Edexcel A2 Biology Unit 4

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.)

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.

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.

• 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

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

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.

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'.

- 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

- 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.

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.

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

Transcription:

- 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

Translation:

- 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

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