Download Unit 1 - Human Cells

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 mRNA molecule carries the code from the DNA in the nucleus
to the ribosomes.
mRNA
mRNA is a single strand of
nucleotides, which has no
folding along its length,
and therefore can have no
base pairing.
The bases found in mRNA are;
A,C,G and U
A tRNA molecule carries a specific amino acid to the ribosomes
• tRNA is a single strand of
nucleotides which folds back on
itself to make a 3D shape
• Base pairing occurs
• The bases found in mRNA 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 mRNA
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
Glycolysis
ATP
phosphofructokinase
The removal of
hydrogen ions from
substrates is brought
about by
Dehydrogenase
enzymes
Pyruvate
Acetyl CoA
Oxaloacetate
Citrate
Citric acid
cycle
ATP
CO2
3NAD
FADH2
FAD
3NADH
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