Chapter 13- Meiosis

QQ 10/5/18Copy the following into notebook:

• Similarities:1.2.3.4.5.

• Differences:1.2.3.4.5.

Figure 13.1

• Transmission of traits from one generation to another- inheritance, or heredity

• With inheritance there are both similarities and differences.

• Genetics is the study of heredity and variation

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

• Parents endow their offspring with genes-heredity “units” of DNA

-Tens of thousands passed on to offspring-All the genes compose the genome

• Human Genome Project (2000)

• Human gene mapping of major genetic conditions

Cystic Fibrosis Gene

• Gametes- the cells used by plants and animals to pass on their genetic info

*sperm/egg (pollen/egg)

• All genes can be found at a specific, corresponding location- Locus– Locus point is specific and identified with

designated (p, q, letters/numbers)

Information on paired genes from each parent (specific locus point)

• Asexual reproduction- A single individual is a parent and passes on ALL of its genetic information by mitosis to a clone.

• Can be single or multicellular organism.• Variation in family lines is caused

by mutations.

Figure 13.2

(a) Hydra (b) Redwoods

Bud

Parent

0.5 mm

Budding in a Hydra

• Sexual reproduction- Variation is caused by both parents passing on a set of their genetic information that then combines. – Meiosis *Makes GAMETES only

• Karyotyping- Matching up pairs of chromosomes from longest to shortest.

• These pairs are called homologouschromosomes or autosomes

*in humans, homologous pairs #1-22 are the autosome; #23 are sex chromosomes- not considered homologous! (XX or XY)

Figure 13.3

Pair of homologousduplicated chromosomes

Centromere

Sisterchromatids

Metaphasechromosome

5 µm

APPLICATION

TECHNIQUE

Karyotypes (humans on left; cat on right)

A Dog’s Karyotype

• There are also two distinct chromosomes that might not match- sex chromosomes- Male- XY- Smaller amount of DNA

- Sperm can either be X or Y- Female- XX

- Egg is ALWAYS X

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Fig. 11.2, p.170

sex chromosome combinationspossible in the new individual

diploidgerm cellsin female

meiosis, gameteformation in bothfemale and male:

fertilization:

eggs sperm

diploidgerm cellsin male

XX XX

XY XY

X

Y

X

X X

Y�

�

X X

• Haploid cell (n)- Single set of chromosomes (in humans, n=23). Offspring receive one set from maternal (egg) side, another from paternal (sperm)

• Diploid cell (2n)- BOTH sets of chromosomes (in humans, 2n= 46)

Cat n=? 2n=? Dog n=? 2n=?

Chapter 13- Meiosis

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Chromosome numbers:

All are even numbers –diploid (2n) sets of homologous chromosomes!

Ploidy = number of copies of each chromosome. Diploidy

Figure 13.5Key

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

• Life Cycle begins when egg meets sperm and is fertilized (forms the zygote)

- Zygote is diploid (2n)

• The only cells not produced by mitosis are the gametes

• AFTER FERTILIZATION = The zygote produces more somatic cells by mitosis and develops into an adult

Figure 13.4

Sister chromatidsof one duplicatedchromosome

KeyMaternal 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)

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

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• Some plants undergo “alternation of generations”- has a diploid AND haploid multicellular stage of life– Diploid stage- Sporophyte (produces

spores, which do not have to fuse to create offspring)

– Haploid stage- Gametophyte (produces gametes)

Alteration of Generations (plants and algae)

Figure 13.6

KeyHaploid (n)Diploid (2n)

Gametes

MEIOSIS FERTILIZATION

Zygote

MitosisDiploidmulticellularorganism

(a) Animals

n

n

n

2n 2n 2n 2n2n

n nn

n n

nn

n

nn

MEIOSIS

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

3 main types of sexual life cycles

Figure 13.6aKeyHaploid (n)Diploid (2n)

Gametes

MEIOSIS FERTILIZATION

Zygote

MitosisDiploidmulticellularorganism

(a) Animals

n

2n

n

n

2n

• The diploid organism, called the sporophyte, makes haploid spores by meiosis.

• Each spore grows by mitosis into a haploid organism called a gametophyte and a gametophyte makes haploid gametes by mitosis! (already haploid so mitosis used to make more IDENTICAL haploid cells!)

• Fertilization of gametes results in a diploid sporophyte!

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Plant and algae sexual life cycle:

Figure 13.6b

2n 2n

n

MEIOSIS FERTILIZATION

Mitosis Mitosis

Mitosis

GametesSpores

Zygote

Haploid multi-cellular organism(gametophyte)

Diploidmulticellularorganism(sporophyte)

(b) Plants and some algae

n n n n

Haploid (n)Diploid (2n)

Key

Moss reproduction video

• Depending on the type of life cycle:– either haploid or diploid cells can divide by

mitosis• However, only diploid cells can

undergo meiosis!

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STEPS OF MEIOSIS VS. MITOSIS

ANIMATION

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KEY DIFFERENCE! Homologous

pairs separate first

32CROSSING OVER!

33Sister

Chromatids separate at centromeres

Meiosis

• Meiosis- reduces the number of chromosome sets from diploid to haploid– Two CONSECUTIVE cell divisions,

Meiosis I and Meiosis II– 4 daughter cells produced

Meiosis

• Interphase- Replication of genome and growth occur. Centrosomes replicate

• Meiosis stage– Prophase I- 90% of total time of meiosis

• Chromosomes begin to condense• Homologous chromosomes pair and

match up by gene (forming a tetrad)

Meiosis• Crossing over- Where ever these

homologous chromosomes match up, genetic information will switch to opposite chromosome

• Centrosome movement toward poles of cell

• Spindle formation and attachment of fibers

• Breakdown of nuclear envelope

Meiosis

• Metaphase I- Tetrads arrange on the metaphase plate, spindle is fully attached

• Anaphase I- Sister chromatids move toward poles– Sister chromatids remain attached

Meiosis

• Telophase I and Cytokenesis I– Each cell will have sister chromatids– Splitting and cytokensis– Chromosomes might unwind, might not

(depending on organism)

Meiosis• Prophase II- Spindle apparatus forms,

movement of sisters towards metaphase plate

• Metaphase II- Spindle fibers attach to sisters at metaphase plate

• Anaphase II- Separation and migration of individual chromosomes toward poles

• Telophase II and Cytokenesis II- Nuclei form and chromosomes unwind

Meiosis (top) vs. mitosis (bottom)

Meiosis

What makes meiosis unique?1) Synapses- process of attachment of

homologous chromosomes- Crossing Over- genetic rearrangement- Chiasma (plural- chiasmata) – physical manifestation of crossing over

Meiosis

2) Tetrads on metaphase plate3) Separation of homologous chromosomes

in Anaphase I, but sisters stay attached to each other

Meiosis

• Three mechanisms that contribute to genetic variation

1) Independent Assortment of chromosomes- random orientation of homologous pairs at Metaphase I

- 50% of the homologous pair is maternal, 50% is paternal

Independent Assortment

Independent Assortment

• Which way is it going to be facing when “pulled” by the spindle?

• Law of Independent Assortment basically says “every chromosome for himself”

• To figure out how many possible combinations we can have, use 2n. Humans = 223 = about 8 million combinations

Meiosis

• Three mechanisms that contribute to genetic variation2) Crossing Over- produces recombinant

chromosomes (from multiple origins)- DNA is switched between maternal and paternal chromosomes- In humans there are roughly 1-3 crossing over events per chromosome

Crossing Over

Meiosis• Three mechanisms that contribute to

genetic variation3) Random Fertilization- The chance that you are sitting here is staggering.

- Paternal side 223, maternal side 223 and we then multiply =about 70 trillion : 1 (and this does NOT take into account crossing over events)

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Mitosis vs. Meiosis

Comparing Mitosis and meiosis Just meiosis!

Figure 13.9b

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

• 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, cohesins are cleaved along the chromosome arms in anaphase I (separation of homologs) and at the centromeres in anaphase II (separation of sister chromatids)

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Origins of Genetic Variation

• Three mechanisms contribute to genetic variation

1. Independent assortment of chromosomes2. Crossing over3. Random fertilization

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Independent assortment

Number of combinations: 2n

e.g. 2 chromosomes in haploid2n = 4; n = 22n = 22 = 4 possible combinations

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Independent assortment

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In humans…

e.g. 23 chromosomes in haploid2n = 46; n = 232n = 223 = ~ 8 million possible combinations!

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Random fertilizationAt least 8 million combinations from Mom,

and another 8 million from Dad …>64 trillion combinations for a diploid

zygote!!!

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Crossing overChiasmata – sites of crossing over synapsis Exchange of genetic material between non-sister chromatids.

Crossing over produces recombinant chromosomes.

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Alterations in chromosome number and individual chromosomes

• Many mutations can occur during mitosis or meiosis that will affect the chromosome numbers or alter the information on individual chromosomes.

• Mutations can be harmful or beneficial to the organism

Types of mutations include:– Nondisjunction– Deletion– Duplication– Inversion– Translocation

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Trisomy 21- Cause of Down Syndrome

Nondisjunction animation

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• Various animations showing chromosomes alterations

Which of the following transmits genes from one generation of a family to another?

a) DNAb) gametesc) somatic cellsd) mitosise) nucleotides

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Which of the following transmits genes from one generation of a family to another?

a) DNAb) gametesc) somatic cellsd) mitosise) nucleotides

© 2011 Pearson Education, Inc.

Fertilization is to zygote as meiosis is to which of the following?

a) mitosisb) diploidc) chromosomed) replicatione) gamete

Fertilization is to zygote as meiosis is to which of the following?

a) mitosisb) diploidc) chromosomed) replicatione) gamete

Privet shrubs and humans each have a diploid number of 46 chromosomes per cell. Why are

the two species so dissimilar?

a) Privet chromosomes undergo only mitosis.b) Privet chromosomes are shaped differently.c) Human chromosomes have genes grouped

together differently.d) The two species have different genes with

different information.

Privet shrubs and humans each have a diploid number of 46 chromosomes per cell. Why are

the two species so dissimilar?

a) Privet chromosomes undergo only mitosis.b) Privet chromosomes are shaped differently.c) Human chromosomes have genes grouped

together differently.d) The two species have different genes with

different information.

Independent Assortment

At what stage do chromosomes undergo independent assortment? How?

a) meiosis I with the pairing of homologsb) anaphase I with the separation of homologsc) meiosis II with the separation of homologsd) meiosis I with metaphase alignment

Independent Assortment

At what stage do chromosomes undergo independent assortment? How?

a) meiosis I with the pairing of homologsb) anaphase I with the separation of homologsc) meiosis II with the separation of homologsd) meiosis I with metaphase alignment

Meiotic Phases

In this cell, what phase is represented?

a) mitotic metaphaseb) meiosis I anaphasec) meiosis I metaphased) meiosis II anaphasee) meiosis II metaphase

Disjunction

What allows sister chromatids to separate in which phase of meiosis?

a) release of cohesin along sister chromatid arms in anaphase I

b) crossing over of chromatids in prophase Ic) release of cohesin at centromeres in

anaphase Id) release of cohesin at centromeres in

anaphase IIe) crossing over of homologues in prophase I

What are 3 ways in which gametes from one individual diploid cell can be different from one

another?

What are 3 ways in which gametes from one individual diploid cell can be different from one

another?

mutation, crossing over, independent assortment

Rate and Process

Prophase I of meiosis is generally the longest phase of meiosis. Why might this be?