Download Supercourse - Scientific Basis for Genetics Part II

Lesson Four
Structure of a Gene
Gene Structure
What is a gene?
 Gene: a unit of DNA on a
chromosome that codes for a
protein(s)

–
–
–
–

Exons
Introns
Promoter sequences
Terminator sequences
Other regulatory sequences
(enhancers, silencers), which may be
far from major components of a gene
Gene Structure

Exons: contain the bases that are
utilized in coding for the protein

Introns: contain bases that are not
utilized in coding for proteins and
intervene between the exons
– Introns are spliced out
Gene Structure

Promoter: bases that provide a
signal to tell the cell’s machinery
where to begin transcription,
usually before or within a gene

Terminator: bases that provide a
signal to tell the cell’s machinery
where to stop transcription, usually
at the end of a gene
Gene Structure

A typical gene might look something like this:
---------= exon
= intron

= promoter
----------
= terminator
This gene has 3 exons and 2 introns
Lesson Five
Transcription
Transcription

The process of using DNA (a gene) as a
template to produce messenger RNA (mRNA)

Occurs in the nucleus

Template strand – the strand of DNA that
is accessed to make mRNA

Coding strand – the strand of DNA that is
NOT accessed to make mRNA. The mRNA
that is made from the template strand will
be identical to the coding strand (with the
exception of U’s for T’s)
RNA Modification

Trimming: removing bases from the 5’ and
3’ ends

Capping: adding a methylated G to the 5’
end
– Necessary for RNA localization to the
ribosome

Tailing: addition of A’s to the 3’ end of
the mRNA
– More A’s = more stabile mRNA

Splicing: removing introns prior to mRNA
transport to the nucleus
Lesson Six
Translation
Translation
The process of using mRNA as a
template to generate a polypeptide
that will eventually become a
mature protein
 Also called protein synthesis
 Requires the ‘genetic code’

– Based on 64 codons, each with 3
nucleotides
– Provides the link between DNA and
protein sequence
Translation Requires Different
Types of DNA

mRNA: messenger RNA; major product
of transcription
– Represents the code for the primary amino
acid sequence of a protein
– Only type of RNA that is translated

tRNA: transfer RNA

rRNA: ribosomal RNA
– Recognizes the mRNA code (tri-nucleotide)
and brings with it (or transfers) the
appropriate amino acid to the protein
– Link between mRNA and protein
– Part of the ribosomes
– Involved with translation by helping to align
the mRNAs and tRNAs
Protein Processing
Final transport
Genomics to Proteomics
Primary control of
gene expression
Lesson Seven
Mutations

Point Mutations
Involves a single base pair
– Substitution, insertion, deletion
– SNPs

May not affect amino acid sequence
– Same sense (silent, neutral, synonymous, same
sense)
– Due to redundancy of the genetic code

May affect amino acid sequence
(nonsynonymous)
– Missense (results from a change in an amino
acid)
– Nonsense (results from a change to a stop
codon – truncated protein)
– Frame shift mutations (insertion or deletion of
1+ bases - alters the reading frame)
Missense Mutation
Sickle Cell Anemia