Download The Genetic Code and Transcription Chapter 12 Honors Genetics

The Genetic Code and
Transcription
Chapter 12
Honors Genetics
Ms. 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
• DNA
Transcription
• mRNA
Translation
Protein
Folding
• Ribosome
There are Different RNAs with Distinct Functions
Transcription is a Key Step in Gene Expression
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.
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.
• 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
RNA polymerase acts here
The enzyme RNA polymerase opens the DNA strands and
synthesizes an RNA complement to only one of the DNA
strands.
A gene
The most important step in the control of gene
expression is WHEN to transcribe a gene.
Eukaryotic Genes are Segmented
Introns are removed from the primary transcript and
exons are spliced together to make mRNA.
In some genes, more than 90% of the pre-mRNA is
destroyed, never to appear in the mRNA.
The Genetic Language Uses 4 Letters Written
Into 3-Letter Words
What the Genetic Code Specifies:
AMINO ACIDS
Two examples
What Translation Accomplishes
The sequence of amino acids
determines the structure, and therefore
the function, of a protein.
During translation, information present in the mRNA is
read by the ribosome to synthesize a polypeptide.
Many
antibiotics
block steps in
translation
within
bacterial cells.
Translation requires:
ribosomes
mRNA
tRNA
amino acids
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.