Genetic Variation

Genetic Variation

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THE lac OPERON
Control, genomes and environment
F215 Unit 5: Cellular control and variation
The bacterium E. coli requires two enzymes, lactose permease (to transport lactose into the bacterial cell) and -galactosidase (to break down
lactose into galactose and glucose), in order to respire the carbohydrate lactose. Interestingly, when E. coli grows in environments lacking lactose,
the bacteria do not produce these two enzymes, as they are unnecessary, but when it is present, the two enzymes are in abundant supply
DNA AND PROTEIN SYNTHESIS The lac operon is a DNA operon (length of DNA comprising structural genes, in this case the Z and Y sites, and control sites, in this case the regulator
gene, promoter region and operator region). Whether lactose is present or not, the regulator gene always expresses the repressor protein
DNA comprises the monomers, nucleotides, which themselves comprise the deoxyribose sugar and phosphate
group (making the sugar-phosphate backbone) and nitrogenous bases: adenine which has an affinity for thymine, I P O Z Y When lactose is absent (left), the repressor protein is able to bind itself to the
and cytosine for guanine. DNA contains thousands of genes which are lengths of DNA coding for polypeptides. DNA operon to the promoter region, also covering part of the operator region. This
ribosome
has a triplet code, whereby every three bases codes for one amino acid at protein synthesis means that the enzyme RNA polymerase is prevented from binding to the
repressor protein binds to operator operon at the operator region, and so cannot transcribe the Z and Y genes
During transcription a single-stranded (RNA) copy of a gene to be expressed is produced, called mRNA (messenger
mRNA region, preventing RNA polymerase (which code for the two enzymes).
RNA) which contains the appropriate coding for a gene. The coding strand of DNA is identical to the mRNA strand,
from binding to the promoter region
and the template strand (used to create the mRNA) is complementary to the coding strand. The DNA double helix
and transcribing the Z site and Y site I P O Z Y
unwinds and unzips, as with replication, and RNA polymerase builds the mRNA strand from the template strand repressor into the two enzymes
protein
The next step is translation, which uses the mRNA strand to express a protein. Each codon triplet on the mRNA RNA polymerase
RNA polymerase transcribes the
corresponds to a complementary anticodon on a tRNA (transfer RNA) molecule. These tRNA molecules carry specific regulator gene and mRNA is
amino acids with them, also. As the mRNA strand enters the ribosome, beginning with AUG, the tRNA with the translated into the repressor protein mRNA lactose
complementary anticodon (UAC) binds to the site, bringing with it the right amino acid (in this case methionine).
When the next one comes along, matching the second codon on the mRNA strand, a peptide bond forms between When lactose is present (right), the repressor protein is produced as
the two amino acids, and the previous tRNA dissociates. This continues, forming a long polypeptide chain normal, but lactose binds to sites on the protein molecule, which causes
the shape of the repressor protein to change. Therefore it cannot bind to RNA polymerase
the operon, and so RNA polymerase is able to attach at the operator transcribes the lactose binds to -galactosidase
MUTATIONS region and transcribe the mRNA for both lactose permease and - regulator gene and repressor protein,
lactose
repressor protein which cannot bind
DNA mutations can occur at any time during DNA replication, by mitosis or meiosis. There are several types: galactosidase permease
is produced as to the operator
substitutions (or point mutations) involve one base pair replacing another ­ only one pair changes The obvious benefit of the lac operon is that energy and resources are normal region due to
insertions and deletions involve either introducing an extra base pair or removing a base pair from the sequence, not wasted producing enzymes which have no use changed structure
which causes a frameshift (alteration in the entire sequence, so all of the triplet codes move)
Point mutations tend to be silent mutations, which have no effect on the actual coding. For example, the codons MEIOSIS
CTA and CTT both code for the same amino acid, so the mutation changing the `A' to a `T' is silent
The process to produce gametes (sex cells) is meiosis. Two gametes will fuse at fertilisation to form a zygote where chromosomes from each haploid
Many mutations can be neutral, where despite a change in the coding, the effects are neither beneficial or harmful parent cell (containing half the number of adult chromosomes, n) become one nucleus in the daughter cell (a diploid cell, 2n)
to the organism; but some can be positive (beneficial) or negative (harmful)
paternal spindle chiasmata homologous chromosomes
homologue chiasma
fibres pulled to opposite poles
APOPTOSIS
centriole
The term apoptosis refers to literally `programmed cell death' and it is a series of biochemical events leading to the
cells divide by nuclear
tidy death of a cell in a way which does not harm neighbouring cells, as opposed to cellular necrosis which is the
cytokinesis envelopes form
improper or unexpected death of cells which proves harmful in most cases
1 enzymes break down the cell's cytoskeleton
2 cell surface membrane creates folds and the bits that stick out are called blebs Metaphase I Anaphase I
3 chromatin from within the nucleus condenses and the nuclear envelope breaks down, and the DNA breaks into maternal nuclear envelope
homologue breaks down
separated nuclear fragments which disperse among the cell
Prophase I
4 the cell packages all of its contents into vesicles
chromatid Telophase I
5 the vesicles are taken up by phagocytes and cellular debris is disposed of without harming neighbouring cells moving
towards the
Apoptosis is usually induced by signals from other cells, including cytokines of the immune system, and hormones
pole
such as nitric oxide, which induces apoptosis by making mitochondrial membranes more permeable to hydrogen
ions, breaking down the carefully balanced chemiosmotic gradient which had been established
haploid
Cells with damaged DNA (or mutations) may not respond at all to these signals, and therefore may not undergo cells
apoptosis, which can lead to the development of tumours. These are able to break away from their initial sites and
enter the blood or lymph systems (this is called metastasis) where they can form secondary cancers
Prophase II
Apoptosis is particularly important during development when mitotic divisions are important. Generally, a cell will Telophase II Metaphase II chromosomes lying on
undergo 50 mitotic divisions on average before apoptosis takes place Anaphase II the equator of the cell

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MEIOSIS AND VARIATION EPISTASIS
prophase I ­ chromosomes shorten and fatten (supercoil) and come together in homologous pairs to form a bivalent, and the Epistasis is different from codominance: this is where one gene masks or suppresses the expression of another.…read more

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