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Cell
division 6.
Sexual reproduction and cell division by meiosis
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Sexual reproduction and cell division by meiosis
Biology exam study revision notes
on meiosis and sexual reproduction, cell division by meiosis, diagrams
explained.
Key words like haploid, diploid, chromosome numbers, zygote
gametes, sperm, eggs, identical twins
Reminders: A chromosome as a thread-like structure of DNA,
carrying genetic information in the form of genes.
A gene is a length of DNA that codes for a protein. An allele
as a version of a gene.
Reminder that in the biological
science of genetics, inheritance is the transmission of genetic
information from one generation to the next generation by chromosomes of
DNA.
Summary of some important points concerning meiosis
Meiosis is nuclear division giving rise to cells that are
genetically different.
Meiosis is involved in the production of gametes (haploid
sex cells).
Meiosis is a reduction division in which the chromosome
number is halved from diploid to haploid resulting in genetically different
cells.
Meiosis explains why variation is produced by the formation
of new combinations of maternal and paternal chromosomes.
Sexual reproduction
Know the cells in reproductive
organs: testes and ovaries in humans, divide to form gametes (see diagrams
below).
Sexual reproduction is when genetic
information is obtained from two organisms - a 'father' and a 'mother' - two
sources of DNA combining to produce offspring which
genetically different from either parent - though individual
characteristics are passed on.
Sexual reproduction means the
offspring inherit features from both the mother and father and the mix
of chromosomes determines the outcome!
It therefore means that you get
variation in the offspring as well as similarities in the
phenotypes.
In sexual reproduction involves sex cells
called gametes which are produced by meiosis, a different type
of cell division than
mitosis described in a previous
section.
Gametes are haploid cells because they only have
one
copy of each chromosome - half the number of chromosomes of a normal
cell (full compliment in a diploid cell) - remember all the DNA genetic
information is bundled together in genes which make up the structure of
chromosomes.
Simplified diagram of sperm cell
The male gamete is the sperm produced
in the testes.
Note: In the sexual reproduction in
plants, the male gametes are called pollen.
Simple diagram of egg cell
Note: In the sexual reproduction in
plants, the female gametes are called ovules.
For more details of egg and
sperm structure see
Stem cells and an introduction to cell specialisation notes.
When gametes fuse together (e.g. egg plus
sperm) you then get the correct number of chromosomes in the fertilised cell
from which the organism develops.
After interphase (during which the
chromosome number has doubled), two meiotic divisions occur.
Body cells have two
sets of chromosomes but sex cells (gametes) have only one set.
Gametes only contain
half the number of chromosomes found in body cells (one chromosome from each
pair).
The female gamete is the egg cell
(ova/ovum) produced in the ovaries.
Meiosis is the process of human cell division of diploid
cells (of 46 chromosomes) that occurs in the testes and ovaries and results
in the formation of the reproductive cells, the sperm and the ovum.
In meiosis four 'daughter cells' are formed after two rounds of division,
each of the sperm cells and egg cells has 23 chromosomes (haploid)
At
fertilisation, maternal and paternal chromosomes pair up, so the
zygote has the normal chromosome number.
A male gamete nucleus fuses with an egg cell
nucleus and the fertilised egg is called the
zygote.
The zygote cell contains the
full set of 46 chromosomes and is therefore a diploid cell.
Chromosomes from the mother pair up
with chromosomes from the father - that's why you end up with the zygote
having a full set of chromosomes.
So in each pair of chromosomes,
one is from the farther, and one is from the mother.
The zygote undergoes multiple cell
division by mitosis as it develops into the
embryo, a ball of
cells that implants itself into the wall of the uterus. (development
of embryo is continued on the notes on human reproduction).
(see the summary diagram above -
details of meiosis further down the page).
Zygote cells are diploid because they have
two copies of each chromosome e.g. human cells have 23 pairs of chromosomes
(46 chromosomes in total).
Gamete cells contain one copy of
each chromosome (23 in human haploid cells).
The zygote/embryo inherits
characteristics from both parents ('father' and 'mother') because it
is derived from a mixture of two sets of chromosomes - therefore
inheritance from two sets of genes, half from the mother and
half from the father.
This mixture of genetic coding
(genetic information of the DNA) produces the variation of
phenotypes in the offspring - just look around at your own family!
The formation of
gametes (sex cells) by meiosis
Unlike mitosis, where two
identical daughter cells are produced with the full compliment of
chromosomes (diploid), meiotic cell division reproduces four
non-identical cells with only half the normal number of chromosomes
(haploid).
Know and understand the type of cell division in
which a cell divides to form gametes is called meiosis
(details below).
In order to make gametes with half the
original chromosomes, cells divide by meiosis, a process which involve two
cell divisions. In humans, this can only happen in the reproductive organs -
sperm cells in the testes of males and egg cells in the female ovaries.
Meiosis is a type of cell division
that produces genetically different cells with half the chromosomes of the
original parent cells.
In humans meiosis only happens in the
reproductive organs - the female ovaries and male testes.
1. Before meiosis can
happen the cell goes through an interphase period in which the
DNA is duplicated - duplicating the genetic information.
The process then starts with a diploid cell in
which the DNA has been replicated to form X-shaped chromosomes (so has two
'two armed' copies of each chromosome).
Each arm of the X-shaped
chromosomes is an exact copy of the other arm.
Note that in the 'starter' parent cell,
half of the chromosomes came from the organism's father and half
from the organism's mother.
Note the numbers of
chromosomes are given in (brackets) in terms of human
organisms.
In the case of human organisms,
the initial number of chromosomes is 46 in the diploid cell, BUT,
temporarily, is doubled to 92 in the interphase prior to the first
meiotic cell division.
2. For
the first meiotic cell division, the chromosomes line up in pairs, held by very
fine fibres in the centre of the cell.
One chromosome in each pair comes
from the mother organism and the other from the father organism.
The first cell division has
involved the prior duplication of the DNA - doubling the
chromosomes.
3. The pairs of chromosomes are then pulled
apart to form two groups, each encased in a nuclear membrane, so forming two
separate nuclei (temporarily within the same cellular membrane).
Each new cell has only one copy
of each chromosome, but, some of the
father's chromosomes and some of the mother's chromosomes go into
each new gamete cell.
This process is much more complicated than shown in the
diagram and the alleles can get quite mixed up creating considerable
genetic variation in the offspring (see also
5.).
4. The cytoplasm divides in two, completing
the first cell division, again, noting that some of the male's chromosomes and some
of the female's chromosomes go into each new cell - this very important
because this creates genetic variation.
This also means that each new
cell only has half the chromosomes of the original parent
cell.
5. The
2nd cell meiotic division is a bit like
mitosis
(full details not shown on the above diagram,
and the daughter cells are NOT identical).
The chromosomes will line up
again in the centre of the cell and the arms of the chromosomes
pulled part.
From each of the two cells from
stage 4., two new nuclei are formed and the cytoplasm divides to
give 4 haploid cells with their own surrounding membranes.
Note that this 2nd cell division
does NOT involve the replication of DNA.
This 2nd cell division produces four haploid gamete cells, each with their own
unique single set of chromosomes.
Each of the gametes is
genetically different from the three others because the
chromosomes get 'shuffled around' in the process of
meiosis as each gamete gets half the chromosomes in a randomised
way.
In the case of human organisms,
each haploid sperm/egg cell has 23 chromosomes - a single set of
chromosomes.
This is a further source of genetic variation when gamete cells
combine in sexual reproduction because each of the four haploid gamete cells is
genetically different
from the others.
So the division of a cell by meiosis
as the production of four daughter cells, each with half the number of
chromosomes, and that this results in the formation of genetically different
haploid gametes.
This double cell division process is called
meiosis and only occurs in the reproductive organs.
Because these haploid gamete cells have
different single sets of chromosomes, it explains why sexual reproduction
produces genetic variation.
Gamete cells contain one copy of
each chromosome (23 in human haploid cells).
In human sexual reproduction two
gametes (sex cells) combine to form a new individual with the full
compliment of chromosomes (46 in human diploid cells, 23 from mother's egg -
female DNA,
23 from father's sperm - male DNA) and, because the offspring cells have a mixture of
the two sets of male and female chromosomes, each new individual is unique
in genetic and phenotype character.
A
new individual then grows and develops by this cell repeatedly dividing by
mitosis.
The fertilised cell has 23 + 23 = 46
chromosomes and inherits characteristics from both parents (male +
female).
Diagram reminder of the overall
process from gametes to offspring
After the gametes have fused together
in the fertilisation process, the resulting fertilised cell divides by
mitosis i.e. it makes a copy of itself.
The mitosis cell division is
repeated many times and all these new cells develop the embryo.
Mitosis occurs rapidly in a newly
fertilised egg.
As an embryo develops, these new
cells begin to differentiate into all the different types of
specialised cell that an organism needs to develop and mature.
Extra notes:
Identical twins from
sexual reproduction are
genetically identical because they are derived from a single zygote cell that
splits in two by mitosis and then two separate embryos develop.
See also
Hormone systems - human reproduction
systems, menstrual cycle and
pregnancy
Genetics - inherited characteristics and human sexual
reproduction
Key points -
Summary of ideas
Based on
the syllabus-specifications for students taking the AQA, Edexcel and OCR
GCSE level biology examinations (~US grades 9-10).
Meiosis and Sexual Reproduction
Meiosis is a type of cell division
that produces gametes (sex cells) for sexual reproduction.
It ensures genetic
variation by creating cells that are genetically different from each other.
This
process is essential for maintaining the correct chromosome number in offspring.
Key Concepts about
sexual reproduction
1.
Chromosome Numbers
-
Diploid (2n):
Cells with two sets of chromosomes (one from each parent). In humans,
diploid cells have 46 chromosomes.
-
Haploid (n):
Cells with only one set of chromosomes. Gametes (sperm and egg cells) are
haploid, containing 23 chromosomes.
2.
Meiosis – The Process
Meiosis occurs in reproductive
organs (testes and ovaries in animals, anthers and ovaries in plants). It
consists of two divisions, resulting in four haploid
daughter cells.
Meiosis I
-
DNA Replication:
Chromosomes duplicate, forming X-shaped structures.
-
Chromosome Pairing:
Homologous chromosomes (one from each parent) pair up.
-
Crossing Over:
Sections of DNA are exchanged between homologous chromosomes, increasing
genetic variation.
-
First Division:
Homologous chromosome pairs are separated into two new cells.
Meiosis II
-
Second Division:
The sister chromatids of each chromosome are separated.
-
Four Haploid Cells:
Each gamete has half the original chromosome number.
3.
Gametes and
Fertilisation
-
Gametes:
Sperm and egg cells in animals; pollen and ova in plants.
-
Fertilisation:
The fusion of a sperm and an egg cell to form a zygote
(diploid cell).
-
Genetic Variation:
Each gamete is unique due to crossing over and independent assortment of
chromosomes.
4.
Identical Twins versus
Fraternal Twins
-
Identical Twins:
Form when a single fertilised egg splits into two embryos. They have
identical genetic material.
-
Fraternal Twins:
Form when two separate eggs are fertilised by two different sperm cells.
They are genetically different.
5.
Importance of Meiosis
-
Maintains Chromosome
Number: Prevents doubling of
chromosomes in each generation.
-
Introduces Genetic
Variation: Essential for
evolution and adaptation.
-
Produces Gametes:
Necessary for sexual reproduction.
Summary of learning objectives and key words or phrases
Be able to describe the process of cell division by meiosis in
sexual reproduction.
Be able to interpret diagrams of meiosis in sexual reproduction.
Be able to use and understand the terms haploid chromosome
numbers, diploid chromosome numbers, zygote, gametes sex cells and sperm
cells and egg cells.
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