Meiosis

Nestor T. Hilvano, M.D., M.P.H.(Images Copyright Discover Biology, 5th ed., Singh-Cundy and Cain,

Textbook, 2012.)

Learning Objectives 1. Distinguish between autosomes and sex

chromosomes; somatic cells and gametes; diploid cells and haploid cells.

2. Define and describe the events of meiosis.

3. Describe key differences between mitosis and meiosis.

4. Define karyotyping and give its importance.

5. Describe the causes and symptoms of Down Syndrome

Inheritance of Genes • ___ is the scientific study of heredity and variation• ___ is the transmission of traits from one generation

to the next• ___ are the units of heredity, and are made up of

segments of DNAa. Heredity b. Genes c. Genetics d. Genomes

• Genes are passed to the next generation through reproductive cells called gametes (sperm and eggs)

• Each gene has a specific location called a locus on a certain chromosome

• Most DNA is packaged into chromosomes• One set of chromosomes is inherited from each

parent

Terminologies • Somatic cells – body cells; in human have 23

pairs of chromosomes (diploid chromosomes) (2n) (2 sets)

• Homologous chromosomes – pair (2 sets) of chromosomes that carry genes.

• Gametes – have 23 chromosomes (haploid) (1n) (1 set); 22 autosomes +X or 22 autosomes + Y

• The sex chromosomes are called X and Y• Human females have a homologous pair of X

chromosomes (XX)• Human males have one X and one Y chromosome (XY)• The 22 pairs of chromosomes that do not determine sex

are called autosomes• Sexual life cycles involve the alternation of haploid and

diploid stages• The fusion of haploid gametes in the process of

fertilization results in the formation of a diploid (2n) zygote (fertilized egg= new child)

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Fig. 13-5Key

Haploid (n)Diploid (2n)

Haploid gametes (n = 23)

Egg (n)

Sperm (n)

MEIOSIS FERTILIZATION

Ovary Testis

Diploidzygote(2n = 46)

Mitosis anddevelopment

Multicellular diploidadults (2n = 46)

Meiosis • Reduces the chromosome number from diploid

to haploid. • But divide twice to form 4 daughter cells (haploid

= gametes).• Cross-over= homologous chromosomes swap to

increase genetic diversity• Consists of:

Meiosis I – homologous chromosomes separate, results in 2 haploid daughter cells with replicated chromosomes

Meiosis II – sister chromatids separate, results in 4 haploid chromosomes

Figure 8.12B

A pair ofhomologouschromosomesin a diploidparent cell

A pair ofduplicatedhomologouschromosomes

Sisterchromatids

1 2 3

INTERPHASE MEIOSIS I MEIOSIS II

Figure 8.11

Pair of homologouschromosomes

Locus

Centromere

Sisterchromatids

One duplicatedchromosome

Meiosis I• Prophase I – chromatin coils up; homologous

pairing (tetrad; with 4 chromatids); crossing over; nucleoli disappear; nuclear envelop breaks; spindle forming

• Metaphase I – chromosomes tetrads aligned on the metaphase plate

• Anaphase I – migration of chromosomes toward the 2 poles; sister chromatids remain attached

• Telophase I and cytokinesis – 2 haploid chromosome; cleavage furrow

Meiosis II

• Prophase II – spindle forms and moves chromosomes toward the middle of cell.

• Metaphase II – chromosomes are aligned on the metaphase plate

• Anaphase II – sister chromatids separate

• Telophase II and cytokinesis – nuclei form; cytokinesis occur; 4 haploid daughter cells (single chromosome)

Comparison between mitosis and meiosis

• Mitosis – results to 2 daughter cells, each with the same chromosomes as the parent cell.

• Meiosis – results to 4 daughter cells, each with half the number of chromosomes as the parent cell.

Figure 8.14

Prophase

Metaphase

Duplicatedchromosome(two sisterchromatids)

MITOSIS

Parent cell(before chromosome duplication)

Chromosomeduplication

Chromosomeduplication

Site ofcrossingover

2n 4

Chromosomesalign at themetaphase plate

Tetrads (homologouspairs) align at themetaphase plate

Tetrad formedby synapsis ofhomologouschromosomes

Metaphase I

Prophase I

MEIOSIS I

AnaphaseTelophase

Sister chromatidsseparate during

anaphase

2n 2n

Daughter cells of mitosisNo furtherchromosomalduplication;sisterchromatidsseparate duringanaphase II

n n n n

Daughter cells of meiosis II

Daughtercells of

meiosis I

Haploidn 2

Anaphase ITelophase I

Homologouschromosomesseparate duringanaphase I;sisterchromatidsremain together

MEIOSIS II

Genetic variations could be due to:

• Random orientation of homologous chromosome pairs at metaphase of meiosis I

• Crossing over creates recombinant chromosomes

Karyotype

• Ordered display of chromosomes in which the :

- genetic sex of an individual can be

determined.

- abnormalities in chromosomal

structure and number can be detected.

Figure 8.19A_1

Trisomy 21

• Trisomy 21 – Down syndrome

• Results from non-disjunction in meiosis

• Increase risk with maternal age

Down syndrome

Figure 8.20B_s3

Normalmeiosis I

MEIOSIS I

MEIOSIS II

Nondisjunction

Abnormal gametesNormal gametes

n 1 n 1 n n

Homework 1. Define – meiosis, haploid chromosome, diploid

chromosome, gametes, somatic cell, autosomes, sex chromosomes, Down syndrome, karyotype, genetics, heredity, genes

2. Describe the events that characterize each phase of meiosis (I & II)

3. Differentiate mitosis and meiosis as to # of cell division, # of daughter cells, # of chromosomes in daughter cells, genetic relationship of daughter cells to parent cell, and functions performed in the human body.

4. Compare sexual and asexual reproduction.