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CIE A Level Biology Your notes
16.1 Passage of Information from Parents to
Offspring
Contents
Haploidy & Diploidy
Homologous Chromosomes
Meiosis in Animal & Plant Cells
Identifying the Stages of Meiosis
Meiosis: Sources of Genetic Variation
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Haploidy & Diploidy
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Haploid & Diploid Cells
A diploid cell is a cell that contains two complete sets of chromosomes (2n)
These chromosomes contain the DNA necessary for protein synthesis and cell function
Nearly all cells in the human body are diploid with 23 pairs (46 individual) of chromosomes in their
nucleus
Haploid cells contain one complete set of chromosomes (n)
In other words they have half the number of chromosomes compared to diploid cells
Humans have haploid cells that contain 23 chromosomes (no pairs) in their nucleus
These haploid cells are called gametes and they are involved in sexual reproduction
For humans they are the female egg and the male sperm
Haploidy and diploidy are terms that can be applied to cells across different species
They describe the number of sets of chromosomes, not the total number of chromosomes
Haploid and Diploid Cells Diagram
Haploid (n) and Diploid (2n) cells
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Exam Tip
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Red blood cells are an exception when it comes to chromosome number as they don’t have a nucleus!
You may be asked to estimate the number of chromosomes that would be present in the haploid cell
of any species. For example, dogs have 78 chromosomes in their diploid cells. When trying to find the
number of chromosomes in their haploid cells simply remember that diploid is 2n and haploid is n,
meaning you just need to divide the number of chromosomes by 2. So dogs have 39 chromosomes in
their haploid cells!
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The Need for Reduction Division during Meiosis
During fertilisation the nuclei of gametes fuse together to form the nucleus of the zygote Your notes
Both gametes must contain the correct number of chromosomes in order for the zygote to be viable. If
a zygote has too many or too few chromosomes it may not survive
For a diploid zygote this means that the gametes must be haploid
n + n = 2n
Where n is the haploid number of chromosomes and 2n is the diploid number of chromosomes
Meiosis produces haploid gametes during sexual reproduction
The first cell division (this is not referring to the first stage) of meiosis is a reduction division
This is a nuclear division that reduces the chromosome number of a cell
In humans the chromosome number is reduced from 46 (diploid) to 23 (haploid)
The reduction in chromosome number during meiosis ensures the gametes formed are haploid
Haploidy and Diploidy Diagram
The maintenance of chromosome number through reduction division in a mammalian life cycle
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Homologous Chromosomes
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Homologous Chromosomes
In diploid cells there are two complete sets of chromosomes in the nucleus
Chromosomes have a characteristic shape
They have a fixed length
The position of the centromere is in a particular location
These characteristic features allow for each chromosome to be identified in a photomicrograph
In photomicrographs chromosomes are often grouped into their homologous pairs
Homologous chromosomes:
Carry the same genes in the same positions (locus)
Are the same shape
During fertilisation a diploid zygote is formed
In a zygote one chromosome of each homologous pair comes from the female gamete and the
other comes from the male gamete
Having the same genes in the same order helps homologous chromosomes line up alongside each
other during meiosis
Homologous Chromosomes Diagram
Human Karyogram showing homologous chromosomes
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Exam Tip
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Although homologous pairs of chromosomes contain the same genes in the same order they don’t
necessarily carry the same alleles (form) of each gene! i.e. We all have the genes to create eyes, some
have the allele to create blue eyes and others the allele for brown eyes, for example.
Page 6 of 20
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Meiosis in Animal & Plant Cells
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Meiosis in Animal & Plant Cells
Meiosis is a form of nuclear division that results in the production of haploid cells from diploid cells
It produces gametes in plants and animals that are used in sexual reproduction
It has many similarities to mitosis however it has two divisions: meiosis I and meiosis II
Within each division there are the following stages: prophase, metaphase, anaphase and telophase
First division of meiosis: Meiosis I
Prophase I
DNA condenses and becomes visible as chromosomes
DNA replication has already occurred so each chromosome consists of two sister chromatids joined
together by a centromere
The chromosomes are arranged side by side in homologous pairs
A pair of homologous chromosomes is called a bivalent
As the homologous chromosomes are very close together the crossing over of non-sister chromatids
may occur. The point at which the crossing over occurs is called the chiasma (chiasmata; plural)
In this stage centrioles migrate to opposite poles and the spindle is formed
The nuclear envelope breaks down and the nucleolus disintegrates
Metaphase I
The bivalents line up along the equator of the spindle, with the spindle fibres attached to the
centromeres
Anaphase I
The homologous pairs of chromosomes are separated as microtubules pull whole chromosomes to
opposite ends of the spindle
The centromeres do not divide
Telophase I
The chromosomes arrive at opposite poles
Spindle fibres start to break down
Nuclear envelopes form around the two groups of chromosomes and nucleoli reform
Some plant cells go straight into meiosis II without reformation of the nucleus in telophase I
Cytokinesis
This is when the division of the cytoplasm occurs
Cell organelles also get distributed between the two developing cells
In animal cells:
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The cell surface membrane pinches inwards creating a cleavage furrow in the middle of the cell
which contracts, dividing the cytoplasm in half
In plant cells: Your notes
Vesicles from the Golgi apparatus gather along the equator of the spindle (the cell plate). The
vesicles merge with each other to form the new cell surface membrane. Layers of cellulose are
laid down to form the primary and secondary walls of the cell
The end product of cytokinesis in meiosis I: two haploid cells
These cells are haploid as they contain half the number of centromeres
Meiosis I Diagram
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Your notes
The different stages of Meiosis I in an animal cell
Second division of Meiosis : Meiosis II
There is no interphase between meiosis I and meiosis II so the DNA is not replicated
The second division of meiosis is almost identical to the stages of mitosis
Prophase II
The nuclear envelope breaks down and chromosomes condense
A spindle forms at a right angle to the old one
Metaphase II
Chromosomes line up in a single file along the equator of the spindle
Anaphase II
Centromeres divide and individual chromatids are pulled to opposite poles
This creates four groups of chromosomes that have half the number of chromosomes compared to
the original parent cell
Telophase II
Nuclear membranes form around each group of chromosomes
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Cytokinesis
Cytoplasm divides as new cell surface membranes are formed creating four haploid cells Your notes
The cells still contain the same number of centromeres as they did at the start of meiosis I but
they now only have half the number of chromosomes (previously chromatids)
Meiosis II Diagram
Prophase II, Metaphase II and Anaphase II in Meiosis II of an animal cell
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Your notes
Telophase II and cytokinesis in Meiosis II of an animal cell
Exam Tip
Understanding the difference between chromosomes and chromatids can be difficult.
We count chromosomes by the number of centromeres present. So when the 46 chromosomes
duplicate during interphase and the amount of DNA in the cell doubles there are still only 46
chromosomes present because there are still only 46 centromeres present. However, there are now
92 chromatids, which are strands of replicated chromosomes.
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Identifying the Stages of Meiosis
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Identifying the Stages of Meiosis
Cells undergoing meiosis can be observed and photographed using specialised microscopes
The different stages of meiosis have distinctive characteristics meaning they can be identified from
photomicrographs or diagrams
Meiosis I or Meiosis II
Homologous chromosomes pair up side by side in meiosis I only
This means if there are pairs of chromosomes in a diagram or photomicrograph meiosis I must be
occurring
The number of cells forming can help distinguish between meiosis I and II
If there are two new cells forming it is meiosis I but if there are four new cells forming it is meiosis II
The distinguishing features at each stage of Meiosis I
Prophase I: Homologous pairs of chromosomes are visible
Metaphase I: Homologous pairs are lined up side by side along the equator of spindle
Anaphase I: Whole chromosomes are being pulled to opposite poles with centromeres intact
Telophase I: There are 2 groups of condensed chromosomes around which nuclei membranes are
forming
Cytokinesis: Cytoplasm is dividing and cell membrane is pinching inwards to form two cells
The distinguishing features at each stage of Meiosis II
Prophase II: Single whole chromosomes are visible
Metaphase II: Single whole chromosomes are lined up along the equator of the spindle in single file (at
90 degree angle to the old spindle)
Anaphase II: Centromeres divide and chromatids are being pulled to opposite poles
Telophase II: Nuclei are forming around the 4 groups of condensed chromosomes
Cytokinesis: Cytoplasm is dividing and four haploid cells are forming
Identifying the stages of meiosis table
Stage Micrograph
Prophase I
One group of chromosomes becomes visible as the DNA
condenses
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Metaphase I
Chromosome pairs are located along the middle of the
spindle Your notes
Anaphase I
Whole chromosomes are being pulled away from the middle
Telophase I
There are two groups of chromosomes at each pole
The nucleus is reforming and the cytoplasm is pinching in
Prophase II
Two groups of chromosomes are visible as the DNA
condenses
Metaphase II
Chromosomes line up along the middle of the spindles in
single-file
Anaphase II
Chromatids are pulled away from the middle of the spindles
Telophase II
There are four groups of chromosomes and the cytoplasm is
pinching in
Meiosis I Photomicrographs
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Prophase I, Metaphase I , Anaphase I and Telophase I as seen in photomicrographs
Meiosis II Photomicrographs Your notes
Prophase II, Metaphase II , Anaphase II and Telophase II as seen in photomicrographs
Exam Tip
The acronym PMAT can help you remember what is happening in each stage:
P for Prophase where chromosomes are Preparing to divide
M for Metaphase for the middle of the spindle and cell which is where the chromosomes will be
lined up.
A for Anaphase, remember A for away from the middle to the poles, which is where the
chromosomes / chromatids are being pulled
T for telophase where we have Two cells (for meiosis I at least!)
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Meiosis: Sources of Genetic Variation
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Meiosis: Sources of Genetic Variation
Having genetically different offspring can be advantageous for natural selction
Meiosis has several mechanisms that increase the genetic diversity of gametes produced
Both crossing over and independent assortment (random orientation) result in different
combinations of alleles in gametes
Crossing over
Crossing over is the process by which non-sister chromatids exchange alleles
Process:
During meiosis I homologous chromosomes pair up and are in very close proximity to each other
The non-sister chromatids can cross over and get entangled
These crossing points are called chiasmata
The entanglement places stress on the DNA molecules
As a result of this, a section of chromatid from one chromosome may break and rejoin with the
chromatid from the other chromosome
This swapping of alleles is significant as it can result in a new combination of alleles on the two
chromosomes
There is usually at least one, if not more, chiasmata present in each bivalent during meiosis
Crossing over is more likely to occur further down the chromosome away from the centromere
Crossing Over Diagram
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Your notes
Crossing over of non-sister chromatids leading to the exchange of genetic material
Independent assortment
Independent assortment is the production of different combinations of alleles in daughter cells due
to the random alignment of homologous pairs along the equator of the spindle during metaphase I
The different combinations of chromosomes in daughter cells increases genetic variation between
gametes
In prophase I homologous chromosomes pair up and in metaphase I they are pulled towards the
equator of the spindle
Each pair can be arranged with either chromosome on top, this is completely random
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The orientation of one homologous pair is independent (unaffected by the orientation of any
other pair)
The homologous chromosomes are then separated and pulled apart to different poles Your notes
The combination of alleles that end up in each daughter cell depends on how the pairs of homologous
chromosomes were lined up
To work out the number of different possible chromosome combinations the formula 2n can be used,
where n corresponds to the number of chromosomes in a haploid cell
For humans this is 223 which calculates as 8 324 608 different combinations
Independent Assortment Diagram
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Your notes
Independent assortment of homologous chromosomes leading to different genetic combinations in
daughter cells
Exam Tip
Several sources of genetic variation have been outlined above. It is also worth remembering that
genetic variation can occur on an even smaller scale than chromosomes. Mutations can occur within
genes. A random mutation that takes place during DNA replication can lead to the production of new
alleles and increased genetic variation.
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Fusion of Gametes
Meiosis creates genetic variation between the gametes produced by an individual through crossing Your notes
over and independent assortment
This means each gamete carries substantially different alleles
During fertilisation any male gamete can fuse with any female gamete to form a zygote
This random fusion of gametes at fertilisation creates genetic variation between zygotes as each will
have a unique combination of alleles
There is an almost zero chance of individual organisms resulting from successive sexual reproduction
being genetically identical
Fusion of Gametes Diagram
How meiosis and the random fusion of gametes affects genetic variation
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Exam Tip
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These sources of genetic variation explain why relatives can differ so much from each other. Even with
the same parents, individuals can be genetically unique due to the processes outlined above.
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