Edexcel A2 Biology Unit 4

A selection of the most difficult topics in unit 4 (Topics 5 and 6)

  • Created by: Eleanor
  • Created on: 12-06-11 18:44

DNA profiling.

Non coding blocks in DNA are called introns, and the coding regions are called exons.

In introns DNA sequences are repeated many times, this is called short tandem repeats (STR's).

A DNA profile is produced using gel electrophoresis, In which DNA fragments produced by restriction enzymes can be separated according to their size.

- A single band shows when a persons maternal and paternal chromosomes have the same number of repeat units.

- two bands occur when the two chromosomes have a different number of repeats at a locus. ( a locus is a place where the same STR's occur.)

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Polymerase chain reaction is used to copy DNA.

Cycle 1:

- A sample of DNA is added to detergent to release the DNA from cells.

- DNA polymerase, DNA primers with fluorescent markers and nucleotides are added.

- (at 95 degrees) the DNA splits into two strands.

- (at 55 degrees) primers attach at the start of the STR repeated sequence.

- (at 70 degrees ) DNA polymerases attach nucleotides are added the DNA sequence is replicated.

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DNA profiling cont.

How a DNA profile is made using gel electrophoresis:

- double-stranded DNA is added to a restriction enzyme which cuts the DNA into fragments.

- fragments of double-stranded DNA are loaded into the wells of an agarose gel in a tank using micropipette.

- the negatively charged DNA moves towards the positive electrode. The fragments separate into the invisible bands.

- DNA is transferred to a nylon or nitrocellulose membrane by solution drawn up through the gel. DNA double strands split and stick to the membrane.

- Membrane placed in a bag with DNA probe. Single-stranded DNA probe binds to fragments with a complimentary sequence.

- if DNA is radioactive x-ray film is used to detect fragments, if fluorescent it is seen using UV light.

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  • are prokaryotic

Made from:

- cell wall

- cell surface membrane

- ribosomes = site of protein synthesis

- capsule = a mucus layer for protection

- flagellum = used for cell movement

- mesosome = in-folding of the cell surface membrane, site of respiration

- main circular DNA

- plasmids = small circles of DNA

pilus = protein tubes that allow bacteria to attach to surfaces

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Consist of:

- a strand of DNA or RNA (Viral DNA can be single or double stranded.)

- protein coat

How viruses reproduce:

- virus attaches to a hosts cell

- virus inserts nucleic acid

- the viral DNA replicates

- viral protien coats are made

- new virus particles are formed

- virus particles are released

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Specific immunity.

B lymphocytes:

- secrete antibodies in response to antigens

- special protein molecules of a class known as immunoglobulins

- B cell produces antibodies which which bind to bacteria with antigens on surface this labels them as 'non self'

- antibody binds to antibody receptor on a macrophage

- macrophage engulfs antibodies and bacterium

- lysosomes fuse with vacuole releasing enzymes which destroy the bacteria.

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Specific immunity cont.

T lymphocytes: 

are produced in the bone marrow. they have specific antigen receptors which bind to antigens with the complimentary shape.

- T helper cells = stimulate B cells to divide and become cells capable of producing antibodies. also enhance the activity of phagocytes.

- T killer cells = destroy any cells with antigens on the surface that have been labeled as 'non self'.

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Activating T helper cells.

- bacterium with antigens on surface

- bacterium engulfed by macrophage

- macrophage presents antigens on its surface and becomes an antigen presenting cell (APC)

- APC binds to T helper cell with complimentary CD4 receptors

- the T helper cell is activated and divides

- clone of T memory cells and clone of active T helper cells are produced

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Clonal selection

- bacterium with antigens on surface

- antigen binds to B cell with complimentary receptor

- B cell becomes an antigen-presenting cell (APC)

- activated T helper cell with complimentary receptor binds to APC and produces cytokines (proteins) that stimulate B cell.

- the B cell divides to give B memory cells and B effector cells

- B effector cells differentiate into plasma cells

- plasma cells secrete antibodies which bind to antigens identifying them for destruction.

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The role of T killer cells.

Bacterium infects cell of host

- the cell presents the antigens and becomes an APC

- T killer cell with complimentary receptor binds to the APC

- the T killer cell divides to form two clones active T killer cells and memory T killer cells. Cytokines from T helper cells stimulate the differentiation.

- The active T killer cells bind to infected cells presenting antigens.

- T killer cell releases chemicals that cause pores to form in the infected cell causing it to explode.

- The infected cell dies.

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HIV invades T helper cells

HIV invades T helper cells within the immune system:

- gp120 bind to the CD4 receptors on the surface of T helper cells

- they then combine with a second receptor allowing the envelope to fuse with the T helper cell membrane

- the viral RNA then enters the cell

- macrophages also have CD4 receptors so the virus can infect them too

How the virus replicates:

-uses an enzyme called reverse transcriptase

- it makes a DNA copy of the RNA

- the DNA is copied to make a double strand that can be inserted into the human genome

- it is then integrated into the hot cell's genome using intergrase

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Transcription and Translation.


- RNA polymerase attaches to the DNA

- hydrogen between the paired bases break and the DNA unwinds

- RNA nucleotides with complimentary bases to the ones on the template strand bond together forming mRNA

- mRNA then leaves the nucleus through a pore in the nuclear envelope


- mRNA attaches to a ribosome

-the anticodons on tRNA are complimentary to the mRNA codons for the amino acid

- free amino acids attach to the correct tRNA molecule which carry it to the ribosome

- the anticodons bind to the codons and form a chain of amino acids which are held together with a peptide bond

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HIV destroys T helpers.

HIV binds to cell receptors virus envelope fuses with cell surface membrane

- virus reverse transcription copies viral RNA into viral DNA

- intergrase inserts viral DNA into host DNA

- transcription occurs

- translation of virus envelope proteins

- virus envelope proteins are incorporated into the cell membrane

- the virus mRNA is translated

- virus particle budding becomes wrapped in cell membrane, forming the virus protein

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