Meiosis

Chapter 13: Meiosis and Sexual Life Cycles

Variation

Living organisms are distinguished by their ability to reproduce their own kind

Genetics is the scientific study of heredity and variation Heredity is the

transmission of traits from one generation to the next

Variation is demonstrated by the differences in appearance that offspring show from parents and siblings

Comparison of Reproductions

In asexual reproduction, one parent produces genetically identical offspring by mitosis A clone is a group of

genetically identical individuals from the same parent

In sexual reproduction, two parents give rise to offspring that have unique combinations of genes inherited from the two parents

A life cycle is the generation-to-generation sequence of stages in the reproductive history of an organism

Genes

Genes are the units of heredity, and are made up of segments of DNA

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

Chromosomes Every organism has its own chromosome number Somatic (body) cell chromosomes come in pairs

Called diploid (2n) number of chromosomes The two chromosomes in each pair are called

homologous chromosomes, or homologs They are the same length and carry genes

controlling the same inherited characters• 1 from mom• 1 from dad

In humans, somatic cells have 46 chromosomes

Chromosomes

Gametes have only 1 of each chromosome Called haploid (n)

number of chromosomes

In humans, sex cells are haploid: n=23

A karyotype is an ordered display of the pairs of chromosomes from a cell

Homologous Chromosomes

Exception 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

The 22 pairs of chromosomes that do not determine sex are called autosomes (found in homologous pairs)

Fertilization Each set of 23 chromosomes

consists of 22 autosomes and a single sex chromosome

In an unfertilized egg (ovum), the sex chromosome is X

In a sperm cell, the sex chromosome may be either X or Y

Fertilization is the union of gametes (the sperm and the egg)

The fertilized egg is called a zygote and has one set of chromosomes from each parent

The zygote produces somatic cells by mitosis and develops into an adult

Meiosis Meiosis is a type of

cell division used to make gametes (sex cells) 2 nuclear divisions

• Meiosis I• Meiosis II

Begins with 1 diploid (2n) cell

Ends with 4 haploid (n) cells

Meiosis Gametes are the only

types of human cells produced by meiosis

Meiosis results in one set of chromosomes in each gamete (23)

Gametes are the only haploid cells in animals

Gametes fuse (23 + 23) to form a diploid zygote (46) that divides by mitosis to develop into a multicellular organism

Cell Cycle Review G1, S, G2, M Mitosis has 4 main

phases Prophase Metaphase Anaphase Telophase

Cells split by cytokinesis

Produce 2 identical cells

Growth and repair

Meiosis I Division in meiosis I occurs in four phases:

Prophase I Metaphase I Anaphase I Telophase I and cytokinesis

Meiosis I results in two haploid daughter cells with replicated chromosomes

Focus is on splitting homologous chromosomes

Meiosis I is preceded by interphase, in which chromosomes are replicated

Each replicated chromosome consists of two identical sister chromatids

Sister chromatids held together by centromere

The sister chromatids are genetically identical

Interphase

Chromosomes begin to condense Nuclear membrane & nucleoli

dissolve Homologous chromosomes pair up

(align gene by gene); this is called synapsis Each pair of chromosomes forms

a tetrad, a group of four chromatids

Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurs

In crossing over, nonsister chromatids exchange DNA segments

Prophase I

Prophase I: Tetrads

Homologous chromosomes (each with sister chromatids)

A Tetrad

In metaphase I, tetrads independently line up across from each other “sandwiching” the equator (metaphase plate)

In mitosis the homologs made one single line on the equator

Microtubules from the poles are attached to the kinetochores of each chromosome of each tetrad

Metaphase I

Pairs of homologous chromosomes separate

One chromosome moves toward each pole, guided by the spindle fibers

Sister chromatids remain attached at the centromere and move as one unit toward the pole

Anaphase I

Reverse of prophase I Spindle fibers breaks

down Chromosomes uncoil Nuclear envelope

reforms In the beginning of

telophase I, each half of the cell has a haploid set of chromosomes Each chromosome still

consists of two sister chromatids

Telophase I

Cytokinesis forms two haploid daughter cells In animal cells, a cleavage furrow forms In plant cells, a cell plate forms

Each new cell has ½ the genetic information as the original 1 chromosome from each pair Need a second division for sister chromatids to split Each chromosome from the pair is still doubled

No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Cytokinesis & Interkinesis

Meiosis I

Meiosis II Division in meiosis II also occurs in four

phases: Prophase II Metaphase II Anaphase II Telophase II and cytokinesis

Meiosis II is very similar to mitosis Focus is on splitting sister chromatids

Prophase II Spindle apparatus forms; nuclear

membrane and nuclelous dissolve Chromosomes condense (each still

composed of two chromatids) and move toward the metaphase plate

Metaphase II Sister chromatids are arranged at

the metaphase plate The two sister chromatids of each

chromosome are no longer genetically identical

The kinetochores of sister chromatids attach to microtubules extending from opposite poles

Prophase II & Metaphase II

Anaphase II Sister chromatids separate

and move to opposite poles

Each chromatid is now its own chromosome

Telophase II Nuclei reform Spindles break down Chromosomes uncoil

Cytokinesis again separates the cytoplasm

Anaphase II & Telophase II

At the end of meiosis, there are four haploid daughter cells Each daughter cell is genetically distinct from the

others and from the parent cell Each has 1 chromosome from each homologous pair Each will mature into eggs or sperm (gametogenesis)

Final Products

Meiosis II

Three events are unique to meiosis, and all three occur in meiosis I: Synapsis and crossing over in prophase I In metaphase I, paired homologous

chromosomes (tetrads) independently arrange on either side of the equator

In anaphase I, homologous chromosomes, instead of sister chromatids, separate

Meiosis Distinctions

Meiosis & Variation Three mechanisms contribute to genetic variation:

Independent assortment of chromosomes• Homologous pairs of chromosomes orient randomly at metaphase I of meiosis

Crossing over• Nonsister chromatids of a tetrad exchange genetic information

Random fertilization• Over 8 million different gametes possible (2^23)• 70 trillion chromosome combinations possible for zygotes! (2^23)x(2^23)

Mitosis vs. Meiosis

Mitosis vs. Meiosis Mitosis

1 division 2 daughter cells Exact copies of

parent cells Diploid to diploid Purpose

• Growth • Repair• Asexual

reproduction

Meiosis 2 divisions

• 1st separates pairs

4 daughter cells Each unique Diploid to haploid Purpose

• Make gametes/ sex cells

• Leads to genetic variation

Review Questions1. Define genetics and differentiate between heredity and variation.2. Differentiate between asexual and sexual forms of reproduction in

regards to the life cycles of various organisms.3. Define the following vocabulary associated with meiosis: gene,

gamete, locus, somatic cell, karyotype, homologous chromosomes, & zygote.

4. Differentiate between autosomes and sex chromosomes.5. Describe the process of fertilization.6. Define meiosis and explain why there must be 2 divisions.7. Define the 4 major phases of meiosis I, along with the important

events that occur during those phases and how they are unique from those phases of mitosis.

8. Explain the relationship between synapsis, tetrads, chiasmata, and crossing over.

9. Define the 4 major phases of meiosis II, along with the important events that occur during those phases and how they are unique from those phases of mitosis.

10.Describe the 3 events that are unique to meiosis.11.Name and describe 3 mechanisms that contribute to genetic

variation.12.Describe 5 major differences between mitosis and meiosis as

processes.