BLG 143 Study Guide - Final Guide: Sister Chromatids, Homologous Chromosome, Allosome

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Chapter 12 – Meiosis

➢ Meiosis is a type of nuclear division resulting in cells that have half as many

chromosomes as the parent cell

➢ In animals, it leads to the formation of eggs and sperm

➢ Each cell produced by meiosis receives a different combination of chromosomes

➢ Since genes are located on chromosomes, each cell produced receives a different

complement of genes

➢ Resulting offspring are genetically distinct form each other and their parents

➢ Leading hypothesis: genetically varied offspring are more likely to include

individuals that can thrive in environments where parasites and disease are

common

➢ If mistakes occur during meiosis, the resulting egg and sperm cells may contain the

wrong number of chromosomes (Trisomy-21 or Down Syndrome)

➢ It’s rare for offspring with incorrect number of chromosomes to develop normally

During sexual reproduction, a sperm and egg (gametes, reproductive cells) unite to form a

new individual  fertilization.

Meiosis  nuclear division that precedes formation of gametes and results in having

chromosome number.

12.1 How Meiosis Occurs

Chromosomes

• Sex chromosome  determines sex of individual, X

• Autosomes  all other types, regular chromosomes

• Homologous chromosomes  XX (girl); carry the same

genes (section of DNA that influences some hereditary

trait/characteristic in an individual)

• Homologs carry same genes in same locations, but gene may contain different alleles

• Allele  different versions of the same gene (eye colour)

• Humans: 46 chromosomes in every cell except gametes, 1 pair of sex chromosomes

(XX or XY) and 22 pairs of autosomes

The Concept of Ploidy

• Karyotype  the number and types of chromosomes present

• Diploid  multicellular species that have 2 copies of each chromosome

• Diploid cells have 2 alleles of each gene. During reproduction, diploid organisms

produce gametes that only have 1 of each chromosome  haploid cells

• Polyploid  more than 2 of each chromosome (some plants, wheat)

 number chromosomes present in a gamete and is the haploid number

human sperm and egg cells: n=23 chromosomes

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Animals are a result of 2 gametes joining together. Gamete

from mother carries the maternal chromosomes and from

father, paternal chromosomes.

Each cell in an animal has twice the number of

chromosomes as a gamete, for a total number of

humans: 2n=46

Meiosis reduces chromosome number by half. In diploid

organisms, products of meiosis are haploid.

An unreplicated chromosome consists of a single DNA

molecule with its associated proteins, while a replicated

chromosome consists of 2 sister chromatids. An

unreplicated chromosome is a single thread; a replicated

chromosome has paired threads.

Unreplicated and replicated chromosomes are both considered single chromosomes, even

though the replicated chromosome comprises 2 sister chromatids.

Just before meiosis,

1. Each chromosome in diploid (2n) parent cell is replicated

2. When replication is complete, each chromosome consists of 2 identical sister

chromatids attached at centromere (cohesion proteins)

Meiosis is 2 Cell Divisions

Meiosis I

• Homologs in each chromosome pair separate from

each other

• One homolog goes to one daughter cell and the other

goes to the other daughter cell

• Each of the 2 daughter cells has one of each type of

chromosome instead of 2

• Diploid (2n) parent cell produces 2 haploid (n)

daughter cells

• Each chromosome still consists of 2 sister chromatids

 chromosomes are still replicated

Meiosis II

• Sister chromatids from each chromosome separate (each one goes to one, different

daughter cell)

• Cells produced (4 haploid daughter cells) have one of each type of chromosome but

now the chromosomes are unreplicated

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Document Summary

Meiosis is a type of nuclear division resulting in cells that have half as many chromosomes as the parent cell. In animals, it leads to the formation of eggs and sperm. Each cell produced by meiosis receives a different combination of chromosomes. Since genes are located on chromosomes, each cell produced receives a different complement of genes. Resulting offspring are genetically distinct form each other and their parents. Leading hypothesis: genetically varied offspring are more likely to include individuals that can thrive in environments where parasites and disease are common. If mistakes occur during meiosis, the resulting egg and sperm cells may contain the wrong number of chromosomes (trisomy-21 or down syndrome) It"s rare for offspring with incorrect number of chromosomes to develop normally. During sexual reproduction, a sperm and egg (gametes, reproductive cells) unite to form a new individual fertilization. Meiosis nuclear division that precedes formation of gametes and results in having chromosome number.

Related Questions

Match the terms to the definitions provided.

Question 1 options:

	gamete
	somatic cells
	zygote
	sexual reproduction

1.	Reproductive life cycle that involves an alternation between haploid and diploid cells
2.	A single cell with a diploid number of chromosomes resulting from the fusion of an egg and a sperm.
3.	Cells (egg or sperm) resulting from a meiotic division of germ-line cells
4.	The non-reproductive cells found in embryos and mature individuals

Question 2 (5 points)

Match the terms to the definitions provided.

Question 2 options:

	syngamy
	haploid
	meiosis
	diploid
	germ-line cells

1.	The cells that will eventually undergo meiosis to produce gametes.
2.	The number of chromosomes found in gametes (eggs or sperm).
3.	The process by which two haploid gametes fuse to form a new diploid cell.
4.	A specialized reduction division that produces cells with half the normal number of chromosomes.
5.	Two sets of chromosomes present in the somatic cells of adult individuals.

Which statements pertaining to genetic variability are correct? Check all that apply (note that wrong answers will be deducted from right answers)!

Crossing over is defined by the exchange of chromosome pieces during metaphase one of meiosis.

If a germ cell (i.e. parent cell before meiosis) had only two pairs of homologous chromosomes (i.e. 4 chromosomes total) there are 2 possible combinations of maternal and paternal chromosomes in gametes.

Siblings produced by sexual reproduction are different because sperms are different from one another and eggs are different from one another, and the combination of an egg and sperm is basically unique.

Random fertilization means that all possible combinations of eggs and sperm are equally likely, i.e. it is pure chance which sperm fuses with which egg.

Individual chromosomes contain thousands of genes. This means that during each crossing over event, more than one gene is exchanged between paternal and maternal chromosomes.

limedonkey586

-Complete the following sentences: metaphase of mitosis, prophase II of meiosis, crossing over, anaphase I of meiosis, variable, random fertilization, identical, random orientation, metaphase I of meiosis, prophase I of meiosis, metaphase II of meiosis, reduction division

Two processes that occur during meiosis produce genetically ____________ cells.

In a process called ____________ , paired homologous chromosomes align randomly at the cell equator. This occurs during this stage of cell division: ____________ .

In a process called ____________ , genetic material is exchanged between homologous chromosomes. This occurs during this stage of cell division: ____________ .

-Complete the following paragraph to describe nondisjunction and the resulting effects on chromosome number.

words: euploidy, primary nondisjunction, nomal, nondisjunction, secondary nondisjunction, homologous chromosomes, abnormal, trisomy, monosomy, sister chromatids, aneuploid

1.During meiosis I, ____________ separate and move towards opposite ends of the poles.

2.During meiosis II, ____________ separate and move towards opposite ends of the poles.

3.During meiosis I, if both members of a homologous pair go into the same daughter cell, ____________ is said to occur and no normal gametes are produced.

4.During meiosis II, if the sister chromatids fail to separate and both daughter chromosomes go into the same gamete, ____________ is said to occur. Half the gametes are aneuploid while the other half are euploid.

5.If an egg that contains 24 chromosomes instead of 23 chromosomes is fertilized by a normal sperm, ____________ is said to have occurred.

6.If an egg that contains 22 chromosomes instead of 23 chromosomes is fertilized by a normal sperm, ____________ is said to have occurred.

BLG 143 Study Guide - Final Guide: Sister Chromatids, Homologous Chromosome, Allosome

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