BIOL #11: Meiosis and Sexual Reproduction

Question 1

Fertilization

Answer 1

During sexual reproduction, a sperm and an egg unite to form a new individual.

Question 2

Meiosis

Answer 2

Meiosis is special nuclear division that precedes the formation of gametes (egg and sperm) and results in a halving of chromosome number.

Question 3

Karyotype

Answer 3

The number and types of chromosomes present in an organism.

A karyotype is determined by ordering images of chromosomes, which are identified by their banding patterns

Can be used to identify chromosomal disorders

Question 4

Sex Chromosomes & Autosomes

Answer 4

Sex chromosomes determine the sex of the individual; all other chromosomes are autosomes.

A karyotype can also be used to determine an individual’s sex or sex chromosome disorder

Question 5

Human Karyotype

Answer 5

Humans have 46 chromosomes in every cell except their gametes.

1 pair of sex chromosomes, 22 pairs of autosomes.

Question 6

Homologous Chromosomes

Answer 6

Chromosomes of the same type are called homologous chromosomes, or homologs.

Question 7

Gene

Answer 7

Chromosomes carry genes. A gene is a section of DNA that influences one or more heritable traits in an individual.

Different versions of a specific gene are called alleles.

Heritable = passed on from parent to offspring

Question 8

The Concept of Ploidy

Answer 8

The haploid number n indicates the number of distinct types of chromosomes present.

A cell’s ploidy (n, 2n, 3n, etc.) indicates the number of each type of chromosome present.

Organisms whose cells contain just one of each type of chromosome are called haploid (n).

Those whose cells contain two versions of each type of chromosome are termed diploid (2n).
- Diploid cells have one paternal chromosome and one maternal chromosome (i.e. homologous pairs).

Organisms with three or more versions of each type of chromosome are called polyploid (3n, 4n, etc.)
- Many flowering plant species are polyploids

Question 9

Zygote

Answer 9

For diploid organisms, fertilization occurs when two haploid gametes fuse
- A full complement of chromosomes is restored. The cell that results from fertilization is diploid and is called a zygote.

Question 10

Maternal & Parental Chromosomes

Answer 10

Each diploid individual receives a haploid chromosome set from both its mother and its father.

Homologs are therefore referred to as being either maternal chromosomes, from the mother, or paternal chromosomes, from the father.

Question 11

Meiosis I

Answer 11

During meiosis I, the diploid (2n) parent cell produces two haploid (n) daughter cells.

The homologs in each chromosome pair separate and go to different daughter cells.

Although the daughter cells are haploid (n), each chromosome still consists of two identical sister chromatids.

Question 12

Meiosis II

Answer 12

During meiosis II, the sister chromatids of each chromosome separate and go to different daughter cells.

The four haploid (n) daughter cells produced by meiosis II also have one of each type of chromosome, but now the chromosomes are unreplicated.

Question 13

Reduction Division

Answer 13

The outcome of meiosis is a reduction in chromosome number. For this reason, meiosis is known as a reduction division.

Question 14

Gametogenesis

Answer 14

In most plants and animals, the original cell is diploid and the four daughter cells are haploid.
- In animals, these daughter cells become gametes via a process called gametogenesis.
+ Gametogenesis in humans produces sperm cells in the testes and egg cells in the ovaries

Question 15

Prophase I

Answer 15

The homologous pairs come together in a pairing process called synapsis. The structure that results from synapsis is called a tetrad, consisting of two homologs.

The chromatids of the homologs are called non-sister chromatids.

These non-sister chromatids begin to separate. Exchange or crossing over between homologous non-sister chromatids occurs where chiasmata are formed during this stage.

Question 16

Metaphase I

Answer 16

The tetrads line up at the metaphase plate.

Question 17

Anaphase I

Answer 17

The paired homologs separate and begin to migrate to opposite ends of the cell.

“Sister” chromatids remain attached

Question 18

Telophase I

Answer 18

The homologs finish migrating to the poles of the cell. Then the cell divides in the process of cytokinesis.

Question 19

Interkinesis

Answer 19

A period between Meiosis I and Meiosis II which is similar to Interphase of the regular cell cycle

In some species, chromosomes de-condense and the nuclear envelope reforms during interkinesis

No chromosome replication occurs between Meiosis I and Meiosis II

Question 20

Prophase II

Answer 20

The spindle apparatus forms and one spindle fiber attaches to the centromere of each sister chromatid.

Question 21

Metaphase II

Answer 21

Replicated chromosomes line up at the metaphase plate.

Question 22

Anaphase II

Answer 22

Sister chromatids separate. The resulting daughter chromosomes begin moving to opposite sides of the cell.

Question 23

Telophase II

Answer 23

Chromosomes arrive at opposite sides of the cell. A nuclear envelope forms around each haploid set of chromosomes, and each cell undergoes cytokinesis.

Question 24

Comparison of Meiosis and Mitosis

Answer 24

Key differences between the two processes:
- Homologs pair in meiosis, but do not in mitosis.
+ Because homologs pair in prophase of meiosis I, they can migrate to the metaphase plate together and then separate during anaphase of meiosis I, resulting in a reduction division.
- Meiosis produces four daughter cells with half the genetic material of the parents, mitosis produces two daughter cells that are genetically identical to the parent cells.

Question 25

The Consequences of Meiosis

Answer 25

The changes in chromosomes produced by meiosis and fertilization are significant because chromosomes contain the cell’s heritable material.

Mechanisms that increase the diversity in combinations of genetic material, which make individuals genetically unique, include (1) independent assortment and (2) crossing over during meiosis, plus (3) random fertilization

Question 26

Independent Assortment

Answer 26

Random separation and distribution of homologous chromosomes during meiosis I can result in a variety of combinations of maternal and paternal chromosomes.

Independent assortment refers to the sorting of maternal and paternal homologs into daughter cells independently of every other homologous pair

Each daughter cell gets a random assortment of maternal and paternal chromosomes, and thus genes, which generates a great deal of genetic diversity in the subsequent gametes.

Humans, with a haploid chromosome number of 23, can produce 223 (~ 8.4 million) different combinations of chromosomes in gametes.

Question 27

Crossing Over

Answer 27

Crossing over produces new combinations of alleles on the same chromosome, combinations that did not exist in each parent.

Crossing over is a form of genetic recombination that increases the genetic variability of gametes produced by meiosis beyond that produced by random assortment of chromosomes.

Question 28

Self-Fertilization

Answer 28

The genetic variation introduced during meiosis ensures that even in self-fertilization, where gametes from the same individual combine, the offspring will be genetically different from the parent.

Organisms that utilize self-fertilization include: many flowering plants and freshwater snails

Question 29

Outcrossing

Answer 29

In many sexually reproducing species, gametes from different individuals combine to form offspring, a process called outcrossing.

Outcrossing increases the genetic diversity of the offspring because chromosomes from two different parents are combined.
- In humans, with independent assortment and random fertilization, this means that two parents can potentially produce 8.4 million  8.4 million = 70.6  1012 genetically distinct offspring.
+ This does not even take additional variation from crossing over into consideration!

Question 30

Asexual Reproduction

Answer 30

In asexual reproduction, a single individual is the sole parent and passes copies of all of its genes to its offspring without the fusion of gametes
- Offspring are exact copies of parent
+ e.g. hydra budding
- Many organisms that can reproduce asexually also have the means to reproduce sexually – why does this occur?
+ e.g. hydra can produce ovaries and testes and use them to produce variable offspring

Question 31

Purifying Selection Hypothesis

Answer 31

Deleterious alleles, alleles with mutation resulting in loss of function, can have lethal effects

• In asexually reproducing individuals, a damaged gene will be inherited by all of that individual’s offspring.
• Sexually reproducing individuals are likely to have some offspring that lack deleterious alleles present in the parent.

Purifying selection = Natural selection against deleterious alleles
- Over time, purifying selection should steadily reduce the numerical advantage of asexual reproduction.

Question 32

Heterogeneous Environment Hypothesis

Answer 32

Darwin recognized that individuals best suited to their local environment leave the most offspring (= higher fitness), thereby transmitting their genes
- Natural Selection results in the accumulation of genetic variations favored by the environment

In a changing (or heterogeneous) environment, it would benefit individuals not to produce exact copies of themselves

• Due to new mutations being mixed and matched during meiosis, at least some of an individual’s offspring are likely to carry the genetic combinations that allow them to cope with new environmental conditions, thereby still transmitting the individual’s genes
• The vast majority of organisms are subject to heterogeneous environments – across time or space or both

The ability to generate genetic diversity is one of the most commonly proposed explanations for the evolutionary persistence of sexual reproduction

Question 33

Red Queen Hypothesis

Answer 33

The ability to generate genetic diversity is one of the most commonly proposed explanations for the evolutionary persistence of sexual reproduction

Increased genetic diversity in offspring would be beneficial in the face of:

• Parasites
• Disease
• Predator-prey interactions

Coevolutionary arms races are an outcome of the Red Queen Hypothesis

• “It takes all the running [organisms] can do, to keep in the same place”
• Organisms must constantly change to compete with other organisms

Question 34

Mistakes in Meiosis

Answer 34

If a mistake occurs during meiosis I and the chromosomes from the parent cells are not properly distributed to each daughter cell, the resulting gametes will contain an abnormal set of chromosomes.
- This failure of chromosomes to separately properly is called nondisjunction

If nondisjunction occurs in meiosis I, two gametes will have an extra copy of a chromosome (causing a condition called trisomy), and two gametes will lack that chromosome (monosomy).
- An example of trisomy is Down syndrome, which is caused by an extra copy of chromosome 21.

Mistakes in meiosis are the leading cause of spontaneous abortion (miscarriage) in humans.

Question 35

Aneuploidy Disorders

Answer 35

Nondisjunction may occur in as many as 10% of meiotic divisions. However, aneuploid zygotes (those with too few or too many chromosomes) typically do not survive to produce viable offspring.

Most instances of aneuploidy in humans involve Down Syndrome (chromosome 21) or the sex chromosomes.

Sex chromosome aneuploidy can occur in many different forms:

• Klinefelter syndrome develops in XXY males.
• Trisomy X (karyotype XXX).
• Females with Turner Syndrome have monosomy – their karyotype is XO (they are lacking a second X chromosome) and are usually sterile.