Transcript Document

LECTURE
CONNECTIONS
13 | Transcription
© 2009 W. H. Freeman and Company
Chapter 13 Outline
• 13.1 RNA Consisting of a Single Strand of Ribonucleotides
Participates in a Variety of Cellular Functions
• 13.2 Transcription Is the Synthesis of an RNA Molecule
from a DNA Template
• 13.3 The Process of Bacterial Transcription Consists of
Initiation, Elongation, and Termination
• 13.4 The Process of Eukaryotic Transcription Is Similar to
Bacterial Transcription but Has Some Important
Differences
• 13.5 Transcription in Archaea Is More Similar to
Transcription in Eukaryotes than to Transcription in
Eubacteria
13.1 RNA Consisting of a Single Strand of Ribonucleotides
Participates in a Variety of Cellular Functions
• RNA is polymer consisting of nucleotides joined
together by ________________________ bonds.
• RNA has a free hydroxyl group on the ___’ carbon atom
of the ribose sugar.
• RNA is degraded rapidly under alkaline conditions.
• RNA’s pyrimidines are cytosine and URACIL.
• RNA is usually single stranded.
The Structure of RNA
• Primary structure
• Secondary structure
RNA molecules can
assume different
structures.
Classes of RNA
• Ribosomal RNA – rRNA
• Messenger RNA – mRNA: carries the coding instructions
for polypeptides chains from DNA to ribosomes.
Pre-messenger RNAs (pre-mRNAs or nRNAs): the
intermediate product of transcription in eukaryotes cells.
• Transfer RNA – tRNA: serve as the link between the coding
sequence of nucleotides in the mRNA and the amino acid
sequence of a polypeptide chain.
Classes of RNA
•
Small nuclear RNAs – snRNAs: some participate in the
processing of RNA.
•
MicroRNAs – miRNAs: carry out RNA interference (RNAi)
•
Small interfering RNAs – siRNAs: triggers degradation of
other RNA molecules
13.2 Transcription Is the Synthesis of an RNA
Molecule from a DNA Template
• Only the gene (s) of interested are transcribed into RNA.
• Transcription is a highly selective process.
• Transcription requires:
- DNA template
- a raw materials needed to build a new RNA molecule
- the transcription apparatus
The Template
• The transcribed strand: template strand.
• During transcription, an RNA molecule that is
complementary and antiparallel to the DNA template
strand is synthesized.
The Template
• The transcription unit
• Promoter: is a DNA sequence that the transcription
apparatus recognizes and binds.
• RNA-coding sequence: a sequence of DNA nucleotides
that is copied into a RNA molecule
• Terminator: a sequence of nucleotides that signals where
transcription is to end.
The transcription apparatus is said to move downstream during
transcription: it binds to the promoter (which is usually upstream of the
start site) and moves toward the terminator (which is downstream of the
start site).
The first nucleotide transcribed is numbered +1.
Nucleotides upstream of the start site are assigned negative numbers.
Concept Check 1
Which of the following phrases does not describe a
function of the promoter?
a. Serves as sequences to which the transcription
apparatus binds
b. Determines the first nucleotide that is transcribed into
RNA
c. Determines which DNA strand is template
d. Signals where transcription ends
Initiation
• The substrate for transcription:
• Ribonucleoside triphosphates – rNTPs added to the 3′
end of the RNA molecule
Nucleotides are added one at a time to the 3’-OH group of the
growing RNA molecule.
Two phosphate groups are cleaved from the incoming rNTP; the
remaining phosphate group participates in a phosphodiester
bond that connects the nucleotide to the growing RNA molecule.
Initiation
• The transcription apparatus:
• Bacterial RNA polymerase: five subunits made up of
the core enzyme:
•
•
•
•
Two copies of α
Single copy of β
Single copy of β′
A stabilize enzyme: ω
• The sigma  factor: binding to the promoter when
transcription starts
Initiation
• The transcription apparatus:
• Eukaryotic RNA polymerases