Chapter 10
Gene Expression
copying DNA instructions into RNA message
& using that message to produce a useable protein
&
Gene Regulation
Control over which genes are used & when they are used
DNA, Genes &Proteins
How are these three related?
DNA is composed of genes (coding regions) and non-coding regions
A gene is information describing how to make a protein
Proteins - perform many functions within a cell, some are enzymes
some are structural material… etc
Gene expression
the process of making proteins from DNA nucleotide message
Two step process
Transcription - the DNA message is written (transcribed) in RNA
using COMPLEMENTARY BASE PAIR RULES (Chargaff’s Rule)
Messenger RNA = mRNA
mRNA leaves the nucleus & travels into the cytoplasm where the
second step takes place...
Translation - mRNA message is read and a protein is made by
a ribosome and tRNA
Transcription of mRNA
This is a form of nucleic acid synthesis - similar to DNA synthesis
Synthesis is performed by an enzyme (Surprise!) - RNA Polymerase
Promoter Sequence = Special region of DNA at the beginning of gene
Promoter sequence recognized by RNA polymerase
RNA Polymerase binds to the promoter and begins making mRNA
This process is directional, it only reads in one direction
(much like you reading this text in one direction)
As you can see in this image
several polymerases can work
at the same time on a single gene
At the end of the
gene a ‘stop’
signal will cause
the polymerase to
drop off
How is RNA written?
The DNA message is written in complementary bases as mRNA
G--> C
C--> G
T--> A
A--> U
RNA has no T instead it uses a U
Terminator
sequence at
end of gene
tells RNA
Polymerase to
stop
mRNA moves to cytoplasm
In the cytoplasm the mRNA finds its way (diffusion) to a ribosome
The ribosomes are sites of protein synthesis
Often in eukaryotes, ribosomes are located on endoplasmic reticulum
Prokaryotes have free-floating ribosomes (no ER)
mRNA/Ribosome interaction
Ribosome serves several roles
Holding station -keeping everything in place
Activation/catalytic site - hooking new amino acids onto
the growing protein
mRNA codons are used to specify amino acids
Codon = 3 mRNA bases = signal for 1 amino acid
mRNA always starts with the same codon - AUG
The anticodon is
complementary to the
codon on the mRNA
G:C, U:A etc...
See table 10-2 on
page 160
RNA Splicing
In eukaryotes things aren’t quite as simple as we’ve just described
Often portions of the original gene are not used in the final product
These ‘extra’ unused portions are called introns they must be removed
before making the protein.
The portions that remain and ARE used are called exons
So, eukaryotes undergo RNA splicing
This is one way a gene can have some variability in its outcome
i.e.. different versions
Eukaryotic vs Prokaryotic
While eukaryotic genes have introns
Prokaryotic genes do not...
No editing is required in prokaryotic genes so they
do not have ‘editing’ enzymes
Because of this, if we want to insert a gene into a bacteria in order
to have it grow a eukaryotic protein we must give it an edited version
of the gene (no introns)