIBOSE sugars and the base URACIL in place of thymine.

• 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.

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

• Considered cis-acting elements– In organic chemistry cis means “next to” or “on the same

side as”

• Additional sequences recognized in E. coli is TTGACA

Transcription

The enzyme RNA polymerase opens the DNA strands and synthesizes an RNA complement to only one of the DNA strands.

RNA polymerase acts here

The most important step in the control of gene expression is WHEN to transcribe a gene.

A gene

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 Genetic Language Uses 4 Letters Written Into 3-Letter Words

What the Genetic Code Specifies:AMINO ACIDS

Two examples

What Translation Accomplishes

During translation, information present in the mRNA is read by the ribosome to synthesize a polypeptide.

The sequence of amino acids determines the structure, and therefore the function, of a protein.

Translation requires:

ribosomes

mRNA

tRNA

amino acids

Many antibiotics block steps in translation within bacterial cells.

Ribosomes are Complicated

Protein Synthesizing

Machines

Translation Is a Cyclic, Multistep Process

Basic Genetic Mechanisms are Universal

The storage of genetic information in DNA, the use of an RNA intermediate that is read in three letter words, and the mechanism of protein synthesis are essentially the same in all organisms.

A tobacco plant expressing the firefly luciferase gene.