12 meiosis is a type of nuclear division. it results in cells that have · · 2014-12-16it...

Chapter 12: Meiosis

Unit 3 AP Biology 1

© 2011 Pearson Education, Inc.

Lectures by Stephanie Scher Pandolfi

BIOLOGICAL SCIENCE

FOURTH EDITION

SCOTT FREEMAN

12Meiosis


Chapter 12: Meiosis part 1 Key Concepts

Meiosis is a type of nuclear division. It results 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. Because genes are located on chromosomes, each

cell produced by meiosis receives a different complement of

genes. Meiosis leads to offspring that are genetically distinct from

each other and from their parents.


Key Concepts

The leading hypothesis to explain meiosis is that genetically

variable offspring are more likely to 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. It is rare

for offspring with an incorrect number of chromosomes to

develop normally.


Introduction

• During sexual reproduction, a sperm and an egg unite to form a

new individual.

– This process is called fertilization.

• Meiosis is nuclear division that precedes the formation of gametes

(egg and sperm) and results in a halving of chromosome number.

Chapter 12: Meiosis

Unit 3 AP Biology 2


Chromosomes Come in Distinct Types

• Each organism has a characteristic number of chromosomes.

• The karyotype is the number and types of chromosomes present in

an organism.

• Sex chromosomes determine the sex of the individual; all other

chromosomes are autosomes.

– Humans have 46 chromosomes in every cell except their

gametes.

– 1 pair of sex chromosomes.

– 22 pairs of autosomes.



Homologous Chromosomes Have the Same Genes

• Chromosomes of the same type are called homologous

chromosomes, or homologs.

• Chromosomes carry genes. A gene is a section of DNA that

influences one or more hereditary traits in an individual.

– Different versions of a specific gene are called alleles.

• Homologs carry the same genes in the same locations, but each one

may contain different alleles.


Chapter 12: Meiosis

Unit 3 AP Biology 3


The Concept of Ploidy

• 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.


Ploidy Varies among Organisms

• 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.

• Organisms with three or more versions of each type of chromosome

are called polyploid (3n, 4n, etc.)

© 2011 Pearson Education, Inc. © 2011 Pearson Education, Inc.

An Overview of Meiosis

• Meiosis reduces chromosome number by half. In diploid

organisms, the products of meiosis are haploid.

• Just before meiosis begins, each chromosome in the diploid (2n)

parent cell is replicated.

– When replication is complete, each chromosome consists of

two identical sister chromatids attached at the centromere.

Chapter 12: Meiosis

Unit 3 AP Biology 4


Meiosis Is Two Cell Divisions

• Meiosis consists of two cell divisions, called meiosis I and meiosis

II.

• The two divisions occur consecutively but differ sharply.


An Overview of Meiosis I

• 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.


An Overview of Meiosis II

• During meiosis II, the sister chromatids of each chromosome

separate and go to different daughter cells.

• The four haploid daughter cells produced by meiosis II also have

one of each type of chromosome, but now the chromosomes are

unreplicated.

Chapter 12: Meiosis

Unit 3 AP Biology 5


Meiosis Is a Reduction Division

The outcome of meiosis is a reduction in chromosome number.

For this reason, meiosis is known as a reduction division.

• 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.


Fertilization Results in a Diploid Zygote

• When two haploid gametes fuse during fertilization, a full

complement of chromosomes is restored. The cell that results from

fertilization is diploid and is called a zygote.

• In this way, 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.


Chapter 12: Meiosis

Unit 3 AP Biology 6


The Life Cycle of a Sexual Organism

• An animal’s life cycle summarizes life from fertilization through

offspring production.

• Meiosis in an adult produces haploid gametes that combine during

fertilization to form a diploid zygote, which develops, through

mitosis, into an adult of the next generation.



The Phases of Meiosis I

• Meiosis I is a continuous process with five distinct phases. These

phases are as follows:

1. Early prophase I

2. Late prophase I

3. Metaphase I

4. Anaphase I

5. Telophase I


Chapter 12: Meiosis

Unit 3 AP Biology 7



Chapter 12: Meiosis

Unit 3 AP Biology 8




• Early Prophase I: The homolog 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.

• Late Prophase I: These non-sister chromatids begin to separate.

Exchange or crossing over between homologous non-sister

chromatids occurs where chiasmata are formed during this stage.

• Metaphase I: The tetrads line up at the metaphase plate.

Chapter 12: Meiosis

Unit 3 AP Biology 9



• Anaphase I: The paired homologs separate and begin to migrate to

opposite ends of the cell.

• Telophase I: The homologs finish migrating to the poles of the cell.

Then the cell divides in the process of cytokinesis.


The Result of Meiosis I

• The end result of meiosis I is that one chromosome of each

homologous pair is distributed to a different daughter cell.

• A reduction division has occurred.

– The daughter cells of meiosis I are haploid and are still in the

form of sister chromatids.


The Phases of Meiosis II

• Like meiosis I, meiosis II is a continuous process, but with four

distinct phases:

1. Prophase II

2. Metaphase II

3. Anaphase II

4. Telophase II


The Phases of Meiosis II

• Prophase II: The spindle apparatus forms and one spindle fiber

attaches to the centromere of each sister chromatid.

• Metaphase II: Replicated chromosomes line up at the metaphase

plate.

• Anaphase II: Sister chromatids separate. The resulting daughter

chromosomes begin moving to opposite sides of the cell.

• Telophase II: Chromosomes arrive at opposite sides of the cell. A

nuclear envelope forms around each haploid set of chromosomes,

and each cell undergoes cytokinesis.

Chapter 12: Meiosis

Unit 3 AP Biology 10


The Result of Meiosis II

• Meiosis II results in four haploid cells, each with one of each type

of chromosome.

– Thus, one diploid cell with replicated chromosomes gives rise

to four haploid cells with unreplicated chromosomes.


Comparison of Meiosis and Mitosis

• The key difference between the two processes is that 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 thus produces four daughter cells with half the genetic

material of the parents, while mitosis produces two daughter cells

that are genetically identical to the parent cells.


Chapter 12: Meiosis




Web Activity: Meiosis

Meiosis


Where Does Crossing Over Occur?

• After replication, sister chromatids stay tightly joined along their

entire length.

• When homologs synapse, two pairs of non-sister chromatids are

brought close together and are held there by a network of proteins

called the synaptonemal complex.

• Crossing over occurs when chromosomal segments are swapped

between adjacent homologs.

Meiosis.html

Meiosis.html

Chapter 12: Meiosis



A Closer Look at Crossing Over

• At each point where crossing over occurs, the non-sister chromatids

from each homolog get physically broken at the same point and

attached to each other. As a result, segments of maternal and

paternal chromosomes are swapped.

• Crossing over can occur at many locations along the length of such

synapsed homologs. Both sets of non-sister chromatids may

undergo crossing over, resulting in the swapping of segments

between maternal and paternal chromosomes.



The Consequences of Meiosis

• Independent shuffling of maternal and paternal chromosomes and

crossing over during meiosis I result in four gametes with a

chromosome composition different from that of the parent cells.

• The changes in chromosomes produced by meiosis and fertilization

are significant because chromosomes contain the cell’s hereditary

material.


Sexual Reproduction Leads to Greater Variation

• Offspring produced during asexual reproduction are clones that

are genetically identical to one another as well as to the parent.

• In contrast, offspring produced by sexual reproduction, the fusion

of gametes, have a chromosome makeup different from that of one

another and from that of either parent.

• Genetic variation in sexual reproduction results from independent

assortment and crossing over.

Chapter 12: Meiosis



Independent Assortment Produces Genetic Variation

• Separation and distribution of homologous chromosomes during

meiosis I can result in a variety of combinations of maternal and

paternal chromosomes.

• 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.



Chapter 12: Meiosis



BLAST Animation: Genetic Variation: Independent Assortment

Genetic Variation: Independent Assortment


The Role of Crossing Over

• 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.


How Does Fertilization Affect Genetic Variation?

Crossing over and the random mixing of maternal and paternal

chromosomes ensure that each gamete is genetically unique.

• 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.


Independent_assortment.html

Independent_assortment.html

Chapter 12: Meiosis




Outcrossing Further Increases Genetic Variation

• 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, 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!


BLAST Animation: Genetic Variation: Fusion of Gametes

Genetic Variation: Fusion of Gametes

Gamete_fusion.html

Gamete_fusion.html

Chapter 12: Meiosis



Why Does Meiosis Exist?

• Sexual reproduction is common among multicellular organisms, but

organisms in most lineages of the tree of life undergo asexual

reproduction.


The Paradox of Sex

• The mathematical model of John Maynard Smith predicts that

asexually reproducing organisms should reproduce faster and

outcompete similar organisms that invest in sexual reproduction.

• Asexual reproduction is much more efficient than sexual

reproduction because no males are produced.


The Purifying Selection Hypothesis

• In asexual reproduction, a damaged gene will be inherited by all of

that individual’s offspring.

• On the other hand, sexually reproducing individuals are likely to

have offspring that lack deleterious alleles present in the parent.

• Natural selection against deleterious alleles is called purifying

selection. Over time, purifying selection should steadily reduce the

numerical advantage of asexual reproduction.

Chapter 12: Meiosis



The Changing Environment Hypothesis

• The changing environment hypothesis states that offspring that are

genetically different from their parents (those produced by sexual

reproduction) may be more likely to survive and produce offspring

in turn if the environment changes than offspring that are

genetically identical to their parents (those produced by asexual

reproduction).


Testing the Changing Environment Hypothesis

If a new strain of disease-causing agent evolves, then all of the

asexually produced offspring are likely to be susceptible to that

new strain. But if the offspring are genetically varied, then it is

likely that at least some offspring will have combinations of

alleles that enable them to fight off the new disease and produce

offspring of their own.

• Studies support the changing environment hypothesis. Thus,

sexual reproduction may be an adaptation that increases the

fitness of individuals in certain environments.


Chapter 12: Meiosis




Mistakes in Meiosis

• 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.


How Do Mistakes Occur?

• For a gamete to get one complete set of chromosomes, each pair of

homologous chromosomes must separate from each other during

the first meiotic division, and the sister chromatids must separate

from each other and move to opposite poles during meiosis II.

• If both homologs or both sister chromatids move to the same pole

of the parent cell, the products of meiosis will be abnormal. This

sort of meiotic error is referred to as nondisjunction.

Chapter 12: Meiosis



Types of 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.


Frequency of Nondisjunction

• Nondisjunction may occur in as many 10 percent of meiotic

divisions. However, aneuploid zygotes (those with too few or too

many chromosomes) typically do not survive to produce viable

offspring.

Mistakes in meiosis are the leading cause of spontaneous abortion

(miscarriage) in humans.


Why Do Mistakes Occur?

• Meiotic errors appear to be accidental, with no genetic

predisposition.

• Maternal age is an important factor in the frequency of trisomy.

Chapter 12: Meiosis



Most Common Aneuploidy Disorders

• 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.


Web Activity: Mistakes in Meiosis

Mistakes in Meiosis

MistakesInMeiosis.html

MistakesInMeiosis.html

12 meiosis is a type of nuclear division. it results in cells that have · · 2014-12-16it...

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