Edexcel IAL Biology A Level
Topic 3: Cell Structure, Reproduction and
Development
Notes
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Scanning electron microscope
concrete
This microscope scans a beam of electrons across the sample which knocks off electrons
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which then form an image.
Advantages: ed
● Higher resolution than optical microscope
● Higher magnification than optical microscope
f ● Forms 3D images
● Can be used on thick specimens devision are
a cell
Chosen by
Disadvantages: divides
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● Lower resolution than TEMs
actively
Transmission electron microscopegg iapayya
dividing
final astage of cell division
This microscope passes electrons through a thin specimen; denser regions bsorb more
when
electrons so less pass through creating a darker area on the image. new cell separates
Advantages:
● Highest resolution
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● Highest magnification
Disadvantages
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● Can see internal structures of organelles mitotic index
may slowdown
● Needsitvery thin specimens or
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● Slides are hard to prepare
Stop cell cycle is controlled by
cyclins
Staining in microscopy ifeng.my
mitotic ind
Stains and dyes are applied to tissue samples and bind to organelles making them easier to
view. Staining increases the contrast in the image formed, this can make it easier to see apart 2
objects that are close together, so increase resolution.
Reproduction chromosomes made
from proven dna thread
Chromosomes histones help package DNA
molee when condense
they
Every cell, except sex cells, has two of each chromosome; one inherited from the mother, one
inherited from the father, this pair is known as a homologous pair. A locus (plural: loci) is the
position of a gene on a chromosome. Homologous chromosomes have the same genes and
loci, however the alleles possessed is different on each chromosome.
Meiosis DNA winds around histone
that form dense clusters are
Key words: nucleons
● Chromatid – When DNA replicates it forms chromosomes made of two identical sister
chromatids, each containing the same copy of genes for that chromosome.
● Gamete – A haploid sex cell
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gametes fuse dig
fertilization to form
● ygote – The diploid cell formed when two gametes fuse zygote
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● Haploid – Describes a cell containing half the usual amount of DNA (for instance sex
cells in humans that contain two chromosomes instead of 46)
● Diploid – Describes a cell containing a complete set of DNA
Variation between organisms that reproduce sexually arises through features of meiosis and
random fertilisation. Meiosis is a type of cell division that gives rise to genetic variation, its role
is to produce haploid gamete cells, which then randomly fuse during fertilisation to form a
zygote with an equal mix of chromosomes from each parent. When the cell is not dividing the DNA
is found as uncondensed strands known as chromatin. Once the DNA has been replicated,
meiosis begins and occurs as follows:
1.
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Prophase I – The chromatin begins to condense and shorten, forming chromosomes.
The nuclear envelope surrounding the DNA breaks down so the chromosomes are free in
n the cytoplasm. Also, spindles (protein strands that move the chromosomes) are made by
the centrioles.
2. Metaphase I – The chromosomes are pushed to the centre of the cell by spindle fibres
and line up in homologous pairs.
3. Anaphase I – The spindle fibres contract and shorten and move the chromosomes to
opposite poles of the cell so one chromosome from each pair is at either end.
4. Telophase I – two nuclear envelopes form around each set of chromosomes.
5. Cytokinesis – the cytoplasm divides to form two cells. when new cell forms
The process then repeats for each of the two cells formed from the first division, this is known as
meiosis II. However, during metaphase II there is only one chromosome from each pair (instead
of two in metaphase I) so the chromosomes line up on their own, and in anaphase II the
enEmbers
chromatids of each chromosome are separated. This results in 4 non-identical, haploid
daughter cells. tr
ined
in gametes from diploid to haploid
Genetic Variation
Two features of meiosis contribute to genetic variation in addition the variation created by random
fertilisation of two unique gametes from different parents.
●
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Independent assortment – occurs during metaphase I – the order the chromosomes line
up in (i.e. which side the maternal and paternal chromosomes line up in) in their pairs is
random, meaning the combinations of chromosomes going into the daughter cells is
random.
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● Crossing over – occurs during prophase I – The relatively rare process whereby
homologous chromosomes swap portions of their chromatids, which results in mixing of
the parental genetic information in offspring chromosomes and new allele combinations.
The structure formed by the homologous chromosomes formed during crossing over is
known as a bivalent.
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Gene linkage
If two genes are located on the same chromosome, then they will be
inherited together, as during meiosis, one whole chromosome is
passed to the gamete (except in the case of crossing over during
meiosis), this is known as chromosome linkage.
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For instance, if a dog has the genes for hair length and hair colour
on the same chromosome; and one of its homologous
chromosomes contains the alleles for long and brown hair, and the
second chromosome contains the alleles for short and blond hair,
then the only allele combinations it can pass on to offspring are still
long and brown, or short and brown - as one chromosome
containing both is passed on to each haploid gamete cell.
Mammalian gametes and fertilisation
Egg cell:
● It contains zona pellucida which is a
protective coating which the sperm have to
penetrate in order for fertilisation to occur, the
main purpose of zona pellucida is to stop
more than one sperm fertilising the egg.
● It contains a haploid nucleus EEso that a full
set of chromosomes is restored at fertilisation
● Cortical granules are organelles that release substances which cause the zona pellucida
to harden.
● Follicle cells form a protective coating around the egg.
Sperm cell:
● Sperm cells contain a lot of mitochondria to
provide the energy for rotation of the
flagellum which enables it to move and swim
towards the egg.
● Acrosome contains digestive enzymes which
break down the zona pellucida and allow
sperm to penetrate the egg.
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Fertilisation in mammals occurs as following:
1. The sperm head meets the protective jelly layer around the egg cell called the zona
pellucida and acrosome reaction occurs – enzymes digest the zona pellucida in order to
enable sperm to reach the egg.
2. The sperm head fuses with the cell membrane of the egg cell thus allowing the sperm
nucleus to enter the egg cell.
3. Cortical reaction occurs which causes the zona pellucida to harden therefore preventing
other sperm from entering the egg cell.
4. The nuclei fuse and a full set of chromosomes is restored thus creating a diploid zygote.
Flowering plants
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Structure of flowers
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The image above illustrates a generalised flower structure. The stamen is the male part of the
plant consisting of a long filament with anthers at the end, which are involved in the production
of male gametes in the form of pollen grains. The carpel is the female part of the plant which is
the site of ovule development.
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The process in which pollen grains produced by anthers are transferred to female reproductive
organs of a plant in the form of stigma is known as pollination. Pollination can either occur with
the help of wind or insects. The product of fertilisation is the seed which then develops into a
fruit.
Double fertilisation
Plant fertilisation occurs as following:
1. Pollen grain composed of the pollen tube cell and the generative cell adheres to the
stigma, where it subsequently germinates to produce a pollen tube.
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2. The pollen tube grows down the stigma, secreting digestive enzyme which digest the
surrounding tissue and use it as a source of nutrients.
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3. The pollen tube grows through a gap between the integuments, known as the microphyle,
into the embryo sac.
4. The generative cell of the pollen divides to produce two sperm cells which enter the
embryo sac.
5. One of the male gametes fuses with the female nucleus to form a zygote.
6. The other male gamete fuses with two polar nuclei to form an endosperm nucleus
which serves as a source of nutrients for the embryo.
7. The fertilised ovule divides by mitosis to form the embryo consisting of the developing
shoot known as the plumule, developing root known as the radicle and one or two
cotyledons. The integuments become the seed coat, the ovule becomes the seed and
ovary becomes the fruit.
Mitosis
Mitosis is the asexual process by which all somatic cells (non-sex cells) divide to produce new
cells, so that organisms can grow and repair and replace damaged cells. Mitosis occurs by the
same stages as meiosis:
1. Prophase – The chromatin begins to condense and shorten, forming chromosomes. The
nuclear envelope surrounding the DNA breaks down so the chromosomes are free in the
cytoplasm. Also, spindles (protein strands that move the chromosomes) are made by the
centrioles.
2. Metaphase – The chromosomes are pushed to the centre of the cell by spindle fibres
and line up on their own.
3. Anaphase – The spindle fibres contract and shorten and move the chromosomes to
opposite poles of the cell so one chromosome from each pair is at either end.
4. Telophase – two nuclear envelopes form around each set of chromosomes.
5. Cytokinesis – the cytoplasm divides to form two cells.
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Mitosis ends after one division so that the two resulting daughter cells are diploid. Since the
resulting cells have no mixing or combining of genetic information, they are genetically
identical to each other and the parent cell, making mitosis the ideal process for normal growth in
organisms.
importance of Mitosis it's how organisms grow
The cell cycle
and replace old Celle MIRGAS
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The cell cycle is the series of stages a cell goes through in its lifetime and consists of 4 main
stages, with cells spending around 10% of the cycle in mitosis, the other 90% in the other
stages that make up interphase:
● Growth 1 – The cell grows, synthesises proteins and carries out its function.
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● S phase – The cell carries out its usual function as well as replicates its DNA in
preparation for mitosis.
● Growth 2 – The cell continues to grow and synthesise proteins, as well as making
proteins needed for cell division such as spindle fibres.
● Mitosis – The cell divides.
Calculating the mitotic index
Mitotic index is a measure of the proportion of cells that are dividing in a tissue sample and is
calculated when observing cells under a light or electron microscope. Cells such as those in skin,
which replace cells quickly, have a high mitotic index.
It is calculated using the following formula:
Number of cells with visible chromosomes ÷ total number of cells observed.
Stem cells
Stem cells are undifferentiated cells which can keep dividing to give rise to other cell types in
a process known as specialisation. There are 4 types of stem cell - totipotent, pluripotent,
multipotent and unipotent stem cells.
Totipotent cells can give rise to all types of specialised
Bandsmani cells including placental cells and
pluripotent cells are able to give rise to many types of specialised cells apart from placental cells.
During development, totipotent cells translate only part of their DNA, resulting in cell
specialisation.
A morula is an early-stage embryo formed 3-4 days after fertilisation. It consists of 16 cells only
and contains totipotent cells that can differentiate into all cells including the placental cells. By 4-5
days after fertilisation the morula has developed into a blastocyst - a mass of 200-300 cells that
contains an inner cell mass which develops into the embryo. Cells present in the blastocyst are
pluripotent.
Totipotent cells only occur in the morula stage in mammalian embryos whereas other type of
stem cells such as pluripotent, multipotent and unipotent cells are found in mature mammals.
Pluripotent stem cells are commonly used in treating human disorders by replacing damaged
tissue. Unipotent cells, such as cardiomyocytes can only differentiate into one cell type. Potency
of cells tends to decrease with age - when you are older, you have fewer pluripotent and
multipotent stem cells.
Sources and uses
Sources of stem cells include embryonic stem cells, adult stem cells and fused cells. Stem
cells can be used to treat a variety of diseases such as diabetes, multiple sclerosis and
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