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Genetics A2 Level Essay

4019 words - 17 pages

DNA & Protein synthesis
A gene is a sequence of Nucleotide bases that code for a specific protein or polypeptide. Genes are
mainly located on the Chromosomes however some can be found in mitochondria. Each gene
occupies a specific LOCUS on the chromosome. Genes are involved in the control of all metabolic
pathways and the synthesis of all non-protein molecules in cells.
The genetic code

Triplet code – every three amino acids or CODON code for an amino acid
There are multiple codons for each amino acid except methionine
Some codons code for ‘STOP’
The codons are widespread but not universal; genes can be used in genetic engineering to
place ...view middle of the document...

3. The ribosome then moves down the mRNA reading it’s codons and binding tRNA molecules
via hydrogen bonds which carry the amino acid specific to the codon in the mRNA.
4. The amino acids are joined by peptide bonds.

5. The polypeptide carries on growing until a stop codon is reached.

Lac operon
For E.Coli to respire lactose as a respiratory substrate two enzymes must be produced, these are

Beta-Galactosidase - converts lactose into glucose and galactase
Lactase permase – allows lactase to be brought through the cell membrane

These two enzymes are triggered by the presence of lactose which acts as the INDUCER. The lac
operon is a section of DNA which account for the production of these two enzymes, it consists of:

The structural genes: these codes for the two enzymes.
The operator region: Can switch the structural genes on or off
Promoter region: Where RNA polymerase binds to begin the transcription of the structural

How the lac operon works
When lactose is absent
1. The regulator gene is expressed, this binds to the operator and promoter region
2. This prevents mRNA binding to the promoter region so the structural genes cannot be
3. B-Galactosidase and lactase permase isn’t produced
When lactose is present
1. Lactose molecules (inducer protein) molecules bind to the repressor protein, changing the
shape of the active site, meaning it can no longer bind to the promoter region and operator
2. RNA transcribes the promoter region and operator region, these allow RNA to transcribe and
translate the structural genes.
3. As a result lactase is allowed into the cell via lactase permase and is broken down into
glucose and galactose to be respired by the cell.

Homeobox genes
The development of organisms in relation to differentiation is controlled by HOMEOBOX GENES.
These genes perform different functions:

Maternal genes affect polarity, anterior and posterior.
Segmentation genes affect the polarity of segments.
Homeotic selector genes, specify the identity of each segment.

Homeobox genes sequence for specific polypeptides that in turn bind to genes and prevent the
expression of them, others act as TRANSCRIPTION FACTORS, which bind to the gene and initiate
transcription in protein synthesis, thus specialising the cell.
MORPHOGENS such as vitamin A or retinoic acid, can affect the transcription of genes leading to

Programmed cell death that occurs in organisms, cells usually undergo 50 mitotic divisions before
apoptosis, it is orderly and tidy.
How does it occur?


Cell signals
Growth factors
Nitric oxide – this induces apoptosis by making the mitochondrial membrane more
permeable, this reduces the concentration gradient meaning the cell can no longer respire
to produce ATP to survive.
Proteins also bind to...

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