3.3 Meiosis Flashcards
(122 cards)
1
Q
What is meiosis?
A
A form of nuclear division that produces four haploid nuclei from one diploid nucleus.
1
Q
How is offspring produced during sexual reproduction?
A
- In sexual reproduction, two diploid individuals each contribute half of their DNA to produce offspring with a new and unique combination of alleles.
- Meiosis makes this possible by producing nuclei containing exactly one copy of each gene.
- These nuclei are found in the reproductive cells (gametes).
2
Q
What happens prior to the start of meiosis?
A
- The DNA of the cell is replicated during the S phase of interphase.
- Thus meiosis begins, as mitosis does, with replicated chromosomes.
3
Q
What steps does meiosis involve?
A
- Two cycles of division, meiosis I and II that are themselves divided into four phases: prophase, metaphase, anaphase, and telophase (similar to mitosis).
- At the end of both meiosis I and meiosis II, cytokinesis occurs.
4
Q
What are the differences between meiosis/mitosis and cytokinesis?
A
- Meiosis and mitosis are nuclear processes.
- Cytokinesis divides the cytoplasm of the parent cell to create two daughter cells.
- Meiosis produces haploid nuclei; cytokinesis produces cells, each containing one of the nuclei.
5
Q
What is meiosis I?
A
Reduction division – Cells begin with two copies of each chromosome and end with only one;
Diploid (2n) → Haploid (n)
6
Q
What are the phases of meiosis I?
A
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I
7
Q
Diagram of prophase I
A
8
Q
Describe the events occurring during prophase I
A
- Chromosomes become visible due to supercoiling. The replicated chromosomes form closely-linked homologous pairs (called tetrads or bivalents), which have two chromosomes and four total chromatids.
- At this stage, non-sister chromatids may cross over at points called chiasmata and exchange equivalent segments of DNA.
- Centrioles, if present, migrate to opposite poles and spindle fibers start to form. The nucleolus and nuclear membrane disintegrate.
9
Q
Diagram of metaphase I
A
10
Q
Describe the events occurring during metaphase I
A
- Homologous pairs move together along the metaphase plate, which lies halfway between the two poles. Maternal and paternal homologues show random orientation towards the poles.
- The spindle fibres attach to the centromeres of each chromosome and gently pull to align them along the equatorial metaphase plate.
- Spindle fibres connect each centromere to one pole only.
11
Q
Diagram of anaphase I
A
12
Q
Describe the events occurring during anaphase I
A
- Spindle microtubules shorten, pulling homologous chromosomes apart towards opposite poles.
- Unlike in mitosis, sister chromatids remain connected at the centromere and move to the same pole.
13
Q
Diagram of telophase I
A
14
Q
Describe the events occurring in telophase I
A
- The first meiotic division effectively ends when the chromosomes arrive at the poles. Note that each chromosome still consists of a pair of chromatids.
- The chromatids partially uncoil and a nuclear membrane then reforms around each nucleus formed.
- Although technically not part of meiosis, cytokinesis usually occurs during telophase I. Cytokinesis results in two daughter cells with haploid nuclei from meiosis.
15
Q
What happens during meiosis II?
A
Separation of chromatids in haploid cells (n → n)
16
Q
What are the stages of meiosis II?
A
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II
17
Q
Diagram of prophase II
A
18
Q
Describe the events occurring during prophase II
A
- Chromosomes condense again.
- Centrioles, if present, migrate to opposite poles and spindle fibers start to form.
- The nucleolus and nuclear membrane disintegrate.
19
Q
Diagram of metaphase II
A
20
Q
Describe the events occurring during metaphase II
A
- The spindle fibers attach to the centromere and connect each centromere to both poles.
- They exert a gentle pull to align the sister chromatids at the equator.
21
Q
Diagram of anaphase II
A
22
Q
Describe the events occurring during anaphase II
A
- Centromeres divide and chromatids are moved to opposite poles by spindle fibers.
- Once sister chromatids are separated, they are called chromosomes.
23
Q
Diagram of telophase II
A
24
Describe the events occurring in telophase II
- Chromosomes reach opposite poles and uncoil. This is followed by nuclear envelope formation and cytokinesis.
- Meiosis is now complete, resulting in four haploid daughter cells. Note that each of the four cells is genetically distinct.
25
Drawing meiosis for the exam
- You should be able to draw diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
- Be certain to label each phase of meiosis as ‘I’ or ‘II’ to receive full marks.
26
A cell in the testis of a male chimpanzee (Pan troglodytes) contains 48 chromosomes. It is about to undergo meiosis.
How many chromosomes will be present in the nucleus of cells formed at the end of meiosis?
24
Meiosis results in the formation of haploid gametes.
Before meiosis, the cell in the testis would be diploid.
If the cell in the testis contains 48 chromosomes, the diploid number is (2n) = 48.
That means that the haploid number (1n) is 48 ÷ 2 = 24 chromosomes.
27
A cell possessing one pair of chromosomes undergoes meiosis.
Which diagram represents anaphase I of meiosis?
1
In anaphase I, the spindle microtubules shorten, pulling homologous chromosomes apart.
This produces haploid nuclei and is a key difference from mitosis.
28
A cell possessing one pair of chromosomes undergoes meiosis.
Which diagram represents anaphase II of meiosis?
5
There is only one copy of the chromosome so the cell is haploid, meaning the cell is in meiosis II.
29
What happens in all the prophase stages of meiosis and mitosis?
Chromosomes condense by supercoiling.
30
What happens in prophase I?
Chromosomes also show a unique and important behavior called crossing over.
31
Diagram of how crossing over in meiosis occurs during prophase I
32
How does a tetrad or bivalent form?
- Homologous chromosomes pair up and form a tetrad or bivalent.
- You may notice that these words have roots that mean ‘4’(tetra-) and ‘2’(bi-).
- That is because the two homologues have two chromatids each for a total of four.
33
Diagram of a pair of homologous chromosomes comprising four total chromatids
34
Describe the structure of chromatids
- Each chromatid is a long, single strand of double-helical DNA organized by histone proteins.
- Sister chromatids are identical and joined at the centromere.
- Non-sister chromatids in the tetrad have the same genes but may have different alleles.
35
When does crossing over occur?
When equivalent portions of the non-sister chromatids are exchanged between homologous chromosomes.
36
What are the chiasmata (singular: chiasma)?
The points at which crossing over occurs.
36
What are the chiasmata (singular: chiasma)?
The points at which crossing over occurs.
37
Can crossing over occur more than once in the same tetrad?
Yes, it can occur multiple times in the same tetrad.
38
Diagram of crossing over during prophase I
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What does crossing over create and why is this important?
- Crossing over creates new combinations of alleles that were not present in either original chromosome.
- This contributes to genetic variation among the gametes produced.
40
Where can crossing over occur?
Almost anywhere along the chromosome (though some areas are more frequent).
41
What are the crossing-over combinations in human chromosomes?
There is a near-infinite number of possible crossing-over combinations in the 23 pairs of human chromosomes, ensuring that every gamete produced is genetically unique.
42
What happens after crossing over has taken place and during and after anaphase I?
- After crossing over has taken place, the tetrads (a pair of homologous chromosomes) complete the process of condensation and move toward the equatorial plate.
- During anaphase I, the homologous chromosomes that formed the tetrad are separated.
- This is followed by the separation of sister chromatids during anaphase II of meiosis II to ultimately produce four haploid nuclei from one diploid nucleus.
43
What is the longest phase of meiosis?
Prophase I
44
What does prophase I involve?
- Pairing of homologous chromosomes
- Crossing over followed by condensation of DNA into highly organized chromosomes.
45
Why is there genetic diversity observed among gametes produced by a parent?
Crossing over occurs in homologous chromosomes at different places each time.
46
Once sister chromatids separate, each is a ___
Complete (unreplicated) chromosome.
47
Between which structures does crossing over typically occur?
Between non-sister homologous chromatids
48
What happens during metaphase I?
The pairs of homologous chromosomes (also called bivalents or tetrads) that crossed over in prophase I align along the equatorial plate of the cell.
49
Diagram of how random orientation of homologous pairs occurs during metaphase I
50
Describe the movement of chromosomes that occurs during metaphase I and II
- Their behaviour as they move to the equator of the cell is unique and important.
- In metaphase of mitosis, chromosomes have no special interactions with their homologue; in metaphase II, chromosomes have no homologue in the cell.
- In both cases, every chromosome lines up individually.
- In contrast, during metaphase I the chromosome pairs line up along the metaphase plate, with the centromere of each chromosome connected to only one pole.
51
Diagram comparing metaphase of mitosis with metaphase I
52
What is the end result of mitosis?
- Two nuclei with the same four chromosomes that the original cell had.
- Mitosis produces genetically identical daughter cells.
53
What are the two important changes that will result from the chromosomes arranging themselves in pairs instead of individually?
- Reduction division
- Random orientation
54
Explain reduction division
- Daughter cells contain only half of the chromosomes that were present in the parent cell.
- In this case a parent cell has four chromosomes; the daughter cell has two.
- The parent cell is diploid.
- The daughter cells will be haploid, having only one version of each chromosome.
55
Explain random orientation
- When pairs of homologous chromosomes line up at the equator of the cell, the paternal copy, for example, has an equal chance of facing either pole.
- The orientation of one pair (e.g. maternal facing north) does not impact the orientation of any other pair.
- Each gamete gets one copy of each chromosome, but a random assortment of the maternally and paternally inherited versions.
56
Clarification of the terms 'maternal' and 'paternal' in meiosis
- It is easy to confuse the use of the terms ‘maternal’ and ‘paternal’ when discussing meiosis because there are actually three generations involved.
- When an ovum is made in a woman’s body, it receives one copy of each chromosome; the woman inherited some of the chromosomes passed to the gamete maternally (from her mother) and some paternally (from her father).
- The ovum fuses with a sperm to create an embryo. The assortment we called ‘maternal’ and ‘paternal’ in the ovum is, from the embryo’s point of view, all maternal. Some come from their maternal grandmother, and some from their maternal grandfather.
- Thus, you are guaranteed to have one version of each chromosome from each parent. However, you probably share a bit more DNA with some grandparents and a bit less from others – it depends on how the homologous pairs lined up on the equator.
57
Why does random orientation contribute to genetic diversity in the gametes?
Because of all the possible combinations of tetrad orientations.
58
A typical sexually-reproducing organism has a diploid chromosome number of 12.
How many chromosomes will it inherit and from whom?
6 chromosomes maternally
The number of chromosomes inherited from each parent will be the haploid number (six in this case).
59
A cell has a diploid number of 2n=4.
If the cell is arranged as shown, what must be true about the position of the chromosomes? (reword)
The chromosome labeled 4 must be oriented toward Pole B.
Chromosome 4 would be attached to chromosome 1. This is how cells create genetic diversity while ensuring that each gamete contains one copy of each gene.
60
Reduction division occurs during ___
Meiosis I
61
What is genetic variation responsible for?
- All the unique and distinctive forms that life has taken on Earth.
- It also accounts for much of the diversity we see within each species.
- Small differences in DNA sequences provide slightly different information to each individual, leading to a variety of shapes, sizes, and other characteristics.
62
What is the original source of all genetic variation?
Mutation
63
What does sexual reproduction allow and why did sex evolve?
- Sexual reproduction allows existing variations to be shuffled into endless new combinations.
- In fact, sex evolved largely as a way to increase the genetic variety in offspring.
64
Example that demonstrates how genetic diversity is an advantage (separate)
- The potatoes in Field A reproduce by cloning.
- The potato clones are very well adapted to the area’s conditions (climate, pests, soil pH, and so on).
- Potatoes in Field B reproduce by sexual reproduction.
- Most of them are well adapted to the area’s conditions, but each generation contains some individuals with less helpful variations.
- Cloned potatoes may have the advantage – until there is a change in the environment.
- Perhaps there is a drought, a new infectious fungus, or acid rain.
- The cloned potatoes are all equally vulnerable to the new threat.
- Without genetic variations, the population cannot adapt and may go extinct.
- The sexually reproducing potatoes, with a range of alleles that affect traits in different ways, may have a few individuals that are more able to handle the threat.
- The individuals carrying the newly beneficial alleles could be the key to the population’s survival.
65
Diagram of the potatoes in Fields A and B
65
Diagram of the potatoes in Fields A and B
66
Genetic variation is the raw material on which ___ acts.
Natural selection
67
Briefly explain natural selection
Helpful variations become more common in a population as harmful variations diminish, enabling the species to evolve.
68
Why can genetic diversity be a crucial advantage?
Because it gives a species resilience and flexibility in a changing environment.
69
What are the two events in meiosis that are especially important in increasing genetic variation in offspring?
- Crossing over during prophase I
- Random orientation of the tetrads during metaphase I
70
Explain how crossing over during prophase I increases genetic variation in offspring
- Homologous chromosomes come together and sections are exchanged between non-sister chromatids.
- This allows the mixing of alleles from the two parental chromosomes to form new, nearly limitless combinations in gametes.
- This occurs during gamete formation in both parents.
71
Explain how random orientation of the tetrads during metaphase I increases genetic variation in offspring
- The orientation of homologous chromosomes at the equatorial plate during metaphase I determines which pole each pair of sister chromatids will move toward during anaphase I.
- The orientation of each pair of chromosomes is independent of other such pairs.
- Thus, the allele inherited for one gene will not affect the allele inherited for another, a phenomenon known as independent assortment.
72
Diagram of random orientation in metaphase I
73
How many possible chromosome combinations in the daughter cells are there in a cell with two pairs of chromosomes (2n = 4) that is undergoing meiosis and how can this be calculated?
- There are four possible chromosome combinations in the daughter cells.
- The number of possible chromosome combinations in the gametes can be calculated using 2^n (where n is the haploid number of chromosomes).
74
Give an example of how the number of possible chromosome combinations in the gametes can be calculated using 2^n
- For a diploid organism where 2n = 16 (i.e. 8 pairs of chromosomes), the number of combinations would be 2^8 = 256 different combinations.
- For humans with 46 chromosomes, that number is 2^23 = 8,388,608 different combinations.
75
How does sexual reproduction increase genetic variation in offspring through the fusion of gametes from different parents?
- The fusion of male and female gametes (sperm and egg) from different parents combines alleles from two different sources in the diploid zygote.
- The sperm may carry alleles that have never before been combined with alleles found in the egg.
- Since the genetic information is coming from two different sources, this creates a much broader range of possibilities.
- Further, which sperm and egg are involved in fertilisation is random.
76
In what three ways does sexual reproduction promote genetic variation?
- Crossing over between homologues in prophase I
- Random orientation of tetrads in metaphase I
- Fusion of gametes from two individuals
77
For a species that has 2n=12 chromosomes, how many different combinations of chromosomes are possible for the gametes?
64
The number of possible combinations is equal to 2 to the power of the number of haploid chromosomes so in this case 2^6=64.
78
Give an example of how errors can occur during meiosis
79
Give an example of how errors can occur during meiosis
- For example, during anaphase I, a pair of homologous chromosomes can fail to separate. During anaphase II, sister chromatids can fail to separate.
- This error is called non-disjunction.
- It leads to a daughter cell, and ultimately a gamete, with two copies of a particular chromosome, and another gamete without any copies.
- When the gamete with the extra chromosome is fertilised by a typical gamete, the zygote will have three copies of a homologue.
- This is called trisomy (tri = three).
79
Give an example of how errors can occur during meiosis
80
Diagram of an overview of meiosis and timing of non-disjunction
81
When can non-disjunction occur?
- At anaphase I or anaphase II.
- Non-disjunction can also occur during anaphase of mitosis, though this usually impacts too few cells to be noticed.
82
What happens if a non-disjunction event occurs very early in embryonic development?
- The individual may have many cells with 46 chromosomes and many with 47 (or 45).
- This is called mosaicism and it accounts for about 2% of Down syndrome cases.
83
Explain what happens when non-disjunction occurs at any of the 23 chromosome pairs
- Non-disjunction can occur at any of the 23 chromosome pairs, resulting in a gamete with an extra or missing copy of that chromosome.
- Down syndrome, which involves chromosome 21 specifically, is well known because the symptoms are relatively mild.
84
What can many trisomies cause?
- Symptoms so severe that the embryo will not develop.
- Monosomy, having one chromosome of a homologous pair, can also occur.
- In humans, there is only one survivable monosomy; having a single X chromosome for the sex chromosome pair.
85
Non-disjunction is a ___ occurrence, but the majority of these gametes do not ___
Frequent
Mature or take part in fertilization.
86
Diagram of non-disjunction takes place in anaphase II and what the consequences for a zygote would be, leading to trisomy
87
One of the most frequently seen results of non-disjunction in humans is ___
Trisomy 21, or Down syndrome.
88
What is trisomy and how is this different than triploid?
- Trisomy is having an extra copy of a single chromosome (2n+1).
- This is different than triploidy or having three copies of every chromosome (3n).
- n = number of chromosome types.
89
Describe the correlation between the age of a mother during pregnancy and the frequency of chromosomal differences
- A number of studies have shown a positive correlation between the age of the mother during pregnancy and the incidence of chromosomal differences.
- The graph in Figure 4 shows that after the age of 30–34 there is a sharp increase in chromosomal anomalies.
- As the mother's age increases, there is a greater chance of non-disjunction.
90
Graph showing the correlation between age of mother and incidence of chromosomal anomalies
91
Describe the correlation between the age of the father and non-disjunction
- There is also some evidence suggesting a similar correlation between the age of the father and non-disjunction.
- However, it is more difficult to establish a clear connection.
- Genetic analysis shows that almost 90% of cases of Down syndrome arise through non-disjunction in the mother, while less than 10% are caused by non-disjunction in the father.
92
Facts about the chromosomal abnormalities in of babies born to 40 year old mothers and children born to mothers younger than 35
Although increased maternal age is an established risk factor for atypical chromosome numbers in offspring, it is also true that:
- Over 98% of children born to 40-year-old mothers will have typical chromosome numbers.
- 75% of children with Down syndrome are born to mothers younger than 35. That is because many more children are born to younger mothers. Although young mothers have a lower risk of chromosomal differences per child, the overall number of children born with these differences is higher.
93
How is information about the influence of parental age on non-disjunction useful?
Couples who are considered high risk for non-disjunction can consider the likelihood of these events and whether to use diagnostic tests (discussed in the next section) to screen for the number and type of chromosomes during a pregnancy.
94
What is non-disjunction?
The failure of sister chromatids to separate during anaphase II.
95
What is the most common cause of Down syndrome?
Non-disjunction during meiosis in the mother
96
What is a karyogram?
An image of a cell’s homologous chromosome pairs ordered by decreasing size.
97
What is karyotyping?
A method used to check for number and type of chromosomes.
98
Why might karyotyping be used?
Sometimes parents want to know a fetal karyotype before birth, especially if there is a high likelihood of a chromosomal abnormality like Down syndrome (trisomy 21) or Patau syndrome (trisomy 13).
99
Diagram of the karyotype of a female with trisomy 13
100
Why is finding an individual’s karyotype generally simple?
Because almost any type of cell will contain the full set of chromosomes.
101
When is karyotyping difficult?
- Before birth, human offspring are isolated inside the closed amniotic sac.
- This protects against infection but also makes it difficult to collect fetal cells.
- During the first 10 weeks of a 40-week pregnancy, the developing human is called an embryo, and after week 10 it is called a fetus.
- There are two commonly accepted ways to obtain fetal material for tests such as karyotyping.
102
What are the two commonly accepted ways to obtain fetal material for tests such as karyotyping?
- Amniocentesis
- Chorionic villus sampling (CVS)
103
Explain how amniocentesis works
- As the fetus develops in the uterus, it is cushioned by amniotic fluid.
- Amniocentesis is usually performed between weeks 14 and 20 of pregnancy.
- A doctor uses ultrasound imagery to guide a syringe needle through the abdomen and uterine wall without piercing the fetus.
- The needle is then used to withdraw a small amount of amniotic fluid.
- Fetal cells floating in the fluid are cultured and karyotyped.
104
Diagram of an amniocentesis
105
What are the risks to the fetus from amniocentesis?
- Risks to the fetus include infection, fetal trauma from the needle, and miscarriage.
- The risk of miscarriage is between 0.1 and 1.0% and varies by practitioner.
106
Explain how chorionic villus sampling (CVS) works
- Early in pregnancy there is not enough amniotic fluid to perform amniocentesis safely; however, during weeks 10–13, CVS can be used.
- As in amniocentesis, ultrasound imaging is used to guide the medical professional during the sampling and avoid harm to the developing embryo or fetus.
- Fetal cells are sampled by inserting a suctioning tool (often a catheter or syringe) through the vagina or abdomen to reach the fetal cells in the chorion.
- The chorion is a membrane that surrounds the fetus and develops into part of the placenta.
107
Diagram of chorionic villus sampling extracts a small amount of chorion-containing fetal cells for karyotyping
108
What are the risks associated with CVS?
- These include bleeding, infection, and miscarriage.
- The risk of miscarriage is 0.5–2.0%, somewhat higher than with amniocentesis.
109
What new technique (other than amniocentesis and CVS) has been developed in recent years fetal karyotyping?
- In the last few years, a technique has been developed that determines fetal karyotype using minute amounts of fetal DNA found in maternal blood.
- This technique would provide the same information as amniocentesis or CVS without the risk of miscarriage.
110
Why may parents choose to terminate a pregnancy after fetal karyotyping?
- These technologies provide information that can help parents and doctors make choices and prepare for the birth of a child that may have particular medical requirements.
- In some places, parents may decide to terminate the pregnancy based on the results of the karyotype.
- Some parents choose to end a pregnancy only if the combination of chromosomes cannot be survived, some if there is a chromosomal difference that could cause developmental challenges, and some based on the biological sex of the child.
- In some places, sons are strongly preferred over daughters, leading to selective abortion of female fetuses.
- These technologies, like many others, raise ethical questions about how they should be used.
111
Purpose of amniocentesis
Obtain fetal cells for karyotyping or other tests
112
Time in pregnancy for amniocentesis
14–20 weeks
113
Source of fetal cells for amniocentesis
Amniotic fluid
114
Risk of miscarriage in amniocentesis
≤1%
115
Purpose of CVS
Obtain fetal cells for karyotyping or other tests
116
Time in pregnancy for CVS
10–13 weeks
117
Source of fetal cells for CVS
Chorion (membrane)
118
Risk of miscarriage in CVS
≤2%
