Cellular Control

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  • Created on: 08-03-13 18:01
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(a) State that genes code for polypeptides, including enzymes
A gene is a length of DNA- sequence of nucleotide bases that code for one or more polypeptides
(b) Explain the meaning of the term genetic code
A sequence of nucleotide bases on a gene- provides a code with instructions for the polypeptide.
It is a triplet code- 3 bases= 1 amino acid
It is a degenerative code- there are a few triplets of bases that code for one amino acid
It is widespread but not universal
(c) Describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino
acid sequence in a polypeptide
Proteins assemble in the cytoplasm so gene has to be transcribed- mRNA. A triplet of bases codes
for one amino acid
(d) Describe, with the aid of diagrams, how the sequence of nucleotides within a gene is used to
construct a polypeptide, including the roles of messenger RNA, transfer RNA and ribosomes
Transcription:
1. The gene to be transcribed unwinds and unzips- this dips into the nucleolus and hydrogen
bonds between the bases break
2. Active RNA nucleotides bind with hydrogen bond to the exposed complimentary bases on
the template strand
This is catalysed by RNA polymerase
3. Two extra phosphate groups are released- release energy
4. mRNA is complimentary to the template strand and a copy of the coding strand
5. mRNA is released from the nuclear pore and passes out of the nucleus.
Translation:
1. mRNA binds to the ribosome- two codons (6 bases) are attached to the small subunit of the
ribosome and exposed to the large subunit
The anti-codon on the tRNA forms hydrogen bonds with the first codon- this uses ATP ad an
enzyme
2. a second tRNA bearing a different amino acid binds to the second exposed codon with its
complementary anticodon
3. a peptide bond forms between the adjacent amino acids- catalysed by an enzyme
4. The ribosome moves along mRNA reading the next codon. The first tRNA leaves and a third
comes
5. the polypeptide chain grows until it reaches a stop codon- no corresponding tRNA
(e) State that mutations cause changes to the sequence of nucleotides in DNA molecules
Mutation- random change to the genetic material
Chromosome mutation- change in the structure to the chromosomes
Certain substances called mutagens may cause mutations- tar, tobacco and UV light
Although the structure of DNA makes it stable mistakes do occur. Mutations may occur during DNA
replication. Mutations associated with mitosis are somatic mutations and are not passed on to
offspring. Mutations associated with meiosis can be inherited

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Types of mutations:
1. point/ substitution ­ one base replaces another
2. insertion- one or more nucleotide bases are inserted into a length of DNA
3.…read more

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Promoter region- where enzyme RNA polymerase binds to to binding the transcription of
the structural genes
When lactose is absent:
Regulator gene is expressed- repressor protein is synthesised and binds to operator region
This covers the promoter region
RNA polymerase cannot attack to the promoter region so the structural genes cannot be
expressed cannot be expressed- enzymes not synthesised
When lactose is absent:
Lactose binds to binding site in the repressor protein
This changes it shape so it can no longer bind to the operator…read more

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It is controlled by cell signalling including cytokines. It is important in development.…read more

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