F215 Cellular Control

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Transcription- the 1st stage of protein synthesis

1. A gene unwinds and unzips - the length of DNA dips into the nucleolus.

2. Hydrogen bonds between complementary bases break.

3. Activated RNA nucleotides bind (H-bonds) to their exposed complementary bases. 
    U-A. G-C. A-T on template strand. RNA polymerase used.

4. Two extra phosphates released, releasing energy for bonding of the adjacent nucleotides.

5. mRNA is produced - complementary to the template strand, a copy of the coding strand.

6. mRNA is released from the DNA and passes out of nucleus to a ribosome.

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Translation -

The assembly of polypeptides at ribosomes.

Amino acids are assembled into a polypeptide in a sequence determined by the sequence of codons on the mRNA.

THINK OF A CABLE TIE - The ribosome has a groove which the mRNA can slide through, reading the code and making the protein. 

tRNA - fold into hairpin shapes, have 3 exposed bases  at the end where a particular amino acid can bind. On the other end is an anticodon which is complementary to the codon on mRNA.

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Translation - the 2nd stage of protein synthesis

1. A molecule of mRNA binds to a ribosome.

2. 2 codons attach to the small subunit of the ribosome, exposing the large subunit. First codon is always AUG. Using ATP energy and an enzyme a tRNA with methionine (AUG) (A-U, U-A, G-C) form H-bonds with this codon and the anticodon.

3. A second tRNA with a different amino acid binds to the second codon with its complementary anticodon.

4. A peptide bond forms between 2 amino acids, catalysed by an enzyme.

5. The ribosome moves along the mRNA, reading the next codon. - Contains 2 tRNAs at once, as first leaves, third joins.

6. Polypeptide chain grows until a stop codon is reached (e.g. UAC), no corresponding tRNA for them so chain is complete.

Some proteins have to be activated by cAMP which changes the 3D shape to better fit their complementary molecules.

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Mutations -

a change in the amount/arrangement of the genetic material in a cell

DNA Mutations - changes to genes due to change in nucleotide base sequences.

Mutations associated with mitosis - not passed on

Mutations associated with meiosis - can be inherited

DNA mutations;

Point Mutations/Substitutions - 1 base pair replaces another.

Insertion/Deletion - 1 or more nucleotide pairs are inserted or deleted from a length of DNA. Cause a frameshift.

Sickle-cell anaemia - point mutation

Cystic fibrosis - deletion of a triplet code

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Effects of mutations

Neautral effect if;

  • the mutation is in a non-coding region of DNA.
  • it is a silent mutation - base triplet has changed but still codes for same amino acid.

Also neutral if  there is a change to structure but it gives no +/- to the organism e.g tongue rolling.

Examples of good/bad effects;

  • Skin colour-dark skin encourages the synthesis of melanin which protects from the harmful effects of ultraviolet light and the synthesis of Vitamin D
  • Pale skin more likely to burn.
  • Can depend on environment whether it is harmful or beneficial.
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The lac operon - Prokayotic

Operon - a length of DNA made up of structural genes and control sites.

Induced by lactose

Promoter region - where RNA polymerase binds so transcription of structural genes can take place.

Operator region - Switches the structural genes on and off.

Z structural gene - codes for the enzyme B-Galactosidase - catalyses the hydrolysis of lactose into glucose and galactose.

Y structural gene - codes for the enzyme lactose permease - transports lactose into the E.Coli cell.

When lactose is not present a repressor protein binds with the operator region and also partially covers the promoter region - structural genes are not made.

When lactose is present it binds to the allosteric site of the repressor protein which changes its shape so it cannot bind to the O region. - structural genes transcribed.

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Genes and body plans

Homeobox genes - control the development of the body plan of organisms - includes polarity and positioning of organs.

The genes are almost identical in all organisms e.g. vertebrates, plants and fungi.

Arranged in clusters called Hox clusters.

Retinoic acid activates homeobox genes in vertebrates - too much or too little causes birth defects and deformities.

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Programmed cell death, multicellular organism

1. Enzymes break down the cell cytoskeleton.
2. Cytoplasm becomes dense, organelles are tightly packed.
3. Cell surface membrane changes and small bits (blebs) form.
4. Chromatin condenses and the nuclear envelope breaks. DNA breaks into fragments.
5. Cell breaks into vesicles that are taken up by phagocytosis. Cellular debris disposed of - does not damage cells. 

Quick process controlled by cell signalling (cytokines, hormones, nitric oxide, growth factors).
Proteins are reased into the cytosol, bind to inhibitor proteins, allow the process to occur. 

Tidier than necrosis and doesn't realease hydrolytic enzymes.

Not enough apoptosis - tumours form.
Too much apoptosis - cell loss and degeneration. 

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Meiosis 1

Reduction division, haploid cells produced for sexual reproduction

Prophase 1 - Homologous chromosomes pair up (synapsis), paired chromosomes (bivalents), centriole divides (animal cells only), migrate to opposite poles of the cell, nuclear envelope disappears, spindle formed.

Metaphase 1 - Bivalents line up at the metaphase plate, each is attached to a spindle fibre from opposite poles, pulled towrds equator.

Anaphase 1 - Spindle fibres contract resulting in chromosomes separating to opposite poles, either maternal or paternal chromosomes can pass into either celll - more variation.

Telephase 1 - the nuceolus and nuclear envelope reform, cells divide into 2.

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Meiosis 2

Prophase 2 - centriole divides, nuclear envelope disappears, chromosomes condense, spindles form.

Metaphase 2 - spindle fibres attach to centromeres, pulled to equator, chromatids randomly assorted.

Anaphase 2 - centromere divides, chromatids separate, chromatids randomly segregate. 

Telophase 2 - Nuclear envelopes reform around haploid daughter nuclei, animals - 2 cells divide to 4 haploid cells, plants - a tetrad of 4 haploid cells is formed.

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