Higher Human Biology Human Cells NAR Revision Key

Higher Human Biology Human Cells NAR Revision

Key Area : Division and differentiation in human cells

• During differentiation, genes which are not needed are switched off and genes which are needed in order to become specialised are switched on. • A specialised cell cannot develop into different types of cell because the genes needed are switched off and can’t be turned back on. • Stem cells are relatively unspecialised cells and can develop into different types of cell because most of their genes are still switched on.

For this reason, stem cells can have therapeutic uses such as; skin grafts bone marrow transplants

corneal grafts • Somatic cells are the differentiated cells that form different types of body tissue, eg. blood, bone, muscle, nerve cells. • Germline cells are the gametes and the cells that produce the gametes.

Cancer cells are cells which do not respond to normal regulatory signals that would instruct them to stop dividing when necessary. They continue to reproduce and form a mass of abnormal cells called a tumour. Secondary tumours develop as a result of cells from a tumour detaching from their neighbours and spreading throughout the body.

Key Area : Structure and replication of DNA

DNA consists of 2 strands, made up of repeating units called NUCLEOTIDES. The nucleotide is made of : • Deoxyribose sugar • Phosphate • Organic Base there are 4 different bases: GUANINE CYTOSINE THYMINE The base pairing rule is; A-T, C-G ADENINE

The backbone of each DNA strand is made up of Deoxyribose and phosphate Deoxyribose + phosphate backbone The two strands run anti-parallel to each other The strands are held together by hydrogen bonds between the bases

Double Helix Hydrogen bond

DNA is tightly coiled and packaged around bundles of protein, which then coil to form chromosomes.

Replication Full replication is complete in 6 stages: 1. DNA double helix unwinds 2. Weak hydrogen bonds break causing the 2 strands to separate 3. Free DNA nucleotide joins complimentary pair on open strand 4. Weak hydrogen bonds reform between base pairs 5. Strong chemical bond forms between both nucleotides, controlled by enzyme called DNA POLYMERASE 6. Newly formed daughter DNA (identical to original) begins to wind into double helix

DNA polymerase is only able to add nucleotides to the free 3’ end of a growing strand, and needs a primer to start things off. The strand that has the 5’ end exposed is replicated in fragments starting at the 3’ end These fragments are then joined together by the enzyme LIGASE

Key Area : Gene expression

RNA and Protein Synthesis A m. RNA molecule carries the code from the DNA in the nucleus to the ribosomes. m. RNA is a single strand of nucleotides, which has no folding along its length, and therefore can have no base pairing. The bases found in m. RNA are; A, C, G and U

A t. RNA molecule carries a specific amino acid to the ribosomes • t. RNA is a single strand of nucleotides which folds back on itself to make a 3 D shape • Base pairing occurs • The bases found in m. RNA are; A, C, G and U H Bonds

Key Area : Genes and proteins in health and disease

Gene Mutations Substitution – one nucleotide swapped for another, so only one amino acid changed Insertion – one nucleotide added, sequence wrong from then on, so all amino acids altered from then on (frameshift) Deletion – one nucleotide lost, sequence wrong from then on, so all amino acids altered from then on (frameshift)

Splice-site mutations - a mutation occurring at a splice site, which results in an intron remaining attached to the m. RNA

Missense mutation - following a substitution, the altered codon codes for an amino acid that still makes sense but not the original sense Nonsense mutation – an amino acid codon is substituted with a stop codon

Key Area : Human genomics

PCR (Polymerase chain reaction) • This technique is used to amplify (make copies) of a specific segment of DNA • This amplification of DNA involves the use of primers • A primer is a piece of single-stranded DNA which is complementary to a specific target sequence on the DNA strand being replicated

Stages; 1. Melting DNA is heated to separate the strands 2. Annealing Cooled for primer binding 3. Extending Heated to allow Heat tolerant DNA polymerase to manufacture complementary strands 4. Repeated cycles of heating and cooling amplify this region of DNA

Key Area : Metabolic pathways

Enzyme Inhibition • Inhibitors are chemicals which slow down or stop an enzyme controlled reaction. • They can be competitive inhibitors. • Or they can be non competitive inhibitors.

Competitive and non-competitive inhibitors summary

Comparative effects of increasing substrate concentration in the presence of inhibitors when enzyme concentration is limited competitive– effect depends on relative concentrations of substrate and inhibitor non-competitive – effect depends only on concentration of inhibitor no inhibitor Rate of reaction competitive inhibitor non-competitive inhibitor 0 10 20 30 40 substrate concentration (mg/l) 50 60

Key Area : Cellular respiration metabolic pathways

Glucose ATP Glycolysis The removal of hydrogen ions from substrates is brought about by Dehydrogenase enzymes Pyruvate Acetyl Co. A Oxaloacetate Citric acid cycle ATP CO 2 NADH Electron transport chain ATP (Largest quantity) water

Remember • In stage 1 of respiration, the use of the enzyme phosphofructokinase to make the intermediate compound is irreversible. • This continues the process of glycolysis and is known as the key regulatory point.

Key Area : energy systems in muscle cells

Slow-twitch muscle fibres are good for endurance activities such as rowing, cycling and long distance running Fast-twitch muscle fibres are good for bursts of activity eg. Sprinting, weight lifting