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The cell cycle
The processes taking place during interphase (G1, S and G2), mitosis and cytokinesis, leading to genetically
identical cells
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In eukaryotic cells the cell cycle has two main phases – interphase and mitotic (division) phase
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Interphase is where cells spend the majority of their time in
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It’s sometimes known as the resting phase – cells aren’t actively dividing but it is a very
active phase as majority of the functions are carried out (preparing for cell division, producing enzymes and hormones
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 S – synthesis phase: replication of DNA in the nucleus and genetic information is checked
 G₂ – the second growth phase: growth of organelles and cell, energy stores increased and the duplicated DNA is
checked for errors
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This is essential because the DNA
will be genetically identical so both daughter cells receive a full copy of the DNA
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The two chromatids are joined
together at the centromere (region at which two chromatids are held together)
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There are two stages:
 Mitosis – Nucleus divides and chromatids separate
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To ensure two genetically identical daughter cells are created from the parent cell, the replicated DNA needs to be errorfree and the chromosomes need to be in the correct position during mitosis
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Checkpoints are the control mechanisms of the cell cycle
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G₁ checkpoint – This checkpoint is at the end of the G₁, before
entry into S phase
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If not, it enters a resting phase (G₀)
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In order for this checkpoint
to be passed, the cell has to check a number of factors
(picture) including whether the DNA has been replicated
without error (DNA damage)
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Spindle assembly checkpoint/Metaphase checkpoint – This checkpoint is at the point in mitosis where all the chromosomes
should be attached to spindles and have aligned
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The main stages of mitosis
Changes in the nuclear envelope, chromosomes, chromatids, centromere, centrioles, spindle fibres and cell
membrane
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Mitosis is the process of nuclear division where two genetically identical nuclei are formed from one parent cell nucleus
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 Most animal cells are capable of mitosis and cytokinesis but only meristem cells in plants can divide this way
 Plant cells don’t have centrioles – tubulin threads are made in the cytoplasm
 In animal cells, cytokinesis starts from the outside bit
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New cell membrane and cell wall material is laid along this cell plate
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The nucleolus disappears
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Spindle fibres begin to form and
two centrioles migrate to opposite poles of the cell
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By the end, the nuclear envelope
disappears
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Anaphase

The centromere, holding together the pairs of
chromatids in each chromosome, divides
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Telophase

Diagram and photo

Two new sets of chromosomes assemble at each pole
and the nuclear envelope reforms around them
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In animals – A cleavage furrow forms around the middle
of the cell
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In plants – Have cell walls so can’t form cleavage
furrows
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Vesicles fuse
with each other and the cell surface membrane, dividing
the cell into two
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Mitosis is necessary when all the daughter cells have to be genetically identical
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It is also necessary for
asexual reproduction, which is the production of genetically identical offspring from one parent
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It also maintains chromosome number in all cells
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Reduction division is cell division resulting in the production of haploid cells from a diploid cell; meiosis
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The gametes (sex
cells) contain half the diploid number of chromosomes of a cell or the chromosomes number of an organism would double
every round of reproduction
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Each gamete contains
half the number of chromosomes as the number of the parent cell (haploid)
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Haploid is
half the normal chromosome number; one chromosome of each type
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This produces new combinations of alleles
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This produces new combinations of alleles
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Meiosis involves two divisions:
Meiosis I – The first division is the reduction division when the pairs of homologous chromosomes are separated into two
cells
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Meiosis II – The second division is similar to mitosis, and the pairs of chromatids present in each daughter cell are
separated, forming two more cells
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Prophase I is similar to prophase in mitosis
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Homologous chromosomes pairing up, forming bivalents
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This is where sections of DNA become entangled
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Homologous pairs of chromosomes assemble along the metaphase plate
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The maternal or paternal chromosomes can end up facing either pole (Independent assortment), and can result in
many different combinations of alleles facing the poles
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Anaphase I is different to anaphase in mitosis
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Sections of DNA on ‘sister’ chromatids, which become entangled during crossing
over, now break off and re-join – sometimes resulting in an exchange of DNA
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When exchange occurs, this forms recombinant chromatids, with genes being exchanged
between chromatids
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Genetic variation arises from this new combination of alleles – the sister chromatids are no longer identical
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The chromosomes assemble at each pole and the nuclear membrane
reforms
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The cell undergoes cytokinesis and divides into two cells
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In prophase II, the chromosomes (still consisting of two chromatids) condense and become visible again
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Metaphase II differs from metaphase I as the individual chromosomes assemble on the metaphase plate, as in metaphase
in mitosis
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Unlike anaphase I, anaphase II results in the chromatids of the individual chromosomes being pulled to opposite poles
after division of the centromeres – the same as in anaphase of mitosis
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The chromosomes uncoil and form
chromatin again
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Cytokinesis results in division of the
cells forming four daughter cells in total
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They will also be geneticall y
different from each other, and from the parent cell, due to the processes of crossing over and independent assortment
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An allele is different versions of the same gene also known as a gene variant
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A homologous pair is a pair of chromosomes containing a maternal and paternal chromatid joined to together in the same
position at the centromeres
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Members of a homologous pair, pair up in meiosis
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Erythrocytes or red blood cells transport oxygen around the body through the blood
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This is essential to their role of transporting oxygen around the
body
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They are also flexible so that they are able to squeeze
through narrow capillaries
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Their multi-lobed nucleus makes it easier for them to squeeze through small gaps to get to the site of infection
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Squamous epithelium tissue is made up of specialised squamous epithelial cells (sometimes known as pavement
epithelium)
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The squamous cells are held in place by the basement membrane
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It is made up of
collagen and glycoproteins
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Ciliated epithelial tissue is made up of ciliate epithelial cells
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g
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Goblet cells present, releasing mucus to trap any unwanted particles present in the air,
preventing bacteria from reaching alveoli once inside the lungs
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Tar in the smoke also damages the cilia
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Their function is to deliver genetic information to the female gamete, the ovum/egg
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The acrosome on the head of the sperm contains digestive enzymes, which are released to digest
the protective layers around the ovum and allow the sperm to penetrate, leading to fertilisation
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Palisade cell are present in the mesophyll and enables photosynthesis to be carried out efficiently
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The chloroplasts can move within the cytoplasm in order to
absorb more light
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They have thin cell walls,
increasing the rate of diffusion of carbon dioxide
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Root hair cells are present in the surfaces of roots near the growing tips and uptake water and minerals from the soil
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Guard cells regulate the rate of transpiration
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When guard cells lose water and become less swollen as a
result of osmotic forces, they change shape and the stoma closes to prevent further water loss from the plant
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The organisation of cells into tissues, organs and organ systems
To include squamous and ciliated epithelia, cartilage, muscle, xylem and phloem as examples of tissues
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Tissues are a group of specialised cells that work together to perform a
particular function
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An organ
system is a group of organs working together to perform an essential function
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It contains elastin and
collagen fibres composed of chondrocyte cells
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Its flexibility
and strength helps with this
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Skeletal muscle fibres contain micro fibrils which contain contractile
proteins allowing movement
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It is composed of vessel
elements, which are elongated dead cells
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Phloem tissue is another type of vascular tissue that transports organic nutrients, particularly sucrose, from leaves and
stems where it is made by photosynthesis to all parts of the plant where it is needed
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The features and differentiation of stem cells
Stem cells as a renewing source of undifferentiated cells
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Once stem cells
become specialised they lose the ability to divide, entering the Gₒ phase of the cell cycle
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If there is uncontrolled division then they form masses of
cells called tumours, which can lead to the development of cancer
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Potency is a stem cell’s ability to differentiate into different cell types
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Stem cells differ depending on the type of cell they can turn into:
 Totipotent – They can differentiate into any type of cell
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 Multipotent – Can only form a range of cells within a certain type of tissue
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After seven days a mass of
cells, called blastocyst, has formed and the cells are now in a pluripotent state
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Tissues (adult) stem cells are Multipotent and present through life from birth
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Stem cells can be harvested from umbilical cords of new born babies
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Meristems are found wherever growth is occurring in plants e
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tips
of roots and shoots (apical meristems)
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Erythrocytes have a short life span of about 120 days due to the lack of nucleus and organelles, so need replacing
constantly
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Neutrophils only live for about 6 hours and the colonies of stem cells in bone marrow produce in the region of 2
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This figure will increase during infection
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Cells originating
from this region differentiate into different cells present in xylem and phloem tissues
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The pluripotent nature of stem cells in the meristems continues throughout the life of the plant
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The repair of damaged tissues, the treatment of neurological conditions such as Alzheimer’s and Parkinson’s, and
research into developmental biology
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 Treatment of burns – stem cells grown on biodegradable meshes that can produce new skin for burn patients, quicker
than taking a graft from another part of the body
 drug trials – potential new drugs can be tested on cultures of stem cells before being tested on animals and humans
 Developmental biology – study of changes that occur as multicellular organisms grow and develop form a single cell, e
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fertilised egg, and why thing sometimes go wrong
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More recently, the use of embryonic stem cells in therapies
and research has led to controversy
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Life beginning at conception means that using embryos is really murder
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The use of umbilical cord stem cells overcomes this because there is greater availability of cord cells so you are more likely
to find donors
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The cells are at their earlier stage of development
so can be stored for future use and slightly mismatched cord cells work almost as well as using bone marrow cells
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Induced pluripotent stem cells (iPSCs) are adult stem cells that have been genetically modified to act like embryonic stem
cells and so are pluripotent
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