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A LEVEL BIOLOGY

EDEXCEL (SNAB)

CELLS
Prokaryotes, Eukaryotes, Fertilisation, Cell Cycle, Protein Trafficking, Stem Cells, Variation
and more
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g
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Scanning electron microcope (SEM)
2
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5-5um / E - 40um
GENETIC MATERIAL: P - DNA circular and free-floating / E - DNA linear and associated
with protein, in the nucleus
CYTOPLASM: P - Contains few organelles / E - contains many organelles
CELL WALL: P - Peptidoglycan / E - No cell wall around animal cells
PROTEIN SYNTHESIS: P - Small free-floating ribosomes / E - Large ribosomes, may be
attached to ER
ORGANELLES: P - No membrane-bound organelles / E - membrane-bound organelles
AEROBIC RESPIRATION: P - Mesosomes / E - Mitochondria
DYNAMIC CELLS:
•
Cell wall not static & constant movement of molecules within it
•
Membrane structures within the cell are continually created and lost
Intracellular - protein used inside cell
Extracellular - protein used outside cell
PROTEIN TRAFFICKING:
1
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mRNA leaves the nucleus via a nuclear pore
3
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Protein moves through the ER assuming 3D en route
5
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Vesicles from rough ER fuse to form the flattened sacs of the Golgi apparatus
7
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Vesicles pinched off the Golgi apparatus contain the modified protein
9
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Sperm reach ovum
2
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Acrosome swells, fusing with cell surface membrane
4
...
Enzymes digest through follicle cells and Zona Pellucida (jelly-like layer)
surrounding ovum
6
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Sperm nucleus enters ovum
8
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Nuclei of ovum and sperm fuse
After fertilisation, cell is zygote
Now diploid with genetic material from mother and father
Now divides by mitosis to make many specialised cells that make up an organism

CELL DIVISION
DNA in eukaryotic cells is linear and found in association with proteins
Coied onto proteins called histones - how tightly coiled it is affects whether or not DNA is
active
When the chromosomes form in prophase, the DNA is tightly coiled onto histones, folded
back and forward onto a protein scaffold to condense
Formation of haploid gametes
Gives rise to genetic variation by 2 main processes:
1
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Crossing over of chromatids
This ensures each gamete is different
MEIOSIS 1:
Both processes occur in Meiosis 1 - pairing of homologous chromosomes occurs
Crossing over involved forming of chiasma between sister chromatids and the exchange of
maternal and paternal chromosomes
Independent assortment occurs when the chromosomes line up on the equator in their
homologous pairs
This is random - any member of the pair can be used in any gamete
Crossing over leads to new combinations of alleles, independent assortment to new
combinations of chromosomes
However - not all genes mix equally, some ‘go together
The position of a gene on a chromosome is its locus
If alleles are on separate chromosomes or far apart on a chromosome they are inherited
independently
LINKAGE:
If the alleles have loci that are close together on the same chromosome they may be
inherited together - linkage
23 chromosomes so 23 linkage groups

Sex linkage occurs when the gene is carried on the X chromosome
Red-green colour blindness: 8% men / 0
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Prophase
2
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Anaphase
4
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Transctription of DNA to mRNA
2
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Protein made on ribosome enter rough ER
4
...
Vesicles pinched off the rough ER contain the protein
6
...
Proteins are modified within the Golgi apparatus
8
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Vesicles fuse with the cell surface membrane releasing the protein
ROOT TIP SQUASH
1
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3
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5
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7
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Coli baceria
Can use different carbohydrates as food sources and only produce the enzymes when
carb is present in surrounding medium
To use lactose, the bacteria must produce B-Galactosidase to convert lactose -> glucose
and galactose
INDUCTION:
•
when lactose not present, a lactose repressor molecule binds to DNA and prevents
transcription of gene
•
when lactose is present, the repressor molecule is prevented from binding to DNA
•
Biochemical gene switching on the gene - lactose
•
Operator + genes = lac operon
CELL ORGANISATION:
Specialised cells group into tissues
Have specific recognition proteins - adhesion molecules - on cell surface membranes
Help cells recognise similar cells and stick to them
Part of recognition protein embedded in cell surface membrane - rest sticks up
Exposed section bins to complementary protein on adjacent cell
If cells from different tissues separated then mixed together, they reform into tissues as
recognition proteins bind
CELL - in multicellular organisms, cells specialised for particular function
TISSUE - group of cells working together to carry out one function
ORGAN - group of tissues working together to cry out one function
ORGAN SYSTEM - group of organs working together to carry out particular function
VARIATION: (nature and nurture)
Phenotype differences caused by:
•
genotype (genetic make-up)
•
combination of environment organism develops in, and genotype
DISCONTINUOUS:
•
characteristics completely controlled by organisms genotype
•
e
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blood group - A B AB O
•
characteristics controlled by single genes
CONTINUOUS:
•
affect by genotype and environment
•
controlled by many genes (polygenes) - polygenetic inheritance
•
e
...
height, skin and hair colour, cancer causes
Height:

•
increasing over recent generations
•
less inbreeding, better nutrition, improved health, end of child labour
•
height determined by interaction of genes and environment - malnutrition
Skin and Hair Coloud:
•
controlled exposure to sunlight helps make vitamin D 0 needed for uptake of
calcium into teeth and bones
•
melanin provides UV damage protection to DNA
•
enzyme tyrosinase catalyses first step in metabolic pathway to convert amino acid
tyrosine to melanin

VARIATION
Melanin is made in melanocytes (special skin cells)
Have receptors for MSH (melanocyte-stimulating hormone) on surface
Process:
Receptors get the MSH and melanin made and put into melanosomes, transferred to skin
cell
Melanosomes collect around nucleus, protecting it from UV light damage
Increased exposure to UV light:
Increases the amount of MSH and MSH receptors making the melanocytes more active
More melanin produces ,so more melanosomes collect around the nucleus of the skin cells
and give more protection
CANCER:
Occur when the rate of cell multiplication is greater than the rate of cell death
Causes the growth of a tumour
Usually caused by damage to the DNA, e
...
UV light, asbestos, or chemical carcinogens
from the environment or cell metabolism
Cancers more likely in older people as they have accumulated more mutations
CELL CYCLE:
Cells go through a fixed sequence:
•
G1
•
S
•
G2
•
M (mitosis)

Progression between phases is controlled
Cancer cells don’t respond to the controls
2 types of genes involved in this control - oncogenes and tumour suppressor genes
ONCOGENES:
Code for proteins which stimulate cell cycle
Mutations in these can cause cell cycle to become continually active
May cause excessive cell divisions resulting in a tumour
TUMOUR SUPPRESSOR GENES:
Produce suppressor proteins that stop the cycle
If these m utter, there will be no break in the cycle, so cell can’t replicate its DNA
Control of cycle is lost - linked to skin, colon, bladder, breast cancers
INHERITED CANCERS:
5% occur due to inherited gene
Mutations in BRCA1 gene predispose a person to breast cancer
Funcitoning allele produces a protein used to repair DNA
Single defective allele = more susceptible, not inevitable
ENVIRONMENTAL CAUSES OF CANCER:
Environmental damage can be physical or chemical
Greatest chemical risk - smoking - increases the likelihood of many cancers, especially
lung
Tar lodges in bronchi and causes damage to DNA in surrounding epithelial cells
UV light can physically damage DNA
If tumour isn’t removed, cancer cells may spread - METASTASIS
ENVIRONMENT - DIET & VIRAL:
Fresh fruit and veg provide antioxidants which destroy radicals
Radicals - chemicals produced by cell’s metabolism which contribute to aging and cancer
through DNA damage
Some cancers caused by viruses - cervical cancer by genital wart virus
ENVIRONMENT & EPIGENOME
DIET:
Methylation of agouti gene in mice (coat colour) means it isn’t expressed and mouse is
healthy
If its not methylated, its expressed, mouse is yellow and obese
If a pregnant yellow mouse is fed a methyl rich diet, her young will be slim, healthy and
brown - suggests an epigenetic change in early development
BEHAVIOUR:
Lab rat grooming

Good mothers groomed pups and were calm (high LG), bad mothers didn’t and were
nervous (low LG)
Pups grew up similar to mothers
Pups swapped from each group - offspring from high LG brought up by low LG, became
nervous (& vice versa)
2 groups of pups are epigenetically different - gene switched in low LG pups due to
methylation
No receptors produced to remove a stress hormone glucocorticoid) from blood stream so
rats are anxious
EPIGENETIC CHANGES DURING CELL DIVISION:
When a cell of a particular type is produced, all genes relevant to that type are switched
on, others switched off
When DNA for this cell is replicated, epigenetic markers copied so same cell type is
formed
Egg and sperm are specialised cells so have epigenetic changes to their genome
When fertilised egg cell divides to form totipotent stem cells, likely epigenetic changes are
removed
Now thought some epigenetic changes pass from parent to child or grandparent to child



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