From Gene to Protein
Genes code for...
Proteins
RNAs
Remember...
a protein is made of monomers called amino acids
amino acids bond together to form a polypeptide - the
primary structure of a protein
DNA vs RNA
Gene expression is
accomplished through two
basic processes...
Transcription = DNA to mRNA
Translation = mRNA to a polypeptide
Some of the RNAs
involved in gene
expression...
mRNA (messenger)
tRNA (transfer)
rRNA (ribosomal)
Transcription
Transcription factors bind to the TATA box on the DNA
strand
RNA polymerase pries the two strands of DNA apart
RNA polymerase joins RNA nucleotides together that
are complementary to the DNA template
RNA polymerase works only in the 5' to 3' direction just like DNA polymerase
Translation
mRNA transcript is brought to the ribosome
Initiation = the rRNA, mRNA transcript, and tRNA
carrying methionine bind together
Elongation = amino acids are added one by one to
create the polypeptide
Termination = when a stop codon is reached on the
mRNA
tRNA
What two processes ensure that the correct amino
acid is added to a growing polypeptide chain?
There are 64 codons. Three are stop codons, so we
can say there are 61 amino acid coding codons.
However, there are only 45 tRNA molecules that exist.
How is this possible?
The need for molecular
recognition...
Pairing of the tRNA anticodon and mRNA codon, but
wobble allows for some mistakes
Pairing of the tRNA with the amino acid, accomplished
by aminoacyl-tRNA synthetases
The Genetic Code
mRNA is read three bases at a time
3 mRNA bases = a codon
The Genetic Code
The Genetic Code
Ribosomes
Bring mRNA and tRNAs together
3 sites = A, P, and E
P= holds the tRNA that is attached to the growing
polypeptide chain
A = holds the tRNA carrying the next amino acid to be
added to the chain
E = is where discharged tRNAs leave the ribosome
How can human cells make 75,000- 100,000 different
proteins, given that there are about 20,000 human
genes?
Transcript Processing
5' Cap and Poly A Tail
5' Cap and Poly A tail
Facilitate the export on mRNA from the nucleus
Help protect the mRNA from degradation from
hydrolysis enzymes in the cytoplasm
Help ribosomes attach to the 5' end of the mRNA prior
to translation
RNA Splicing
The average transcription unit is 27,000 bases long.
The average protein is 400 amino acids long, requiring
only 1,200 RNA bases.
This means that most mRNA transcripts are initially
too long and contain a lot of non-coding segments.
RNA Splicing continued...
Non-coding segments (introns) of the mRNA transcript
are dispersed among the coding segments (exons)
Introns are removed from the mRNA transcript prior to
it leaving the nucleus.
This forms a mRNA transcript with a continuous
coding sequence
RNA Splicing continued...
The signal for RNA splicing is a short nucleotide
sequence at each end of an intron
snRNPs (small nuclear ribonucleoproteins) recognize
these sequences
Splicesomes
Includes multiple snRNPs and
proteins
Cuts out introns and pastes
together exons
A Note...
A single gene can code for more than one type of
polypeptide
This is because of alternative RNA splicing
Whether mRNA segments are treated as introns or exons
depends on the polypeptide product being expressed
For this reason the number of protein products an
organism produces can be much greater than its number
of genes
Mutations!
The ultimate source of new genes and, therefore,
genetic diversity.
Types of mutations...
Mutations are caused
by...
Physical mutagens (e.g. radiation)
Chemical mutagens (e.g. tobacco)
Chemical mutagens that cause cancer are called
carcinogens
Exposure to mutagens increases over an organism's
lifetime
So, what is a gene?
A discrete unit of inheritance that affects phenotype
A specific segment of a chromosome
A specific nucleotide sequence on a DNA molecule
A DNA sequence that codes for a specific polypeptide
product
Let's use...
Gene = a region of DNA that can be expressed to
produce a final functional product that is either a
polypeptide or an RNA molecule
Mutations
Silent mutations do not change what the gene
expresses
Frameshift mutations can change what, if anything,
the gene expresses
Exit Slip
How are transcription and translation similar? How
are they different?