LECTURE PRESENTATIONSFor CAMPBELL BIOLOGY, NINTH EDITION

Jane B. Reece, Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky, Robert B. Jackson

© 2011 Pearson Education, Inc.

Lectures byErin Barley

Kathleen Fitzpatrick

Meiosis and Sexual Life Cycles

Chapter 13

Overview: Variations on a Theme

• living organisms are distinguished by their ability to reproduce their own kind

• Genetics =the scientific study of heredity and variation

• Heredity = the transmission of traits from one generation to the next

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

Inheritance of Genes

• in a literal sense, children do not inherit particular physical traits from their parents

– it is genes that are actually inherited• Genes are the units of heredity

– made up of segments of DNA– passed to the next generation via reproductive cells

called gametes (sperm and eggs) – somatic cells – any other cell other than the gamete or

its precursor• each gene has a specific location called a locus on a

certain chromosome

Comparison of Asexual and Sexual Reproduction

• asexual reproduction - a single individual passes genes to its offspring without the fusion of gametes

– produces an exact copy of the parent – a clone– done through fission or mitosis – following duplication of DNA

• clone = a group of genetically identical individuals from the same parent

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

Sets of Chromosomes in Human Cells• a life cycle = generation-to-

generation sequence of stages in the reproductive history of an organism

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

– human somatic cells have 23 pairs of chromosomes

• the two chromosomes in each pair are called homologous chromosomes, or homologs

– chromosomes in a homologous pair have the same length, the same centromere position

– also carry genes at the same loci controlling the same inherited characters

Pair of homologousduplicated chromosomes

Centromere

Sisterchromatids

Metaphasechromosome

5 m

APPLICATION

TECHNIQUE

• sex chromosomes = X and Y– human females have a

homologous pair of X chromosomes (XX)

– human males have one X and one Y chromosome

• gamete (sperm or egg) contains a single set of chromosomes haploid (n)

– humans, the haploid number is 23 (n = 23)

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

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

Some Definitions You Know Already

• remaining 22 pairs of chromosomes are called autosomes

• each pair of homologous chromosomes includes one chromosome from each parent

• a diploid cell (2n) has two sets of chromosomes

– humans, the diploid number is 46 (2n = 46)

– dogs, the diploid number is 72! (2n = 72)

– Drosophila – 2n = 4

Some Definitions You Know Already

• in a cell in which DNA synthesis has occurred - each chromosome has been replicated

• each duplicated chromosome consists of two identical sister chromatids joined at a centromere

Sister chromatidsof one duplicatedchromosome

Key

Maternal set ofchromosomes (n 3)Paternal set ofchromosomes (n 3)

Key

2n 6

Centromere

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes (one from each set)

• chromosome = organized structure of DNA, protein and RNA found in the nucleus or nucleoid region

– single piece of coiled DNA containing genes, regulatory elements and other nucleotide sequences

– associated with DNA binding proteins for the packaging of the DNA and control of gene expression

– true definition – DNA organized into chromatin• two forms in interphase – euchromatin

(active form) and heterochromatin (inactive form)

– replicated DNA condenses during the early stages of mitosis and meiosis to form two sister chromatids joined at a centromere

Let’s Remind Ourselves Shall We?

1. chromatid2. centromere3. p arm (short arm)4. q arm (long arm)

• chromosome = organized structure of DNA, protein and RNA found in the nucleus or nucleoid region

– circular (prokaryotes) or linear (eukaryotes)• prokaryotic “chromosome” known as a

genophore – not organized as chromatin– genes organized into operons – no introns

• smaller, circular genophores = plasmids• circular “chromosomes” found in eukaryotic

mitochondria and chloroplasts

Let’s Remind Ourselves Shall We?

• centromere = point where the two sister chromatids join– physical role – act as the site of assembly for the kinetochore– indirect role – control the separation of chromatids

• kinetochore – complex group of proteins that attaches the centromere to the spindle microtubules and is responsible for chromatid separation (signals that all chromosomes are attached, aligned and are ready for separation)

• two kinetochore regions– inner plate: associates with the centromere DNA sequences

» modified histone proteins that interact with DNA– outer plate: associates with the microtubules

Centromeres

** the sister chromatids of a chromosome are linked all along theirlength by cohesin proteins-cutting of cohesins happens during prophase until metaphase – only point of connection is the cohesins in the centromere region

• centromere = point where the two sister chromatids join

– two types: Point and Regional• Point: smaller and more compact (e.g. yeasts)

– DNA sequence is necessary for the formation of the centromere

– bind to specific kinetochore proteins• Regional: most centromeres (e.g. humans)

– DNA sequence contributes to formation of the centromere

– not an actual defined sequence of DNA but is an array of repetitive sequences of “satellite DNA” that are similar to one another

– DNA of the centromere is heterochromatin form– heterochromatin form is critical for the adherence of

cohesin proteins in that region– end up with a high concentration of cohesins in the

centromere region

Centromeres 1. chromatid2. centromere3. p arm (short arm)4. q arm (long arm)

• so what makes up a centromere?– 1. DNA regions – two strands of DNA interact through base

pairing • DNA is heterochromatin

– 2. cohesion proteins – high concentration• interacts with the DNA

– 3. kinetochore – inner and outer plates• for attachment to the microtubules and chromatin separation

Centromeres

• centromere positions– Metacentric: typical X shaped

chromosome• two arms (p and q) are equal in length• humans – chr 1 & 3

– Submetacentric: arms are unequal• p is shorter

– Acrocentric: p arm is so short it is hard to observe• humans: chr 13,14,15,21,22 and Y

– Telocentric: centromere is located at the end of the chromosome• none in humans

– Holocentric: the entire length of the chromosome acts as the centromere• plants and many invertebrates (nematodes)

Centromereschromosome 1

chromosome 13

• the centromere position in humans is inherited– daughter chromosomes will assemble their centromeres

in the same positions as the parent chromosome• epigenetic – nothing to do with DNA sequence

– has to do with interactions with histones• role of H3 histone ? – H3 in the histones that interact

with the centromere region is replaced with a related protein called Centromere Protein A/CENP-A (humans)

Centromeres

Centromeres – How’s this for cool?

• the tension receptors in the interzone sense the attachment of the kinetochore to microtubules

• cdc20 promotes the activation of the Anaphase Promoting Complex (APC) – a group of proteins that initiates anaphase & triggers the separation of chromatids during anaphase

It gets even cooler! Yes it does!

APC

S and M cyclins

ubiquitin

Metaphase AnaphaseTransition

Degradation

ubiquination

Securin

Separase

Separase

Degradationof SecurinSeparase release

Cohesin degradationSister chromatid release

• cdc20 promotes the activation of the Anaphase Promoting Complex (APC) – a group of proteins that initiates anaphase & triggers the separation of chromatids during anaphase

– APC is a ubiquitin ligase – promotes the attachment of ubiquitin to protein targets degradation

– ubiquitin is attached to S and M cyclins – degradation results in transition from metaphase to anaphase

– ubiquitin is also attached to a protein complex made of proteins called securin & separase• ubiquitin dependent degradation of

securin “releases” separase from the complex

• separase now targets the cohesin protein complexes that link chromatids

• once separated - dynein motor proteins in the outer plate “walk” the separated chromatids along the microtubule to the opposite centrioles of the cell

Separase

SeparaseSeparase

MEIOSIS

PM Lecture

GO HAVE LUNCH!

• Fertilization is the union of gametes - the sperm and the egg

• the gametes have one set of chromosome - haploid

– produced via meiosis from a diploid germ cell

– gametes are the only types of human cells produced by meiosis rather than mitosis

– has one set of chromosomes from each parent

• the zygote produces somatic cells by mitosis and develops into an adult

• fertilization and meiosis alternate in sexual life cycles to maintain chromosome number

Behavior of Chromosome Sets in the Human Life Cycle Key

Haploid (n)Diploid (2n)

Egg (n)

Haploid gametes (n 23)

Sperm (n)

Ovary Testis

Mitosis anddevelopment

Diploidzygote(2n 46)

Multicellular diploidadults (2n 46)

MEIOSIS FERTILIZATION

The Variety of Sexual Life Cycles• the alternation of meiosis and fertilization is common to

all organisms that reproduce sexually• the three main types of sexual life cycles differ in the

timing of meiosis and fertilization Key

Haploid (n)

Diploid (2n)

Gametes

MEIOSIS FERTILIZATION

Zygote

MitosisDiploidmulticellularorganism

(a) Animals

n

n

n

2n 2n2n

2n2n

n n

n

n n

nn

n

n

nMEIOSIS

MEIOSIS

FERTILIZATION

FERTILIZATION

Mitosis Mitosis

Mitosis

Mitosis Mitosis

GametesSpores

Gametes

Zygote

Zygote

Haploid multi-cellular organism(gametophyte)

Diploidmulticellularorganism(sporophyte)

Haploid unicellular ormulticellular organism

(b) Plants and some algae (c) Most fungi and some protists

• gametes are the only haploid cells in animals

– produced by meiosis – undergo no further cell

division before fertilization – fuse to form a diploid zygote

that divides by mitosis to develop into a multicellular organism

Key

Haploid (n)

Diploid (2n)

Gametes

MEIOSIS FERTILIZATION

Zygote

MitosisDiploidmulticellularorganism

(a) Animals

n

2n

n

n

2n

Animals

• plants and some algae exhibit an alternation of generations

– their life cycle includes both diploid and haploid multicellular stages

– diploid organism - called the sporophyte - makes haploid spores by meiosis

– the spore grows (via mitosis) into a haploid organism called a gametophyte• gametophyte bears the reproductive

parts of the organism OR is the reproductive part

– gametophyte makes haploid gametes by mitosis

– fertilization of gametes produces a new diploid sporophyte

2n2n

n

MEIOSIS FERTILIZATION

Mitosis Mitosis

Mitosis

Gametes

Spores

Zygote

Haploid multi-cellular organism(gametophyte)

Diploidmulticellularorganism(sporophyte)

(b) Plants and some algae

n n nn

Haploid (n)

Diploid (2n)

KeyPlants

• in most fungi and some protists, the only diploid stage is the single-celled zygote

• there is no multicellular diploid stage

• the zygote produces haploid spores by meiosis

• each haploid cell grows by mitosis into a haploid multicellular organism - fungus

• the haploid adult produces gametes by mitosis

Key

Haploid (n)

Diploid (2n)

2n

n n

n

n

n

MEIOSIS FERTILIZATION

Mitosis Mitosis

Gametes

Zygote

Haploid unicellular ormulticellular organism

(c) Most fungi and some protists

Fungi

• depending on the type of life cycle, either haploid or diploid cells can divide by mitosis

– in animals – diploid cells undergo mitosis so that the organism grows into a diploid organism• specialized diploid germ cells undergo meiosis to produce gametes• gamete fusion produces a new diploid organism

– in plants & fungus – haploid spores undergo mitosis so the organism grows into a haploid organism (e.g. gametophyte)• haploid organism produces gametes via mitosis• gamete fusion produces a diploid organism that produces haploid spores

via meiosis (e.g. sporophyte)• only diploid cells can undergo meiosis• in all three life cycles, the halving and doubling of chromosomes

contributes to genetic variation in offspring

Meiosis reduces the number of chromosome sets from diploid to haploid

• meiosis is preceded by the replication of chromosomes

– just like mitosis• meiosis takes place in two sets of cell

divisions, called meiosis I and meiosis II• the two half as many chromosomes as

the parent cell• cell divisions result in four daughter

cells, each with

Meiosis - sperm

The Stages of Meiosis• after chromosomes duplicate,

two divisions follow– Meiosis I (reductional

division): homologs pair up and separate, resulting in two haploid daughter cells with replicated chromosomes

– Meiosis II (equational division): sister chromatids separate

• the result is four haploid daughter cells with half the number of unreplicated chromosomes as the parental cell

Pair of homologouschromosomes indiploid parent cell

Duplicated pairof homologouschromosomes

Chromosomesduplicate

Sisterchromatids Diploid cell with

duplicatedchromosomes

Homologouschromosomes separate

Haploid cells withduplicated chromosomes

Sister chromatidsseparate

Haploid cells with unduplicated chromosomes

Interphase

Meiosis I

Meiosis II

2

1

Prophase I Metaphase I Anaphase I Telophase I andCytokinesis

Centrosome(with centriole pair)

Sisterchromatids

Chiasmata

Spindle

Homologouschromosomes

Fragmentsof nuclearenvelope

Duplicated homologouschromosomes (red and blue)pair and exchange segments;2n 6 in this example.

Centromere(with kinetochore)

Metaphaseplate

Microtubuleattached tokinetochore

Chromosomes line upby homologous pairs.

Sister chromatidsremain attached

Homologouschromosomesseparate

Each pair of homologous chromosomes separates.

Cleavagefurrow

Two haploid cells form; each chromosomestill consists of two sister chromatids.

• Division in meiosis I occurs in four phases:– Prophase I– Metaphase I– Anaphase I– Telophase I and cytokinesis

Prophase I• prophase I typically occupies more than 90% of the time required for meiosis• many similarities with mitosis prophase

– chromosomes begin to condense within the nucleus– the centrioles migrate and the spindle begins to form– chromosomes attach to the spindle

• BUT a unique event happens – synapsis = pairing of two homologous chromosomes– ends of the chromosomes attach to proteins in the nuclear envelope– other proteins in the nuclear matrix help the 2 chromosomes align gene by gene– the paired chromosomes are called a tetrad– synapsis is followed by crossing over

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__stages_of_meiosis.html

Crossing Over– synapsis in Prophase I is followed by crossing over

crossing over meiosis: https://www.youtube.com/watch?v=rqPMp0U0HOA

Metaphase I• the nuclear envelope is gone and the spindle is completing its formation• tetrads line up at the metaphase plate - with one chromosome facing each pole

– microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad

– microtubules from the other pole are attached to the kinetochore of the other chromosome

– different from mitosis – spindle attached to both kinetochores of one chromosome

Inner Plate

Outer Plate

Microtubules

MITOSIS

Anaphase I• pairs of homologous chromosomes separate• one chromosome moves toward each pole - guided by the

spindle apparatus• sister chromatids remain attached at the centromere and

move as one unit toward the pole

Telophase I and Cytokinesis• in the beginning of telophase I - each half of the cell has a

haploid set of chromosomes– BUT each chromosome still consists of two sister

chromatids• cytokinesis forms two haploid daughter cells• in animal cells - a cleavage furrow forms• in plant cells - a cell plate forms• no chromosome replication occurs between the end of

meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Prophase II Metaphase II Anaphase II Telophase II andCytokinesis

Sister chromatidsseparate

Haploid daughtercells forming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing unduplicated chromosomes.

• 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

Prophase II• the spindle apparatus re-forms• in late prophase II, chromosomes move toward the metaphase plate• line up like mitosis

Metaphase II• the sister chromatids are arranged at the metaphase plate• BUT - because of crossing over in meiosis I - two sister

chromatids of each chromosome are no longer genetically identical

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

– like mitosis

Anaphase II• the sister chromatids separate – like mitosis• the sister chromatids of each chromosome now move as two newly

individual chromosomes toward opposite poles – like mitosis

• sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I

• protein complexes called cohesins are responsible for this cohesion– in mitosis: cohesins are cleaved at the end of metaphase– in meiosis I (anaphase I): cohesins are cleaved along the

chromosome arms in anaphase I separation of homologs and at the centromeres

– in meiosis II (anaphase II) separation of sister chromatids

Anaphase II & Cohesins

• sister chromatid cohesion allows sister chromatids of a single chromosome to stay together through meiosis I

• protein complexes called cohesins are responsible for this cohesion• cohesins – complex of 4 protein subunits (SMC1, SMC3, SSC1 & SSC3)

– 4 protein subunits form a ring-like structure that encircles both chromatids• forms during the S phase of interphase

– functions:• 1. holds sister chromatids together during metaphase and regulates their

separation during anaphase of mitosis or meiosis• 2. facilitates chromatid attachment to spindle – interacts with the

kinetochore and helps position the chromosomes at the metaphase plate• 3. facilitates recombination (crossing-over)• 4. other functions – e.g. transcriptional regulation

Anaphase II & Cohesins• cohesins are cleaved by the enzyme called separase• in mitosis: all cohesins are cleaved at the end of metaphase• in meiosis I (anaphase I): cohesins are cleaved along the chromosome arms in

anaphase I but not at the centromere separation of homologs • in meiosis II (anaphase II) separation of sister chromatids just like mitosis• mechanism:

– in meiosis I – the cohesin complex is protected from separase when in tetrad formation• only one kinetochore of the two per chromosome is attached to a microtubule at

metaphase I protection of cohesins from separase & chromosome separation– in meiosis II & mitosis – both kinetochores in the chromosome contact a

microtubule and cohesins are susceptible to separase action

Metaphase I chromosomes

Telophase II and Cytokinesis• just like mitosis

– the chromosomes arrive at opposite poles– nuclei form, and the chromosomes begin decondensing– Cytokinesis separates the cytoplasm

• at the end of meiosis, there are four daughter cells - each with a haploid set of unreplicated chromosomes (half the number as the parent cell)

• BUT - each daughter cell is genetically distinct from the others and from the parent cell

A Comparison of Mitosis and Meiosis• Mitosis conserves the number of chromosome sets, producing cells that are

genetically identical to the parent cell• Meiosis reduces the number of chromosomes sets from two (diploid) to one

(haploid), producing cells that differ genetically from each other and from the parent cell

SUMMARY

Property Mitosis Meiosis

DNAreplication

Number ofdivisions

Synapsis ofhomologouschromosomes

Number of daughter cellsand geneticcomposition

Role in the animal body

Occurs during interphase beforemitosis begins

One, including prophase, metaphase,anaphase, and telophase

Does not occur

Two, each diploid (2n) and geneticallyidentical to the parent cell

Enables multicellular adult to arise fromzygote; produces cells for growth, repair,and, in some species, asexual reproduction

Occurs during interphase before meiosis I begins

Two, each including prophase, metaphase, anaphase,and telophase

Occurs during prophase I along with crossing overbetween nonsister chromatids; resulting chiasmatahold pairs together due to sister chromatid cohesion

Four, each haploid (n), containing half as manychromosomes as the parent cell; genetically differentfrom the parent cell and from each other

Produces gametes; reduces number of chromosomesby half and introduces genetic variability among the gametes

Prophase

Duplicatedchromosome

MITOSIS

Chromosomeduplication

Parent cell

2n 6

Metaphase

AnaphaseTelophase

2n 2n

Daughter cellsof mitosis

MEIOSIS

MEIOSIS I

MEIOSIS II

Prophase I

Metaphase I

Anaphase ITelophase I

Haploidn 3

Chiasma

Chromosomeduplication Homologous

chromosome pair

Daughter cells of

meiosis I

Daughter cells of meiosis II

n n n n

SUMMARY

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html

• Three events are unique to meiosis, and all three occur in meiosis l– Synapsis and crossing over in prophase I:

Homologous chromosomes physically connect and exchange genetic information

– At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes

– At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate

Meiosis: A summary

Genetic variation produced in sexual life cycles contributes to evolution

• Mutations (changes in an organism’s DNA) are the original source of genetic diversity– mutation is a change in the DNA sequence that lasts through

rounds of mitosis or meiosis (i.e. perpetuates in the daughter cells)

• mutations create different versions of genes called alleles• gametes are produced through meiosis – each with a unique

complement of alleles• reshuffling of alleles during sexual reproduction produces genetic

variation

Origins of Genetic Variation Among Offspring

• the behavior of chromosomes during meiosis and fertilization is responsible for most of the variation that arises in each generation

• three mechanisms contribute to genetic variation– Independent assortment of chromosomes– Crossing over– Random fertilization

Independent Assortment of Chromosomes• Homologous pairs of chromosomes orient randomly at metaphase I of

meiosis– red chromosomes from “Dad”; blue chromosomes from “Mom”– when you undergo meiosis – multiple possibilities exist – see possibility 1 vs. 2

• in independent assortment - each pair of chromosomes sorts maternal and paternal homologs into daughter cells independently of the other pairs

– so there is a 50% chance a daughter cell at the end of meiosis will get the maternal chromosome and a 50% chance it will get the paternal homolog

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Possibility 1 Possibility 2

Two equally probablearrangements ofchromosomes at

metaphase I

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

• the number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number

– for humans (n = 23), there are more than 8 million (223) possible combinations of chromosomes

Crossing OverProphase Iof meiosis

Nonsister chromatidsheld togetherduring synapsis

Pair of homologs

Chiasma

Centromere

TEM

Anaphase I

Anaphase II

Daughtercells

Recombinant chromosomes

• crossing over produces recombinant chromosomes, which combine DNA inherited from each parent

• begins very early in prophase I, as homologous chromosomes pair up gene by gene

– homologous portions of two non-sister chromatids trade places

• contributes to genetic variation by combining DNA from two parents into a single chromosome

• in crossing over the nonsister chromatids exchange DNA segments– also known as genetic recombination– produces recombinant chromosomes

• the segments that are exchanged are similar in sequence and are on the same locations on either chromosome

• the crossed region is called a chiasma – region where the two chromosomes are physically joined

– each tetrad usually has more than one• requires an enzyme called a recombinase• major source of genetic variation

http://www.tokyo-med.ac.jp/genet/anm/mimov.gif

• crossing over can be unequal• major source of gene duplication and deletions between

chromosomes

“equal”cross oversite

Random Fertilization• random fertilization adds to genetic

variation because any sperm can fuse with any ovum (unfertilized egg)

– the fusion of two gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of about 70 trillion diploid combinations

• crossing over adds even more variation• each zygote has a unique genetic identity

Random Fertilization

The Evolutionary Significance of Genetic Variation Within Populations

• Natural selection results in the accumulation of genetic variations favored by the environment

• Sexual reproduction contributes to the genetic variation in a population, which originates from mutations