Chapter 3.3

Question 1

Meiosis

Answer 1

A form of nuclear division that produces four haploid nuclei from one diploid nucleus.

Question 2

When does meiosis happen

Answer 2

Meiosis is done after DNA replication in the S phase of interphase.

Question 3

Prophase I

Answer 3

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.Crossing over is followed by condensation of DNA into highly organised chromosomes.

Centrioles, if present, migrate to opposite poles and spindle fibres start to form. The nucleolus and nuclear membrane disintegrate.

Question 4

Metaphase I

Answer 4

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.

Question 5

Anaphase I

Answer 5

Spindle microtubules shorten, pulling homologous chromosomes apart towards opposite poles. Unlike mitosis, sister chromatids remain connected at the centromere and move to the same pole.

Question 6

Telophase I

Answer 6

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.

Question 7

Prophase II

Answer 7

Chromosomes condense again. Centrioles, if present, migrate to opposite poles and spindle fibres start to form. The nucleolus and nuclear membrane disintegrate.

Question 8

Metaphase II

Answer 8

The spindle fibres attach to the centromere and connect each centromere to both poles. They exert a gentle pull to align the sister chromatids at the equator.

Question 9

Anaphase II

Answer 9

Centromeres divide and chromatids are moved to opposite poles by spindle fibres.

Once sister chromatids are separated, they are called chromosomes.

Question 10

Telophase II

Answer 10

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 are genetically distinct.

Question 11

Draw all stages of meiosis

Answer 11

https://app.kognity.com/study/app/2024-dp1-biology-hlsl/sid-47-cid-176856/book/early-stages-core-id-1813/

Question 12

Chiasmata and crossing over

Answer 12

Crossing over occurs when equivalent portions of the non-sister chromatids are exchanged between homologous chromosomes. The points at which crossing over occurs are called chiasmata. Crossing over creates new combinations of alleles that were not present in either original chromosome. This contributes to genetic variation among the gametes produced. Further, crossing over can occur almost anywhere along the chromosome. 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.

Question 13

Two important changes that result from chromosomes being in pairs instead of individually

Answer 13

1. Reduction division: Daughter cells contain only half of the chromosomes that were present in the parent cell. The parent cell is diploid. The daughter cells will be haploid, having only one version of each chromosome.
2. Random orientation: When pairs of homologous chromosomes line up at the equator of the cell, the paternal copy has an equal chance of facing either pole. The orientation of one pair 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. Because of all the possible combinations of tetrad orientations, random orientation contributes to genetic diversity in the gametes.

Question 14

What does sexual reproduction allow?

Answer 14

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.

Question 15

Advantage of sexual reproduction

Answer 15

Genetic variation is the raw material on which natural selection acts – helpful variations become more common in a population as harmful variations diminish, enabling the species to evolve. Genetic diversity can be a crucial advantage because it gives a species resilience and flexibility in a changing environment.

Question 16

What in sexual reproduction causes genetic diversity

Answer 16

1. crossing over
2. random orientation
3. fusion of gametes between maternal and paternal parents - The fusion of male and female gametes from different parents combines alleles from two different sources in the diploid zygote. 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.

Question 17

non-disjunction

Answer 17

During anaphase I and II, the homologous chromosomes can fail to separate. 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.

Question 18

Age affects chromosomal differences

Answer 18

Age > higher the age of the mother, the more likely the chance of chromosomal differences
Couples who are considered high-risk for non-disjunction can consider the likelihood of these events and whether to use diagnostic tests to screen for the number and type of chromosomes during a pregnancy.

Question 19

karyotyping and chromosomal differences

Answer 19

Karyotyping can look at the chromosomes in a cell to determine certain differences. Parents use it to determine whether or not their child may have Patau’s or Down syndrome.

Question 20

Amniocentesis

Answer 20

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. 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.

Question 21

Chorionic villus sampling (CVS)

Answer 21

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. The risks associated with CVS include bleeding, infection and miscarriage. The risk of miscarriage is 0.5–2.0%, somewhat higher than with amniocentesis.

Question 22

Maternal blood and karyotyping

Answer 22

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.