Gene expression, mutations
& Recombinant DNA- Part one
(E4, E3, D3,D4)
Control of Gene Expression
p. 487
Review Genes and Chromosomes
Genes: the units of heredity in a living organism.
They are encoded in the organism's genetic material (usually
DNA or RNA), and control the physical development and
behavior of the organism.
During reproduction, the genetic material is passed on from the
parent(s) to the offspring. Genetic material can also be passed
between un-related individuals (e.g. via transfection, or on
viruses).
 Genes are located on chromosomes and consist of
segments (sequence) of DNA molecules
 A gene consists of a sequence of about 1000 DNA basepairs (though there is considerable variation in this length).
About 175,000 genes compose the DNA molecule of a
single human chromosome. The genes act in pairs that
dictate traits.
 Genes encode the information necessary to construct the
chemicals (proteins etc.) needed for the organism to
function.
1
 Genes control cellular chemical reactions, by directing the
formation of enzymes.
 Genes always occur in pairs. Half of each person's genes
come from the mother and half from the father. Most
ordinary characteristics like height and eye color are
determined by combinations of several different genes.
Gene expression = protein product
 Many levels to control gene expression exist within cells
4 main levels of control
1. Transcriptional control (nucleus)
 Determine which genes are transcribed and rate through:
1. organization of CHROMATIN (access to gene)
2. TRANSCRIPTION FACTORS (molecules required to
bind DNA & start transcription)
2. Posttranscriptional control (nucleus)
 mRNA
a. between primary mRNA and mature mRNA
b. speed at which mature mRNA is allowed to
2
leave nucleus
3. Translational control (cytoplasm)
 Between mature mRNA & translation
4. Posttranslational control (cytoplasm)
 The final touches to protein product
After protein synthesis- before protein has undergone
additional changes to make it biologically functional
Transcriptional Control in Prokaryotes
(fig 25.14)
An OPERON includes the following elements…..ALL ARE
SHORT SEQUENCES OF DNA
Regulator gene- a gene that codes for repressor protein (DNA
to mRNA to protein that returns to nucleus)
Promoter -site where RNA polymerase attaches and is the
starting point for transcription
Operator -site where repressor protein binds. prevents RNA
polymerase from binding (on/off switch)
3
Structural gene -one or several genes of a metabolic pathway
that are transcribed as a unit.
Note: structural genes are translated to proteins that function
in cytoplasm. Regulator genes are translated to proteins that
return to the nucleus to control what structural genes are
transcribed.
DNA----------Regulator gene---promoter---operator----structural
gene
Transcriptional Control in Eukaryotes- 2 ways
-each cell, regardless of type (muscle, blood, liver), contains an
identical set of genes. What varies however is which of those
genes are “turned on or off” (transcribed) in each. Therefore
dictating what protein products will result.
1. Activated Chromatin
-chromosome must decompact before transcription can begin
-lampbrush chromosomes (fig 25.15 p. 489)
4
2. Transcription Factors-(TF)
-substances that attach to DNA and ‘turn on’ particular genes.
-once the right combo of TFs have bound, RNA polymerase
can bind and transcription begins
Gene Mutations
(25.4 p. 490)
During the molecular maneuvering that occurs with DNA
replication, if nucleotides are lost, rearranged, or paired in error,
the resulting change in instruction of the genetic code could
lead to a protein that does not function properly when the
DNA’s code is translated.
Gene mutation: a change in nucleotide (sugar, phosphate,
base) sequence. Results from a chemical change in the
structure of a gene.
Mutations can be caused by internal and external factors.
Any factor that can cause a mutation is called a MUTAGEN
5
(e.g. benzene, dioxins, UV light, asbestos, DDT, cigarette
smoke, x-rays, etc.)
Changes (if not repaired), will first be reflected in RNA copy,
then in the enzyme or other protein that the RNA codes for,
and finally in the appearance of new traits in the living
organism.
Two main categories of mutations
Mutations occur b/c of the alteration in one or more base
pairs of the DNA molecule, garbling the existing genetic code.
1. GENE MUTATION (affect only one gene)
May be caused by a change (e.g. substitution, deletion,
addition) in a single nucleotide. The effect on the individual
depends on the gene’s role.
Frameshift mutations: one or more nucleotides is either
ADDED or DELETED from DNA.
The result can be a completely non-functional protein…doesn’t
make sense.
6
Normal ‘Reading frame’ =normal sequence of DNA
i.e.
--- THE CAT ATE THE RAT--If you delete a base (letter) ‘C’
---THE ATA TET HER ATH----Original message:
THE BIG DOG BIT TED AND RAN OFF
--- ---
ADDITION/FRAMESHIFT
THE BIG FDO
GBI TTE DAN DRA NOF F--
---
Now each 3 base pair sequence or CODON has changed….
affecting the tRNA matching ANTICON and therefore the
particular amino acid, and ultimately the resulting PROTEIN.
Point mutation: one base is SUBSTITUTED for another.
Change in DNA that causes a change in the significant part of
the mRNA codon(s), a different amino acid will be translated,
and a different protein will be made.
Results can vary. Sometimes “neutral”, usually harmful but very
occasionally beneficial. (About 1 time in a million, the change
might actually improve the protein-this is called a beneficial
7
mutation and while infrequent, drive evolution of species!)
Ex. Sickle-cell anemia is a genetic disorder caused by gene
mutation. (Figure 26.16 p. 490)
2. CHROMOSOMAL MUATIONS (affect many genes)
Occur after chromosomes are broken (e.g. due to exposure to
radiation, addictive drugs, pesticides) and REFORM
ABONORMALLY. Pieces of chromosomes can be lost, added,
or whole chromosomes can be lost or added
(i.e. Down’s Syndrome or Trisomy 21-extra chromosome #21).
Extra sex chromosome causes abnormalities. Most common is
Kleinfelter’s Syndrome (occurs in 1 out of 700 males). Have 3,
instead of 2 sex chromosomes. 2 X and 1 Y. (normal male=XY,
normal female XX)
MUTATION REPAIR
PROOFREADING
The cell RARELY makes a mistake during DNA replication.
8
However occasionally they DO occur.
The cell has a PROOFREADING mechanism in place.
The enzyme, DNA polymerase that carries out replication, also
proofreads the new strand against the old one. If it detects
mismatched base pairs, it will cut them out and replace them
with the correct nucleotide.
9