Meiosis Honors Biology. Introduction to Heredity Offspring acquire genes from parents by inheriting...

Meiosis

Honors Biology

Introduction to Heredity

Offspring acquire genes from parents by inheriting chromosomes

Inheritance is possible because:

– Sperm and ova carrying each parent’s genes are combined in the nucleus of the fertilized egg

Actual transmission of genes depends on the behavior of chromosomes

•Chromosomes-organizational unit of hereditary material in the nucleus of eukaryotic organisms•Contain hundreds of thousands of genes, each of

which is a specific region of the DNA molecule, or locus

Human Life Cycle

Each somatic cell (body cell) has 46 chromosomes or 23 matching pairs (diploid)

Karyotype: male

Autosomes: non-sex chromosomes

Sex chromosomes:determine gender (XX; XY)

Human Life Cycle

Gametes (sex cells) have a single set of 22 autosomes and a single sex chromosome, either X or Y

With 23 chromosomes, they are haploid

haploid number: n = 23

diploid number: 2n = 46

Haploid sperm + haploid ova zygote (2n)fertilization

Meiosis

Reduces chromosome number from diploid to haploid

Increases genetic variation among offspring

Steps resemble steps in mitosis

Single replication of DNA is followed by 2 consecutive cell divisions Meiosis I Meiosis II

Produces 4 different daughter cells which have half the number of chromosomes as the original cell

In the first division, meiosis I, homologous chromosomes are paired While they are paired, they cross over and exchange

genetic information The homologous pairs are then separated, and two

daughter cells are produced

Interphase I

Chromosomes replicate (still as chromatin)

Duplicated chromosomes consist of 2 identical sister chromatids attached by centromere

Centriole pairs replicate

Figure 8.14, part 1

MEIOSIS I: Homologous chromosomes separate

INTERPHASE PROPHASE I METAPHASE I ANAPHASE I

Centrosomes(withcentriolepairs)

Nuclearenvelope

Chromatin

Sites of crossing over

Spindle

Sisterchromatids

Tetrad

Microtubules attached tokinetochore

Metaphaseplate

Centromere(with kinetochore)

Sister chromatidsremain attached

Homologouschromosomes separate

Meiosis I

This cell division separates the 2 chromosomes of each homologous pair and reduce the chromosome number by one-half

Prophase I

Chromosomes condense

Synapsis occurs (homologues pair)

Chromosomes seen as distinct structures; each chromosome has 2 chromatids, so each synapsis forms a tetrad

Prophase I

Sister chromatids held together by centromeres; non-sister chromatids held together by chiasmata where crossing-over occurs (exchange of DNA)

Late Prophase I

Centriole pairs move apart and spindle fibers formNuclear envelope disappears and nucleoli disperse

Prophase I

Metaphase I

Homologous chromosomes line up along metaphase plate

Metaphase I

Anaphase I

Homologous chromosomes separate, independently from others

Anaphase I

Telophase I and Cytokinesis

Each pole now has a haploid set of chromosomes (each with 2 sister chromatids)Usually, cytokinesis occurs simultaneously with telophase I, forming 2 haploid daughter cells (cleavage furrow forms in animals; cell plate forms in plants)

Telophase I

Meiosis II is essentially the same as mitosis The sister chromatids of each chromosome separate The result is four haploid daughter cells

Figure 8.14, part 2

MEIOSIS II: Sister chromatids separate

TELOPHASE IAND CYTOKINESIS PROPHASE II METAPHASE II ANAPHASE II

Cleavagefurrow

Sister chromatidsseparate

TELOPHASE IIAND CYTOKINESIS

Haploiddaughter cellsforming

Meiosis II

This cell division separates the 2 sister chromatids of each chromosome

Prophase II

Spindle apparatus forms and chromosomes move toward metaphase II plate

Prophase II

Metaphase II

Chromosomes align singly on the metaphase plate

Metaphase II

Anaphase II

Sister chromatids of each pair (now individual chromosomes) separate and move toward opposite poles of the cell

Anaphase II

Telophase II and Cytokinesis

Nuclei form at opposite poles of the cell

Cytokinesis occurs producing 4 haploid daughter cells (each genetically different)

Telophase II

Key Differences Between Mitosis and Meiosis

Meiosis is a reduction division Mitotic cells produce clones (same xsome #) Meiosis produces haploid cells

Meiosis creates genetic variation Mitosis produces 2 identical daughter cells Meiosis produces 4 genetically different

daughter cells Meiosis is 2 successive nuclear divisions Mitosis has one division

Spermatogenesis

Process of sperm production

Results in 4 viable sperm

Oogenesis

Process of egg (ova) production

Results in 1 viable egg and 3 polar bodies that will not survive

Polar bodies result from an uneven division of cytoplasm

Mechanisms of Genetic Variation

Independent assortment—each pair of homologous chromosomes separate independently Results in gametes with different gene combinations

Crossing-over—exchange of genetic material between non-sister chromatids Results in genetic recombination

Random fertilization—random joining of two gametes

Independent Assortment

Figure 8.16

POSSIBILITY 1 POSSIBILITY 2

Two equally probable

arrangements of chromosomes at

metaphase I

Metaphase II

Gametes

Combination 1 Combination 2 Combination 3 Combination 4

Figure 8.18A

TetradChaisma

Centromere

Figure 8.17A, B

Coat-color genes Eye-color genes

Brown Black

White Pink

Tetrad in parent cell(homologous pair of

duplicated chromosomes)

Chromosomes ofthe four gametes

How crossing over leads to genetic recombination

Figure 8.18B

Tetrad(homologous pair ofchromosomes in synapsis)

Breakage of homologous chromatids

Joining of homologous chromatids

Chiasma

Separation of homologouschromosomes at anaphase I

Separation of chromatids atanaphase II and completion of meiosis

Parental type of chromosome

Recombinant chromosome

Parental type of chromosome

Gametes of four genetic types

Coat-colorgenes

Eye-colorgenes

Crossing Over

In Prophase I of Meiosis I, synapsis occurs allowing the crossing over of genetic material between non-sister chromatids

Creates new combinations of genes not seen in either parent

Random Fertilization

Random as to which gametes join and form a gamete

Importance of Genetic Variation

Essential to evolution (change over time)

Variation can cause changes that leads to different traits Some favorable Some unfavorable

Errors and Exceptions in Chromosomal Inheritance

Alterations in chromosome number or structure causes some genetic disorders Physical and chemical disturbances Errors during meiosis

To study human chromosomes microscopically, researchers stain and display them as a karyotype A karyotype usually shows 22 pairs of autosomes

and one pair of sex chromosomes

ALTERATIONS OF CHROMOSOME NUMBER AND STRUCTURE

Preparation of a karyotype

Figure 8.19

Blood culture

Centrifuge

Packed redAnd white blood cells

Hypotonic solution

Fixative

WhiteBloodcells

Centromere

Sisterchromatids

Pair of homologouschromosomes

Human female bands

Figure 8.19x1

Human female karyotype

Figure 8.19x2

Human male bands

Figure 8.19x3

Human male karyotype

Figure 8.19x4

Alterations of Chromosome Numbers

Nondisjunction—pair of homologues do not move apart during Meiosis I, or sister chromatids do not separate during Meiosis II Results in uneven distribution of chromosomes to

daughter cells

Alterations of Chromosome Numbers

Aneuploidy: abnormal chromosome number Trisomy: three copies of chromosomes Monosomy: one copy of a chromosome Trisomy and monosomy are usually lethal

Abnormal chromosome count is a result of nondisjunction Either

homologous pairs fail to separate during meiosis I

Accidents during meiosis can alter chromosome number

Figure 8.21A

Nondisjunctionin meiosis I

Normalmeiosis II

Gametes

n + 1 n + 1 n – 1 n – 1

Number of chromosomes

Or sister chromatids fail to separate during meiosis II

Figure 8.21B

Normalmeiosis I

Nondisjunctionin meiosis II

Gametes

n + 1 n – 1 n n

Number of chromosomes

Fertilization after nondisjunction in the mother results in a zygote with an extra chromosome

Figure 8.21C

Eggcell

Spermcell

n (normal)

Zygote2n + 1

Trisomy 21 (Down Syndrome)*Short stature, characteristic facial features, and heart defects (varying severity)*Most common serious birth defect (1 out of 700 births)*Mothers 35+ years of age have higher chance of having a Down baby

The chance of having a Down syndrome child goes up with maternal age

Figure 8.20C

Down syndrome karyotype

Figure 8.20Ax

Nondisjunction with Sex Chromosomes

Table 8.22

Klinefelter’s karyotype

Figure 8.22Ax

XYY karyotype

Figure 8.22x

Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion: chromosomal fragment is lost during

cell division Duplication: fragment may join to the

homologous chromosome Inversion: fragment may reattach to the original

chromosome but in the reverse orientation Translocation: fragment joins a nonhomologous

chromosome

Alterations of Chromosome Structure

Chromosome Mutation: Deletion

Deleted region

BeforeDeletion

AfterDeletion

Cri du Chat Syndrome: Partial deletion 5p

Before inversion

After inversion

Inverted region

Chromosome Mutation: Inversion

Chromosome 4

Chromosome 4Chromosome 20

Chromosome 20

Regionbeingmoved

Before TranslocationAfter Translocation

Chromosome Mutation: Translocation

Chromosomal changes in a somatic cell can cause cancer

Figure 8.23C

Chromosome 9

A chromosomal translocation in the bone marrow is associated with chronic myelogenous leukemia

Chromosome 22Reciprocaltranslocation

“Philadelphia chromosome”

Activated cancer-causing gene

Philadelphia Chromosomet(9,22)

Translocation

Figure 8.23Bx

Acknowledgements

Unless otherwise noted, illustrations are credited to Pearson Education which have been borrowed from BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2003. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.BIOLOGY: CONCEPTS AND CONNECTIONS 4th Edition, by Campbell, Reece, Mitchell, and Taylor, ©2001. These images have been produced from the originals by permission of the publisher. These illustrations may not be reproduced in any format for any purpose without express written permission from the publisher.

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