D2.1: Cell and Nuclear Division Flashcards
1
Q
List implications of the idea that new cells are only produced from a pre-existing cell.
A
- we can trace the origin of all the cells in our body back to the first cell; zygote produced by the fertilisation of a sperm and egg
- origins of all cells can be traced back through billions of years of evolution to LUCA of all life on Earth
- There must have been a 1st cell that arose from non-living material
2
Q
Define cytokinesis.
A
The physical division of a the cytoplasm and organelles of a cell into 2 daughter cells
3
Q
State the difference between mitosis and cytokinesis.
A
Mitosis: division of the nucleus and DNA into 2 daughter cells
Cytokinesis: division of the cytoplasm and organelles into 2 daughter cells
Mitosis and cytokinesis can occur at the same time
4
Q
Explain cytokinesis in plant cells
A
- Golgi buds off vesicles that move towards the cell equator
- Vesicles fuse creating a cell plate which extends until it fuses with the sides of the parent cell thus, separating the 2 new daughter cells
- Both daughter cells release cellulose by exocytosis into the space betw. the 2 membranes which builds the cell wall of each daughter cell
5
Q
Explain cytokinesis in animal cells
A
- A network of actin and myosin proteins form a contractile ring at the cell equator that pinches the cell membrane together to form a cleavage furrow
- Cell membrane cleavage furrow pinches in until the cell splits into 2 daughter cells
6
Q
Describe the formation of the cleavage furrow in animal cell cytokinesis.
A
A network of actin and myosin proteins form a contractile ring at the cell equator that pinches the cell membrane together to form a cleavage furrow
7
Q
Describe the formation of the cell wall in plant cell cytokinesis.
A
Both daughter cells release cellulose by exocytosis into the space betw. the 2 membranes which builds the cell wall of each daughter cell
8
Q
What does cytokinesis result in?
A
Results in equal division of the cytoplasm and organelles between the daughter cells to form 2 equal sized cells
9
Q
State the reason why daughter cells must receive at least one mitochondria during cytokinesis.
A
Mitochondria undergo their own division to repopulate the new daughter cells
10
Q
Outline unequal cytokinesis in yeast budding.
A
- Budding is an asymmetric division mechanism used by most yeasts to reproduce asexually
- The nucleus divides by mitosis then during the cytokinesis the daughter cell receives only a small portion of the cytoplasm
- Daughter cell initially remains attached to parent cell
- Cells are separated and the daughter cell matures into a new yeast cell
11
Q
Outline unequal cytokinesis during human oogenesis.
A
- Oogenesis is the production of an egg cell
- Cytoplasm is divided unevenly during cytokinesis to produce 1 large egg cell and 3 small polar bodies
- A single large egg cell contains the cytoplasm of all 4 daughter cells
- The large vol. of cytoplasm in the egg cell is used to provide all the organelles and stores energy to sustain the developing embryo
12
Q
What is mitosis and what does it produce
A
- nuclear division resulting in continuity of the chromosome number and genome. Produces:
- 2 identical daughter cells
- A full set of genetic material
- organelles
13
Q
Define histone
A
Protein around which eukaryotic DNA is wrapped
14
Q
Define nucleosome
A
The structure that results from DNA wrapping around histone proteins
15
Q
Define chromatin
A
A collection of loose nucleosomes; DNA is present in its chromatin form during interphase
16
Q
What is meiosis?
A
nuclear division that results in the reduction of the chromosome number and diversity between genomes.
17
Q
Outline the cause and consequence of anucleate cells.
A
Cause:
- Loss of nucleus occurs during maturation
Consequence:
- Loss of nucleus means loss of ability to reproduce and therefore, die
- More space to carry hemoglobin therefore carrying more O2. (RBC Only)
- gives biconcave shape (RBC Only)
(E.g: RBC)
18
Q
What process occurs before both mitosis and meiosis?
A
DNA replication occurs before both mitosis and meiosis.
19
Q
in mitosis, when is the nuclear membrane not present
A
- Metaphase
- Anaphase
20
Q
When does DNA replication take place?
A
DNA replication occurs in the S-phase of interphase.
21
Q
Explain how replicated DNA molecules are held together, with reference to chromatid, replicated chromosome, centromere and cohesin.
A
At the centromere, located in the DNA it:
- adheres sister chromatids to one another
- it’s the site of kinetochore and microtubule attachment for movement of chromosomes
Protein complex cohesin, established in interphase before mitosis and meiosis:
- Holds sister chromatids together until anaphase
Cohesin holding the chromatids are removed by the start of anaphase and are able to move to opposite poles of the cell ensuring each daughter cell will have a complete copy of the genetic material
22
Q
Explain how and why chromosomes condense during mitosis and meiosis.
A
How:
- Mitosis: During prophase, Chromatin condenses into chromosomes each consisting of sister chromatids without the chromosomes pairing up
- Meiosis: During prophase I, Chromatin condenses into chromosomes each consisting of 2 sister chromatids. These homologous chromosomes pair up forming bivalents. Then, crossing over occurs between the non-sister chromatids (one of the chromatids of each chromosome are non-sister chromatids), forming recombinant chromosomes.
BOTH CONTAIN SISTER CHROMATIDS BUT IN MITOSIS, THEY DON’T PAIR UP
Why:
- to facilitate accurate segregation of genetic material.
23
Q
State the role of microtubules and kinetochore motor proteins.
A
(mitotic spindle microtubules) Microtubules:
- Long and thin cylindrical fibrous proteins that form the spindle apparatus during cell division
Kinetochore motor proteins (like Kinesin):
- The kinetochores link the chromatids to the microtubules thus, pulling the chromosomes toward the poles during anaphase
24
Q
State the names of the four phases of mitosis.
A
- Prophase
- Metaphase
- Anaphase
- Telophase
25
Draw typical eukaryotic cells as they would appear during the interphase and the four phases of mitosis.
pk
26
Outline four events that occur during prophase in mitosis
- Chromatin condenses into homologous chromosomes whilst remaining unpaired
- Nuclear envelop begins dissolving
- Centrosomes move towards the opposite ends of the poles thus, establishing the poles of the mitotic spindle
- Microtubules, aka spindle fibers, begin growing from the centromeres at the poles of the cell
27
Outline the process of metaphase, inclusive of the role of microtubules and the kinetochore, in mitosis
- Microtubules attach to the kinetochore of each chromatid and attach to opposite poles.
- Cohesin loops still attach the chromatids
- Chromosomes line up across the metaphase plate and are equidistant from the 2 poles
28
Outline the process of anaphase in mitosis
- Chromatids separate as Cohesin loops get cut and become separate chromosomes
- Microtubules link each chromosome to the pol
- Chromosomes move towards opposite poles of the cell due to spindle fibres shortening
29
Outline four events that occur during telophase in mitosis (complete opposite of prophase in mitosis)
- Two diploid nuclei form.
- Chromosomes decondense; chromosomes uncoil back into chromatin
- Spindle fibres disintegrate
- Cell elongates in preparation for cytokinesis
30
What can result from mitosis occurring when it shouldn't? What regulates the progression through the phases of mitosis?
- Cancer
- Proteins called cyclins regulate the progression through the phases of mitosis
31
Determine the phase of mitosis of a cell viewed in a diagram, micrograph or with a microscope.
pk
32
Explain the need for meiosis as part of a sexual life cycle.
Meiosis must occur at some stage in a sexual life cycle to avoid increases in the chromosome number
33
Explain what it means for chromosomes to be “homologous.”
Alleles can be reshuffled during meiosis to produce new combinations and together with gene mutations
34
Define diploid.
- indicated by 2n
- A nucleus, cell or organism with 2 sets of chromosomes and therefore, homologous pairs of chromosomes
35
State the human cell diploid number.
46 chromosomes
36
State an advantage of being diploid.
- Contains the normal number of chromosomes
37
Define haploid.
- indicated by n
- A nucleus, cell or organism with a single set of chromosomes which are all non-homologous
38
State the human cell haploid number.
23 chromosomes
39
List example haploid cells.
sperm and egg cells
40
Given a diploid number (for example 2n=4), outline the movement and structure of DNA through the stages of meiosis I.
Meiosis I: The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated.
- Prophase I: Chromatin condenses into chromosomes by supercoiling. Homologous chromosomes pair up (synapsis) and crossing over occurs resulting in recombinant chromosomes (a tetrad or bivalent)
- Metaphase I: Microtubules from the 2 poles link to different homologous chromosomes in each pair. Homologous chromosome pairs line up across the cell equator by random orientation
- Anaphase I: Homologous chromosomes get separated and get pulled towards the opposite poles of the cell due to spindle fibres shortening
- Telophase I: Two haploid nuclei form and both prepare for the 2nd phase of meiosis
41
Given a diploid number (for example 2n=4), outline the movement and structure of DNA through the stages of meiosis II.
Meiosis II: The second division is a non-reductionist division (haploid → diploid) in which chromosomes are formed.
- Prophase II: chromosomes begin to condense.
- Metaphase II: The individual chromosomes align at the metaphase plate by random orientation.
- Anaphase II: Identical sister chromatids separate and move to opposite poles of the cell.
- Telophase II: two more haploid nuclei form and thus, 4 haploid nuclei form in total.
42
Explain why meiosis I is a reductive division.
- RD prevents the number of chromosomes from doubling with each generation in a species as gametes must be haploid.
- The chromosome number is halved from diploid to haploid which counteracts the doubling effect of the male and female gametes fusing
43
Are the cells diploid or haploid at the end of meiosis I
the 2 cells are haploid at the end of meiosis I
44
Compare meiosis II with mitosis
In both, sister chromatids separate and move to different poles during anapahse
In both, no crossing over occurs during prophase
- Meiosis II: haploid cell in metaphase II, creates gametes
- Mitosis: diploid cell in metaphase, creates daughter cells
45
Compare divisions of meiosis I and meiosis II.
Divisions in Meiosis I:
- Reductive division (diploid to haploid)
- Results in two haploid cells
- Chromosomes remain duplicated
- Crossing over occurs
- Proceeding by interphase with DNA replication
Divisions in Meiosis II
- Non-reductive division (haploid to diploid)
- Results in four haploid cells
- Chromatids of a chromosomes separate
- No crossing over occurs
- Proceeding by interkinesis, without DNA replication
46
Outline the events of prophase, metaphase, anaphase and telophase in meiosis I
- Prophase I: replicated chromatin condenses into chromosomes, homologous chromosomes pair up, crossing over between homologous non-sister chromatids occurs
- Metaphase I: homologous pairs of chromosomes line up at the metaphase plate (independent assortment) after having been moved by microtubules
- Anaphase I: homologous chromosomes are pulled to opposite poles of the cell
- Telophase I: in some cell types, 2 haploid nuclei form; cytokinesis forms 2 daughter cells
47
Outline the events of prophase, metaphase, anaphase and telophase in meiosis II
- Prophase II: nuclear membrane breaks down, chromosomes condense, the spindle fibers form
- Metaphase II: replicated chromosomes are moved by microtubules to the metaphase plate (the haploid cell makes it impossible for chromosomes to be paired)
- Anaphase II: sister chromatids are pulled to opposite poles
- Telophase II: chromosomes decondense, 4 haploid nuclei formed; cytokinesis makes 4 haploid cells
48
Define nondisjunction.
- Failure of homologous chromosomes to separate correctly during meiosis I
- or failure of sister chromatids to separate correctly during meiosis II.
49
State the result of nondisjunction during anaphase I and anaphase II.
- This leads to gametes with one extra chromosome to chromosome number 21
- or one missing chromosome.
50
Describe the cause and symptoms of Down syndrome.
cause:
- Extra copy of chromosome 21
consequence:
- Poor muscle tone, heart defects and delayed development
51
Explain how meiosis leads to genetic variation in gametes.
Random orientation of bivalents
- increases the number of potential combinations of alleles
Crossing over
- There is a mutual exchange of DNA between two non-sister chromatids in the pair of homologous chromosomes and therefore, genes between the two chromatids too.
52
Define bivalent (Tetrad)
A pair of homologous chromosomes that come together during prophase I of meiosis.
53
Describe the process and result of crossing over between bivalents during prophase I of meiosis.
Process:
- Crossing over takes place whilst the chromatids are still elongated
- 2 non-sister chromatids are brought together at the same point along their gene sequences
- 2 strands of their DNA double helices are cut one at a time and are rejoined with the equivalent strand in the other chromatid
- Thus, there is a mutual exchange of DNA and therefore, genes between the 2 chromatids too
Result:
- new recombinant chromosomes and near infinite genetic variability
54
Define what chiasma are
The point at which there is a physical link between two non-sister chromatids belonging to homologous chromatids
55
Draw a diagram to illustrate the formation of new allele combinations as a result of crossing over.
Homologous Pair ---> Crossing Over ---> Gametes
56
Describe the process and result of random orientation of bivalents during metaphase I of meiosis.
Process:
- pairs of homologous chromosomes line up on the equator and are oriented randomly
- This means there is no relationship betw. the position of one maternal chromosome and the position of another
Result:
- there is an equal probability of the daughter cell having either the maternal or paternal chromosome
57
Draw a diagram to illustrate the formation of different chromosome combinations that result from random orientation during meiosis.
metaphase I ---> telophase I
58
What is the number of chromosome combinations possible due to random orientation?
2^n.
59
Define cell proliferation.
- A process in multicellular organisms by which a cell grows and divides to produce 2 daughter cells
60
List three processes which require cell proliferation.
- Organism growth
- Replacement of dead cells
- Tissue repair
61
Define what meristems are
A region of a plant with undifferentiated cells that undergo mitosis
62
State the function of root and shoot apical meristems
Root apical meristems:
- Create the cells for lengthening the roots
Shoot apical meristems:
- Create cells for elongating the stem creating leaves and forming flowers
63
State the function of lateral meristems
Widens the stem, adding thickness
64
Outline cell proliferation during growth at plant meristems and early-stage animal embryos.
During Growth at Plant Meristems:
- Root apical meristems create the cells for lengthening the roots
- Shoot apical meristems create cells for elongating the stem, creating leaves and forming flowers
- Laternal meristem widens the stem, adding thickness
During Early-Stage Animal Embryos:
- Embryonic cell proliferation is rapid; the number of the cells increases but the size of individual cells gets smaller.
- Egg cells are typically very large to supply nutrients and a cytoplasm
- During juvenile growth: Cells continue to proliferate with "spurts" of growth
65
Describe skin cell proliferation during cell replacement
Cell replacement:
- More cells are produced to replace the cells that die
- e.g: Replacement of RBC as they die naturally - Hematopoietic stem cell niche found in bone marrow protects the stem cells and provides the physical and chemical signals necessary for the cell to proliferate and differentiate into the different types of blood cells
- e.g: Skin cells that are replaced throughout an animal's life - New skin cells are created, they mature and the mature skin cells push old cells to the surface, mature cells deteriorate and dead cells arrive at the top and flake off from the surface
66
Describe skin cell proliferation during tissue repair
Tissue Repair:
- cell proliferation occurs to replace damaged cells of injured tissues
- e.g: This is especially important for skin and blood vessels that protect and bring O2 & nutrients to other cells in the body
- e.g: Cell proliferation can replace blood lost through bleeding and in most organisms, cell proliferation can replace entire lost limbs or tails
a cut or burn of skin will trigger skin cell proliferation by Basal cells in the epidermis and multipoint stem cells in the hair follicle (for more severer damage)
67
List the phases of the cell cycle.
1. Interphase: G1, S, G2
2. Mitosis: PMAT
3. Cytokinesis
68
Distinguish between interphase, mitosis and cytokinesis.
Interphase: protein synthesis occurs, ATP is produced, increase in # of organelles and vol. of cytoplasm, and DNA replication occurs (PAID).
Mitosis: division of the nucleus and DNA into 2 daughter cells
Cytokinesis: division of the cytoplasm and organelles into 2 daughter cells
69
Outline events of the G1, S, and G2 phases of interphase.
G1: Cell grows larger
S: DNA Replication occurs creating 2 genetically identical DNA strands (sister chromatids)
G2: Cell grows larger and there is an increase in the number of organelles to allow the cell to split into 2 cells during Mitosis
70
Outline the fate of cells that leave the cell cycle.
- These cells enter a non-dividing state called G0: Cells in G0 are alive and perform differentiated roles but do not divide again.
E.g:
- Differentiated bone cells
- Skeletal muscle fibre cells
- Neurons
71
Outline the structures that must be produced by a cell as it grows prior to division (duringh G1 at interphase)
- ribosomes
- golgi body
- mitochondria
72
List example metabolic reactions occurring during cell interphase.
1. DNA replication in the nucleus
2. protein synthesis in cytoplasm
3. Reactions of cellular respiration
73
State the functions of cell cycle checkpoints.
The cell checks for errors or defects before proceeding to the next stage of the cell cycle
74
Outline events of the M checkpoint
M:
- At the end of metaphase
- Determines whether all the sister chromatids are correctly attached to the spindle microtubules.
- If not, Anaphase will not proceed until kinetochores of each pair of sister chromatids are firmly anchored to at least 2 spindle fibres from the opp. poles of the cell
75
Outline the role of cyclins in controlling the cell cycle.
- cyclins bind to kinases and activate them. Kinases phosphorylate other proteins and thus, the phosphorylated proteins perform specific functions within the cell cycle.
- Different conc. of cyclins rises and falls through the cell cycle
- Cyclins regulate the sequence of the cell cycle
76
State what cyclins are controlled by
Genes
77
State how many types of cyclins there are
- 4 types
- D, E, A, B
- The conc. of the different cyclins rises and falls at diff. times of the cell cycle
78
Interpret a graph of cyclin concentrations throughout the cell cycle.
Cyclin D:
- Cyclin D is high throughout the whole process of mitosis as it triggers the movement of cells from G0 to G1 and the S phase.
Cyclin E:
- peaks at the beginning of the S phase. During the S phase, the DNA is replicated
Cyclin A:
- is a checkpoint to make sure DNA is replicated only once.
Cyclin B:
- peaks at metaphase and promotes the assembly of the mitotic spindle.
79
Describe how cancer arises, referring to accumulation of mutations over time.
- Cancer arises when a mass of cells mutate (divide) at abnormally fast rates (called primary tumor) and spread to other parts of the body
80
How are tumor formations initiated
- Random errors
- Mutagens
81
Define and list mutagens.
Definition:
- Mutagens are substances capable of causing mutations in DNA that controls the cell's progression through cell cycle
E.g:
- Radiations (UV, Xrays, Gamma rays)
- Chemicals (Carcinogens)
- Infectious agents (virus or bacteria)
82
Explain how mutations to proto-oncogenes and tumor suppressor genes can lead to the development of cancer.
Proto-oncogenes:
- code for proteins that progress the cell cycle like genes that code for cyclins
- after mutations, it becomes cancer-causing alleles called oncogenes which get permanently activated leading to overproduction of cyclins and thus, continuous activation of cyclin-depended kinases resulting in cell division from occurring when it shouldn't and out of control --> Cancer
Tumor Suppressor Gene
- code for proteins that inhibit cell division or tell cells when to undergo apoptosis (die)
- after mutations, more cyclin-dependent kinases are inactivated and increased cyclin production leading to cell division from occurring when it shouldn't and out of control --> Cancer
83
Compare the rates of cell division and growth and the capacity for metastasis and invasion of neighboring tissues between normal cells and cancerous cells (Outline the 4 steps involved in cancer development - IPPM)
Initiation:
- Normal cell transforms into a cancerous cell due to mutations to genes that regulate cell cycles
Promotion:
- Initiated cell divides --> large number of daughter cells containing the mutation (this group is called the primary tumour)
Progression:
- Cancerous cells become aneuploid (having the wrong # of chromosomes) and invade neighbouring tissues
Metastasis:
- Cancer cells detach from primary tumour, travel through blood thus, forming secondary tumours in other parts of bodies
84
Define primary tumour, secondary tumour, benign, malignant, metastasis and cancer.
Primary Tumour:
- A mass of cells that are dividing at abnormally fast rates
Cancer:
- The disease that results when the primary tumour spreads to other parts of the body
Metastasis:
- The spreading of cancerous cells through the body via the blood or other mechanism
Secondary Tumour:
- the tumour that has spread from the primary tumour is called the secondary tumour
Benign (classification of a tumour):
- Cells in the tumor adhere to each other and remain in a single mass --> no cancer
Malignant (classification of a tumour):
- cells in the tumor can detach and invade neighbouring tissues --> Cancer
85
State the formula for calculation of a mitotic index and define it
- of all cells in mitosis / total number of all cells x 100
- It is a measure of the proportion of cells in a population that are actively dividing
86
In what tissues is the mitotic index usually elevated
- In tumorous tissue because cancerous cells proliferate quicker than normal cells
87
Outline the use of mitotic index calculations in diagnosis and treatment of cancer.
Mitotic index calculations:
- # of all cells in mitosis / total # of all cells x 100
- Low mitotic index = treatment is efficient to cure cancer
Treatment of cancer:
- Chemotherapeutic Drugs = kill dividing cells by damaging part of the cell's control centre which makes cells divide
88
Define Recombinant chromosomes
The recombinant chromosomes have new combinations of alleles as a result of the crossing over
89
Define what sister chromatids are
genetically identical strands
90
Outline what is crossing over and random orientation of chromosomes
- Random orientation of homologous chromosomes
- Crossing over between homologous chromosomes
91
Define what chromatids are
sister chromatids are 2 dna molecules formed by dna replication
92
Where is the kinetochore and microtubules
microtubules: spindle fibres extending from the centrosomes (at opp. poles of the cell) outwards towards chromosomes
kinetochore: centre of chromosomes where microtubules are attached to
93
What 3 processes result in genetic variation
- Random fertilisation: Fusion of gametes from different parents
- Random Orientation
- Crossing over
94
The point on chromosomes where crossing over takes place is called
Chiasmata (Chiasma is plural)
95
Define sister chromatids
- Two identical copies of the same chromosome attached by centromere
(2 chromatids make up a chromosome)
96
Define non-sister chromatids
Either of the 2 chromatids of any of the homologous chromosomes
97
Define crossing over of non-sister chromatids
a process that produces new combinations of genes by exchanging segments between non-sister chromatids of homologous chromosomes to create recombinant DNA.
98
Define Gamete
Gametes are reproductive cells that carry genetic information
99
in which stage of meiosis could non-disjunction happen
- Anaphase
- Because sister chromatids may not separate and move to opposite poles of the cell, pulled by microtubules
100
Define monosomy
the condition in which only one chromosome from a pair is present in cells
101
Define Trisomy
When a diploid organism has three copies of one of its chromosomes instead of two.
102
Define zygote
the first diploid cell that is formed by the fusion of male and female gametes resulting in the formation of an embryo
103
What is true about the chromosomes initially in meiosis and mitosis
Mitosis:
- No homologous pairs of chromosomes instead, chromatin condenses into chromosomes forming sister chromatids
Meiosis:
- Homologous chromosomes pair up forming bivalents by crossing over
104
How do cyclins control the cell cycle
By activating cyclin-dependent kinases.
105
What do cyclin dependent kinases do
Phosphorylate target proteins that do jobs related to moving the cell through the phases of the cell cycle
106
What is another name for random orientation and define it. Where does this occur (mitosis or meiosis)?
- Independent assortment
- This is defined to be homologous chromosomes lining up along the equator of the cell in pairs
- This occurs during Metaphase I of Meiosis I however, doesn't occur during Mitosis.
- In Mitosis, chromosomes align individually along the equator of the cell, but there is no pairing of homologous chromosomes, so there is no opportunity for random orientation or independent assortment to occur.
107
Distinguish between sister chromatids and chromosomes
- Sister Chromatids: 2 chromatids make up 1 chromosome
- Chromosomes: condensed by supercoiling
108
Where are tumours most likely to occur
- Breasts,
- Skin
- lung
- large intestine
109
Explain how crossing over results in the formation of genetically unique nuclei (3)
- crossing over is an exchange of the alleles between homologous chromosomes
- crossing over can happen on one chromatid out of the pair
- after crossing over sister chromatids are not identical
- random orientation in metaphase I
110
Similarities and differences between homologous chromosomes
Similarities:
- both have centromeres in the same position
- both have the same sequence of genes
- both have the same length
- both are linear DNA molecules
Differences:
- One chromosome is inherited maternally while the other is inherited paternally
111
outline the use of a karyogram during pregnancy
- Chromosomes get stained and photographed to appear on karyograms
- Karyograms start with the longest pair and end with the shortest
- Karyograms examine individuals' karyotypes: This can reveal the sex of an individual by the appearance of the sex chromosome pair and test for chromosomal abnormalities
112
State what chromosomes determine which sex
XX = Female
XY = Male
113
Why are the chromosomes at the start of meiosis all double stranded?
DNA replication produces a copy of all the DNA before meiosis.
114
Outline the events which would lead to an individual being born with Down syndrome, which is characterised by the presence of three copies of chromosome 21.
- Down syndrome is caused due to an extra chromosome being present which is the result of non-disjunction; where homologous pairs of chromosomes are unable to separate properly during Anaphase I
- As the mother's age increases, the chances of non-disjunction increases
- Both chromosomes from a homologous pair move to the opposite poles of the cell
