OCR Biology F215: Cellular Control

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(a) State that:

Genes code for polypeptides, including enzymes

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(b) Explain the meaning of the term Genetic code

The sequence of nucleotides in DNA or RNA that determines the specific amino acid sequence in the synthesis of proteins..
The sequence of the bases on a gene is a code with instructions for the construction of proteins. It has a number of characteristics:
--> It is a triplet code - 3 bases code for an amino acid
--> It is a degenerate code - all amino acids, bar one, have more than one code
--> Some codes code for 'stop' codons which indicate the end of the polypeptide chain
--> It is widespread but not universal - codons generally always code for the same amino acid in every organism, but this isn't always the case.

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(c) Describe, with the aid of diagrams, the way in which the nucleotide sequence codes for the amino acid sequence in a polypeptide

Protein synthesis has 2 stages: Transcription and translation (point d).

--> occurs in the nucleus
--> mRNA is synthesised, using one of the DNA strands as a template, from RNA nucleutides in the nucleus
--> the genetic code is 'transcribed' onto mRNA from ONA
--> DNA helicase 'unzips' DNA by breaking hydrogen bonds between bases (synoptic - hydrolysis)
--> this leaves the DNA strands exposed/unpaired
--> one of the strands is used as a template for the synthesis of mRNA
--> there are free nucleutides in the nucleus which come and align with the unpaired DNA bases
(synoptic - DNA base pairing) on the template/coding strand, by the rules of complementary base pairing
--> RNA polymerase catalyses the consensation reaction btwn the mRNA nucleotides & ensures the correct base is present
--> end of gene reached? The mRNA is fully formed & leaves the nucleus

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(d) describe, with the aid of diagrams, how the sequence of nucleotides within a gene is used to construct a polypeptide, inc the roles of mRNA, tRNA & ribosomes

--> occurs in the cytoplasm
--> mRNA (messenger RNA) attaches to a ribosome
--> 2 codons (6 bases) can fit into the 'information proscessing region' of the ribosome
-->tRNA (transfer RNA) molecules have an anti-codon at one end & can pick up an amino acid at the other end
--> again using the rules of complementary base pairing, the tRNA with the complementary anti-codon to the code in the ribosome, comes to the ribosome, bringing its amino acid with it

--> when 2 amino acids are next to each other, they undergo a condensation reaction (synoptic) & peptide links are formed btwn them, each time this happends, the polypeptide chain gets longer
--> when a 'stop' codon is reached, mRNA detaches from the ribosome
--> the chain of amino acids pass into the R.E.R (synoptic?) where it will be folded into a precise shape & bonds are formed that form the secondary/tertiary structure (synoptic)

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(e) State that

Mutations cause changes to the sequence of nucleotides in DNA molecules

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(f) Explain how mutations can have beneficial, neutral or harmful effects on the way a protein functions

A mutation is a change in the order of bases in DNA = different sequence of bases in RNA = different sequence of amino acids which determins the folding of proteins = different shapes.
This is done by 'deletion' (frame shift), 'addition'
(frame shift) or 'substitution' (only changes that triplet codon but might not change the amino acid because they're degenerate)

--> Beneficial - the organism gains an advantagous characteristic from the change in sequence of amino acids. E.g tolerance to high cholesterol
--> Neutral - this is a mutation in the non-coding region of DNA. It is a silent mutation - although the triplet has changed, it still codes for the same amino acid & the protien isn't changed (degenerate?). May not affect the final codon? Almost impossible to detect because there is no change to the phenotype
--> Harmful - the organism gains a characteristic which is harmful from the change in sequence of amino acids. E.g. sickle cell mutation

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(g) State that

Cyclic AMP (cAMP) activates proteins by altering their three-dimensional structure

(.'. it is a better shape to fit their substrate)

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(h) Explain genetic control of protein production in a prokaryote using the lac operon

E.coli absorbs & repires glucose as it's main respiratory substrate. If grown in a medium containing lactose it's able to produce 2 enzymes: lactose permease & B-galactosidase. The genes that code for these enzymes are only expressed/ switched when needed i.e. when lactose is available

The lac operon is a length of DNA that is responsible for the production of lactose permease & B-galactosidase. It is made up of a number of parts


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(h) cont...

No lactose?
--> The regulator gene is expressed & the repressor protein is synthesised. It has 2 binding sites. One binds to lactose & the other binds to the operator region
--> When binded to the operator region, it covers part of the promotor region where RNA polymerase normally binds
--> RNA polymerase can't bind to the promotor region so the structural geners can't be transcribed into mRNA
--> Without mRNA the genes can''t be translated & the enzymes can't be synthesised
When lactose is added?

--> Lactose binds to the other site on the repressor protein, causing the molecule to change shape. This prevents the other binding site from binding to the operator region. The repressor dissociates from the operator region (no longer fits)
--> The promotor region is now unblocked & RNA polymerase can now bind to it & initiate the transcription of mRNA
--> The operator-repressor-inducer system acts as a molecule switch. It allows the synthesis of structual genes
--> The bacteria can now use the lactose permease enzyme to take up lactose from the medium into its cells. They can convert it to glucose & galactose using the B-galactosidase enzyme. These sugars can then be used for respiration

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(i) Explain the the genes the control development of body plans are similar in plants, animals & fungi, with reference to homeobox genes

Homeobox genes:
--> are present in animals, plants & fungi, they determine the pattern of early development i.e. the basic pattern to the body
--> the homeobox is a sequence of DNA that codes for a region of 60 amino acides (180 bases (triplet codons etc)) (protein domain/ homedomain), & the resulting protein is found in most, if not all eukaryotes
-->The region binds to DNA so that they can regulate transcription
--> In animals the homeobox is common in genes concerned with the control of developmental events, such as segmentation, the establishment of the anterior-posterior axis & the activation of genes coding for body parts such as limbs

--> mice & humans have 39 hox genes on 4 chromosomes

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(j) Outline how apoptosis (programmed cell death) can act as a mechanism to change body plans

--> apoptosis isi an integral part of plant & animal tissue development. It is a series of biochemical events that leads to an orderly & tidy cell death, in contrast to cell necrosis, which leads to the release of harmful hydrolytic enzymes. Apoptosis ensures the rate of cells produced by mitosis is the same as the rate of cells dying = number remains constant. Not enough apoptosis leads to cancer. Apoptosis causes the toes & fingers to separate from each other.

--> cell shrinks & loses contact with neighbouring cells
--> chromatin condenses & begins to degrade
--> plasma membrane forms 'blebs' on its surface
--> nuclear membrane degrades
--> chromatin clumps into 'chromatin bodies'
--> nucleus breaks up
--> cells break up into numerous 'apoptic bodies'
--> a particular phospholipid is placed on the surface of apoptic bodies
--> phagocytoc cells have protein receptors in the membranes that recognise the phospholipid
--> binding occurs & fragments are engulfed

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(j) in short..

--> Enzymes break down the cell cytoskeleton
--> the cytoplasm becomes deanse with organelles tightly packed
--> the cell suface membrane changes & 'blebs' form

--> the chromatin condenses & the nuclear envelope breaks. DNA breaks into fragments
--> the cell breaks down into vesicles that are taken up by phagocytosis. the cellular debris is deposed of so that it doesn't damage other cells or tissue

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These are really good revision notes. Thanks :)

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