F215 - Cellular Control

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Cellular Control

Key definitions:

A gene: A length of DNA that codes for one or more polypeptides.

A genome: The entire DNA sequence for that organism.

A protein: A large polypeptide consisting of many amino acids.

Ribosomes: They consist of a small sub-unit and a large sub-unit, the mRNA strand is held in the large sub-unit whilst the anti-codon (tRNA) is held in the small sub-unit.

tRNA: This is formed in the nucleus and passes out into the cytoplasm forming an anticodon which has 3 unpaired bases on its end.

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DNA coding for proteins - Protein Synthesis

The first stage is Transcription which occur in the nucleolus. This is the formation of an mRNA molecule. Here are the following stages of the process:

1. The gene to be transcribed unwinds and unzips. The hydrogen bonds between the complementary bases are hydrolysed and break.

2. Activated RNA nucleotides bind with hydrogen bonds to thier exposed complementary bases through base pairing rules. This is catalysed by the enzyme RNA polymerase.

3. The mRNA strand produced is complementary to the nucleotide base sequence on the template strand of the DNA so its a copy of the coding strand.

4. The mRNA is released from the DNA and passes out of the nuclues, through a nuclear pore going to a ribosome.

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Translation

The second stage of protein synthesis; takes place in the ribosome:

Ribosomes: They consist of a small sub-unit and a large sub-unit, the mRNA strand is held in the large sub-unit whilst the anti-codon (tRNA) is held in the small sub-unit.

tRNA: This is formed in the nuclues and passes out into the cytoplasm forming an anticodon which has 3 unpaired bases on its end.

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Assembly of a polypeptide

1. A molecule of mRNA binds to the ribosomes at the large sub-unit. Two codons enter the small sub-unit of the ribosome and are exposed to the mRNA molecule. The first mRNA codon is always AUG.

2. A second tRNA binds to the second exposed codon with its complimentary anti-codon

3. A peptide bond forms between the two adjacent amino acids.

4. The ribosome moves along reading the next codon and another tRNA passes along bringing in another amino acid. A peptide bond forms and as the ribosome moves along the first tRNA is allowed to leave and collect another amino acid to carry on the chain.

5. The polypeptide chain grows until a stop codon is reached. The polypeptide chain is now complete.

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Mutations

A mutation is a change in the arrangement or the amount of genetic material in a cell.

Chromosome mutation: is a change in parts of the chromosomes

DNA mutation: is a change in the nucleotide sequence of a gene.

Frameshift: Every triplet of nucleotide bases after the mutation is different

There are two main classes of DNA mutations:

  • Point mutations: where one base pair replaces another. This is also knows as a substitution
  • Insertion/deletion mutations: where one or more nucleotide bases are inserted or deleted from a length of DNA. This causes a frameshift.
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The Lac Operon

The lac operon is a section of DNA within the bacterium's DNA (e.g. E.Coli). It consists of a number of parts:

  • The structural genes:  Z - this codes for the enzyme beta-galactosidase, and Y codes for the enzyme lactose permease. Each consists of a sequence of base pairs which can be transcribed into a length of mRNA.
  • The operator region: O - this is a length of DNA next to the structural genes which can be switched on and off
  • The promoter region: P - A length of DNA where the enzyme RNA polymerase can bind to begin transcription of Z and Y
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How the Lac Operon works

When lactose is absent from the growth medium:

1. The regulator gene is expressed and the repressor protein is synthesised. It has two binding sites, one that binds to lactose and one that binds to the operator region.

2. The repressor protein binds to the operator region, when this occurs it slightly covers part of the promoter region where the RNA polymerase normally attaches.

3. The RNA polymerase can no longer bind to the promoter region, so the genes cannot be transcribed into mRNA.

4. Without mRNA these genes cannot be translated and the enzymes beta-galactosidase and lactose permease cannot be synthesised.

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How the Lac Operon works

When lactose is present in the growth medium:

1. When there is lactose present it binds to the site on the repressor protein. This causes the molecules of the repressor protein to change shape so that its other binding site no longer fits the operator region.

2. The promoter region is not covered meaning the RNA polymerase can bind and intiate transcription for mRNA for genes Z and Y.

3. The enzymes beta-galactosidase and lactose permease can be produced as a result.

Beta-galactosidase: converts lactose into glucose and galactose

Lactose permease: allows the E.Coli to take up the lactose into their cells

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Apoptosis

Apoptosis: Programmed Cell Death

How it occurs:

  • Enzymes break down the cytoskeleton.
  • The cytoplasm becomes dense, with organelles tightly packed together.
  • The cell surface membrane changes and forms small bits called blebs.
  • The chromatin condenses and the nuclear envelope breaks down. The DNA breaks down into fragments.
  • Cell breaks down into vesicles that are taken up by phagocytosis

How is it controlled?

Normally cell signalling indicates if there is a problem with the cell or it is not required (e.g. mutation, old cells, formation of fingers and toes etc). Hormones can also be used to trigger apoptosis.

Nitric Oxide can induce apoptosis by making the inner mitochondrial membrane more permeable stopping chemiosmosis.

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