Unit 3 Biology

Gene Expression in Prokaryotic Cells

Circular chromosome; occurs at the same time

RNA vs DNA

Single stranded, contains a ribose instead of a deoxyribose, and can contain more complex structures

rRNTp vs dNTP

Differences: 2’ —OH vs 2’ —H

Similarities: 3 Phosphates, added to 3’ -OH

Types of Prokaryotic RNA

mRNA and ncRNA

mRNA (how long?)

Messanger RNA; Long (average 3000 nucleotides), codes for proteins. Coding region is 1200 BPs

Contains a promoter sequence, regulatory sequence, coding sequence, and transcription terminator

Types of sequences in mRNA

Promoter: Determines where transcription starts

Regulatory Sequence: Determines when, where, how much mRNA is made

Coding Sequence: ORF, opening region frame

Transcription Terminator: Where transcription ends

ncRNA

Includes tRNA (transfer RNA), rRNA (ribosomal RNA), and sRNA (small RNA)

Gene

Unit of heredity, carries the genetic information for a protein (polypeptide) or RNA

Opening Reading Frame

Stretches of DNA that lack a stop codon; every 3 nucleotides encode for amino acid during translation of mRNA to form a proteins; there are 3 reading frames possible per DNA strand

Basic Structure of a Bacterial Protein-Coding Gene (DRAW)

Promoter: From -10 to -35 site, where RNA polymerase binds. Determines transcription start

Transcription Start Site (TSS): From +1

Opening Reading Frame (ORF): Protein coding sequence

Start and Stop Codons

Start; ATG

Stop: TAG, TGG, TGA

Template vs Coding Strand

Template strand is use for mRNA synthesis

Coding strand is identical to mRNA strand, except A is replaced with U

Transcription Overview

RNA binds to single stranded DNA template at the promoter

DNA helix unwinds

RNA is synthesized from rNTPS from 5’ to 3’ direction

5’ end of transcript is displaced from template as polymerase moves

How to know which strand is template strand for a gene? Draw

RNA vs DNA Polymerase

Doesn’t require a primer, only one nucleotide polymer is built, no proofreading

How many total DNA strands are possibe?

Operons

Are in prokaryotic cells. Multiple ORF in one sequence, each has its own ATG and STOP sequence.

RNA Polymerase Structure

6 subunits, one leaves

Initiation

Consensus sequence upstream of TSS in the promoter

Sigma subunits binds to the -10 and -35 sequences to correctly position the enzyme

After initation, the sigma subunit disassociate and RNA polymerase can continue elongation

Elongation

RNA polymerase synthesizes. 17 bp are unwound in the transcription bubble. Moves at 50 nucleotides per second and the error rate is 10^4 or 10^5

Intrinsic Termination

G-C rich stem-loop structure of mRNA followed by a rich string of As. RNA polymerase dissasociates when it encounters the G-C loop.

Rho Dependent Termination

Rho binds to rut sequence upstream of termination site and acts as helicase to pause the RNA-DNA hybrid and facilitates the release of mRNA