A2 OCR Biology: Cellular Control and Variation

A2 OCR Biology: Cellular Control and Variation

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  • Created on: 25-05-11 00:08
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Cellular Control and Variation
A gene determines the sequence of amino acids in a polypeptide, for example hormones.
Many proteins are composed of several polypeptides and have quaternary structures such
as:
A molecule of catalase is made of four identical polypeptides
a molecule of adult haemoglobin is made of two alpha-globin and two beta-globin
chains
The four bases; Adenine, Cytosine, Thymine and Guanine, code for 20 different amino acids.
The triplet code for the amino acids is called the genetic code and is shown as the RNA
code or as one of the DNA codes. Each group of three bases in mRNA that codes of an
amino acid is known as the codon.
Protein Synthesis
The three stages are Transcription, Amino Acid Activation and Translation.
Transcription
1) Hydrogen bonds between the nucleotide bases in the polynucleotide chains break in
the area of DNA that is needed for a specific gene
2) One chain acts as a template for the synthesis of the mRNA
3) Free RNA nucleotides in the nucleus pair up with the exposed bases on the template
polynucleotide chain.
4) The nucleotides are joined together to form a polynucleotide ­ mRNA. This process
is catalysed by RNA polymerase.
5) mRNA leaves the nucleus through the nuclear pore.
Amino Acid Activation
Amino acids are identified by combining them with molecules of tRNA. The nucleotide,
tRNA, is the anticodon. Enzymes in the cytoplasm attach amino acids to specific tRNA
molecules. The hydrolysis of ATP provides the energy for the process.
Translation
1) The mRNA molecule joins with a ribosome in the cytoplasm
2) Each ribosome has two sites to hold two tRNA molecules at the same time. Each
tRNA molecule is attached to the amino acid.
3) Each tRNA has an anticodon that pairs with the codon on the mRNA, following the
rules of base pairing.
4) A condensation reaction occurs between amino acids to form a peptide bond
5) The ribosome moves along the mRNA, reading the sequence of bases

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During this, a new polypeptide grows by the attachment of amino acids.
7) When an amino acid has joined the polypeptide chain, its tRNA molecule moves onto
another amino acid to attach to.
8) When the ribosome reaches a STOP codon, the polypeptide breaks away and
begins to fold spontaneously into a secondary structure and eventually a tertiary
structure.
9) The cell processes the polypeptide, perhaps by combining it with another
polypeptide to form a protein (e.g.…read more

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E.coli. The production of the enzyme is controlled along with two other
proteins that are required for the absorption and metabolism of lactose. Three genes
are transcribed together giving one mRNA transcript.…read more

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Genes in Development
Homeobox genes code for transcription factors that bind to DNA. The homeobox
sequences of organisms are almost identical because they all have the same function ­
coding for transcription factors.
Apoptosis is programmed cell death. Cells respond to external and internal signals that
trigger an ordered sequence of changes in the cytoplasm ensuring that cells are removed
efficiently without the release of hydrolytic enzymes that would damage the surrounding
tissue.…read more

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