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Unit 18 Assignment 2 Mitosis and Meiosis
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- PEARSON (PEARSON)
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Mitosis and MeiosisAssignment 2
Mitosis is the term which is given to the division of the cell nucleus into two equal sets of
chromosomes and is used to describe the distribution and duplication of chromosomes,
chromosomes are the structures which carry genetic information of the body. Mitosis is a process
whereby one cell (the mother cell) divides to produce two new cells (the daughter cells) which are
genetically identical to itself. Most cell divisions that occur in the body involve mitosis. An organism’s
body will become populated with cells throughout an organism’s life through the mitosis process
majorly through growth and development as it replaces worn-out old cells with new ones. The aim
of mitosis is to ensure each daughter cells gets a full set of perfect chromosomes. This is important
as too little or too many chromosomes normally have an affect on functions, they may either cause
cancer or may not even survive. Cells which are undergoing mitosis don’t just divide their DNA
randomly and go into two daughter cell piles. Therefore, to try and prevent this as much as possible
they split up the duplicated chromosomes in a series of steps which are carefully organised.
DNA replication is followed by mitosis, so when mitosis is beginning each chromosome has already
been replicated. Firstly, the beginning of mitosis gradually occurs when each chromatin thread coils
into a structure which is more compact, this cannot be viewed at first with the light microscope but
as the packing proceeds, the chromosomes will become visible and will be able to be viewed as long,
thin threads which are tangled together. Packing proceeds even further and this causes
chromosomes to become more identifiable and looking thicker and shorter. Chromosomes each can
be viewed to consists of two sister chromatids which are twisted around one another. A chromatid is
one copy of a chromosome which is newly copied and is still joined to the original chromosome by a
single centromere (region of the chromosome.) During the time the chromosomes are becoming
shorter, a structure created from thin fibres of proteins start to form alongside the nucleus within
the cytoplasm. Usually this structure cannot be identified under an ordinary light microscope this is
due to the fibres being so thin. The shape of the structure which forms in the cytoplasm is known as
a spindle. The spindle is a microtubule-based bipolar structure which main function is to segregate
the chromosomes in mitosis. Its primary function is playing a crucial role in manoeuvring
chromosomes during mitosis. It contains poles which consist of centrosomes. Protein fibres which
radiate out from the poles of the spindle are known as the “lines of longitude”. The equator is the
middle widest point present in the “lines of longitude” this is where chromosomes are all lined up to
ensure their segregation and bi-orientation is correct. The length of spindle fibres vary as some may
stretch all the way from one pole to another, whilst some extend to just about half ways whilst
others may become attached to the chromosomes. A nuclear envelope is also present, a nuclear
envelope is a membrane barrier which contains many proteins and is deemed as highly regulated it
separates the cytoplasm from the nucleus in eukaryotic cells. This nuclear envelope begins to break
up whilst the spindle forming. Due to the nuclear envelope breaking up he chromosomes are no
longer confined and have more space to move.
Each chromosome, two sister chromatids are attached to one another at a point called the
centromere, chromosomes also attach to the spindle fibres here. One fibre is attached to each side
of the centromere whilst each fibre is then stretched to one of the poles. Therefore, each
chromosome becomes suspended from two poles off the spindle. Centromeres are pulled by the
spindle fibres this way and this keeps on occurring until the centromeres are pulled into the middle
and become arranged around the spindle’s equator. At this point the chromosomes are at their
thickest and shortest point. Depending on the orientation of the spindle sometimes chromosomes
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