Meiosis A reduction division. Meiosis Multi-celled organisms Multi-celled organisms produce gametes produce gametes (cells) through the process (cells)

MeiosisMeiosis

A reduction divisionA reduction division

MeiosisMeiosis Multi-celled organisms Multi-celled organisms produce gametesproduce gametes

(cells) through the process (cells) through the process

of meiosis. of meiosis. In humans and many other organisms, these In humans and many other organisms, these

cells are called ova(eggs) and sperm.cells are called ova(eggs) and sperm. When an ovum and a sperm combine, a new When an ovum and a sperm combine, a new

organism is formed with a full set of organism is formed with a full set of chromosomes, half of which came from the chromosomes, half of which came from the father, and half of which came from the father, and half of which came from the mother.mother.

So what is a So what is a chromosomchromosome anyway?e anyway?

CHROMOSOMECHROMOSOME

the DNA-containing the DNA-containing structures that carry structures that carry the organism’s the organism’s genetic informationgenetic information

Looking at ChromosomesLooking at Chromosomes

Chromosomes are arranged in Chromosomes are arranged in matching pairs – matching pairs –

Homologous pairsHomologous pairs

A normal set of chromosomes will A normal set of chromosomes will have 23 pairshave 23 pairs

The first 22 are completely The first 22 are completely homologous – Autosomeshomologous – Autosomes

The 23The 23rdrd pair are sex chromosomes pair are sex chromosomes

KaryotypeKaryotype

Meiosis is a type of cell Meiosis is a type of cell division where one cell divides division where one cell divides to form new cells, each of to form new cells, each of which have half the number of which have half the number of chromosomes as the original chromosomes as the original (ex – Humans have ? (ex – Humans have ? normally?). normally?).

This condition is called This condition is called haploid.haploid.

Cells with the original number Cells with the original number of chromosomes are called of chromosomes are called diploid.diploid.

Another way to say this is that Another way to say this is that each new daughter cell gets each new daughter cell gets half of the parent cellhalf of the parent cell’’s DNA. s DNA.

Humans have 23 Humans have 23 pairs of pairs of chromosomes, for a chromosomes, for a total of 46. total of 46.

In each pair, one In each pair, one chromosome comes chromosome comes from mom and one from mom and one comes from dad. comes from dad.

Both members of the Both members of the chromosome pair are chromosome pair are the same size, shape, the same size, shape, and carry the same and carry the same genetic information. genetic information.

There are two main stages of There are two main stages of meiosis: Meiosis I and Meiosis meiosis: Meiosis I and Meiosis II. II.

Each of these is divided into Each of these is divided into four stages: Prophase, four stages: Prophase, Metaphase, Anaphase and Metaphase, Anaphase and Telophase Telophase

Before the cell starts dividing, it Before the cell starts dividing, it has to complete Interphase – has to complete Interphase – What happens during this very What happens during this very important time? important time?

MeiosisMeiosis We refer to the 23 We refer to the 23

pairs as homologous pairs as homologous pairs. Why?pairs. Why?

During the earliest During the earliest

stage of meiosis, the stage of meiosis, the members of each pair members of each pair will temporarily attach will temporarily attach together to form a together to form a structure called a structure called a tetrad. Why are there tetrad. Why are there four instead of two?four instead of two?

homologous pairhomologous pair

tetradtetrad

Prophase IProphase I Very similar to Prophase of Very similar to Prophase of

mitosis.mitosis. What happens during What happens during

Prophase?Prophase? Chromatin does what?Chromatin does what? Nucleus does what?Nucleus does what? Spindle fibers?Spindle fibers? Chromosomes (tetrads) do what?Chromosomes (tetrads) do what? Centrioles do what?Centrioles do what?

There are a few extras to add!There are a few extras to add! The tetrads actually exchange The tetrads actually exchange

some of their DNA in a process some of their DNA in a process called crossing-over (Swapping called crossing-over (Swapping spit – Disgusting!!)spit – Disgusting!!)

Metaphase IMetaphase I

______ , not ______ , not individual individual chromosomes, line chromosomes, line up on the equator up on the equator of the cell.of the cell.

_______ fibers _______ fibers attach to the attach to the centromeres of centromeres of chromosomes.chromosomes.

Tetrads Tetrads

SpindleSpindle

Anaphase IAnaphase I

Spindle fibers Spindle fibers contract and pull contract and pull ______ apart.______ apart.

Entire double Entire double stranded stranded chromosome pulled chromosome pulled to opposite ends of to opposite ends of the cell.the cell.

Chromosome Chromosome number is now half number is now half of original (haploid).of original (haploid).

TetradsTetrads

Telophase ITelophase I

Cell plate (plant) or Cell plate (plant) or cleavage furrow cleavage furrow (animal) divides the (animal) divides the _________._________.

Cell usually goes Cell usually goes directly into Prophase directly into Prophase II, but if not, II, but if not, chromosomes relax, chromosomes relax, nuclei reform, and nuclei reform, and spindle fibers break spindle fibers break down.down. Each daughter cell has _____ Each daughter cell has _____ of the number of of the number of chromosomes as the chromosomes as the original.original.

cytoplasmcytoplasm

halfhalfChromosomes not to scaleChromosomes not to scale

Summary of Meiosis IISummary of Meiosis II

The mechanics of Meiosis II are The mechanics of Meiosis II are _______ to Mitosis._______ to Mitosis.

Each haploid daughter cell produced Each haploid daughter cell produced by Meiosis I divides again, so that by Meiosis I divides again, so that four haploid cells are produced four haploid cells are produced altogether.altogether.

In _____________, all four cells survive In _____________, all four cells survive to produce viable sperm.to produce viable sperm.

In _________, only one cell survives to In _________, only one cell survives to produce a viable ovum.produce a viable ovum.

spermatogenesisspermatogenesis

oogenesisoogenesis

identicalidentical

OverviewOverview Tetrads form (Pro I)Tetrads form (Pro I) Crossing over can Crossing over can

occur (Pro I)occur (Pro I) Tetrads line up and Tetrads line up and

are separated (Meta are separated (Meta I)I)

Daughter cells are Daughter cells are haploidhaploid

Produces gametes Produces gametes (ova and sperm)(ova and sperm)

How is How is mitosismitosis

different from different from meiosismeiosis??

How is mitosis different from How is mitosis different from meiosis?meiosis?

MitosisMitosis makes makes two identical two identical daughter cellsdaughter cells which have the which have the same number of chromosomes as same number of chromosomes as parent cell. parent cell. Makes body cellsMakes body cells..

MeiosisMeiosis makes makes four different four different daughter cells daughter cells which have half the which have half the number of chromosomes as number of chromosomes as parent cell. parent cell. Makes sex cells (or Makes sex cells (or gametes)gametes)..

CompareCompare Critical differences Critical differences

between Mitosis between Mitosis and and Meiosis:Meiosis:

Tetrads form (Pro I)Tetrads form (Pro I) Crossing over can Crossing over can

occur (Pro I)occur (Pro I) Tetrads line up and Tetrads line up and

are separated are separated (Meta I)(Meta I)

Daughter cells are Daughter cells are haploidhaploid

Only occurs in Only occurs in germ cellsgerm cells

Produces gametes Produces gametes (ova and sperm)(ova and sperm)

Mitosis vs. Meiosis IMitosis vs. Meiosis I

Left: Metaphase of MITOSIS: no pairing!!!!

Right: Metaphase I of Meiosis: homologous chromosomes Pair UP!

What are the two What are the two gametes?gametes?

Egg cell – ovaEgg cell – ova

Sperm cellSperm cell

How many How many chromosomes do chromosomes do human gametes human gametes

have? have? How many How many

chromosomes do chromosomes do human body cells human body cells

have? have?

(haploid - 23)

(diploid - 46)

What is the What is the difference between difference between diploid and haploid? diploid and haploid?

How are these How are these

related to mitosis, related to mitosis, meiosis and number meiosis and number

of chromosomes?of chromosomes?

Associated Terms..Associated Terms..Fertilization –Fertilization –

Zygote - Zygote -

Homologous pair –Homologous pair – Sister Chromatids - Sister Chromatids -

A haploid sperm A haploid sperm cell fuses with a cell fuses with a haploid egg cellhaploid egg cell

The resulting fertilized The resulting fertilized eggegg

Two Two chromosomes, chromosomes,

with matching size with matching size and genesand genesAn identical An identical copy of the copy of the

chromosomechromosome

Independent AssortmentIndependent Assortment

Independent AssortmentIndependent AssortmentResults of alternative arrangements of

chromosomes at metaphase of meiosis I. The positioning of each homologous pair of

chromosomes is random; the two red chromosomes can be on the same side

(possibility 1) or on opposite sides (possibility 2).

The arrangement of chromosomes at metaphase I determines which chromosomes

will be packaged together in the haploid gametes.

Because possibilities 1 and 2 are equally likely, the four possible types of gametes will

be made in approximately equal numbers.

How do do we get How do do we get variety?variety?

The tetrads actually The tetrads actually exchange some of their exchange some of their DNA in a process called DNA in a process called crossing-over.crossing-over.

During Prophase I During Prophase I homologous homologous chromosomes line up chromosomes line up gene-by-gene and may gene-by-gene and may exchange segments.exchange segments.

Crossing OverCrossing Over

The exchange of segments allows

for genetic variability.

The resulting chromosomes

are called “recombinant”

When When MeiosisMeiosis goes goes

WRONG? WRONG?

Remember Mutations?Remember Mutations?

Chromosome Mutations Chromosome Mutations

are spontaneous changes are spontaneous changes involving an involving an

entire chromosomeentire chromosome

or or piecespieces

of a chromosomeof a chromosome

Chromosome MutationsChromosome Mutations Can occur during Can occur during

meiosismeiosis Some harmful, Some harmful,

some notsome not BiggerBigger effect than effect than

gene mutationsgene mutations 2 types: Structural 2 types: Structural

mutations and mutations and Numerical Numerical MutationsMutations

Structural MutationsStructural Mutations

change in change in arrangement of arrangement of genes on a genes on a chromosomechromosome

For example, For example, DeletionDeletion- one - one or more genes or more genes are lostare lost


Another Another example…. example…. InversionInversion- - piece of a piece of a chromosome chromosome breaks off and breaks off and reattaches itself reattaches itself backwardsbackwards


Another Another example, example, Duplication-Duplication- extra pieceextra piece duplicated duplicated genesgenes


Another Another example, example, TranslocationTranslocation- - piece of a piece of a chromosome chromosome breaks off of breaks off of one & attaches one & attaches to another to another different different chromosomechromosome

WhatWhat’’s the Mutation?s the Mutation? QRSTUV QRSTUV toto QRUTSVQRUTSV InversionInversion UVWXYUVWXY toto ABCDUVWXYABCDUVWXY Insertion/AdditionInsertion/Addition LMNOPLMNOP toto LMNLMNOPLMNLMNOP DuplicationDuplication ABCDEFABCDEF toto ABEFABEF DeletionDeletion What TYPE of mutations are these?What TYPE of mutations are these? Structural - change in arrangement or # of genesStructural - change in arrangement or # of genes What causes them?What causes them? Problems with crossing overProblems with crossing over

Numerical MutationsNumerical Mutations changes in the number changes in the number

of chromosomes in a of chromosomes in a zygotezygote

Usually results from Usually results from nondisjunctionnondisjunction- failure - failure of chromosomes to of chromosomes to separateseparate

Can occur in Meiosis I Can occur in Meiosis I or IIor II

Chromosome pair fail to Chromosome pair fail to separate in Anaphaseseparate in Anaphase

Numerical MutationsNumerical Mutations

Monosomy- zygote receives only 1 of a particular chromosomeEX: female being XO or getting 1 # 6 chromosome

Numerical MutationsNumerical Mutations

Trisomy-Trisomy- zygote zygote receives 3 receives 3 chromosomes of chromosomes of a particular kinda particular kind

DownDown’’s s SyndromeSyndrome - - extra 21st extra 21st chromosomechromosome

KlinefelterKlinefelter’’s Syndromes Syndrome

XXY - 47 MalesXXY - 47 Males

The most common The most common sexsexchromosome chromosome disorder.disorder.

~1/1000 males~1/1000 males

Males are usually Males are usually infertile.infertile.

Numerical ChangesNumerical Changes PolyploidyPolyploidy- none - none

of the of the chromosomes chromosomes separate and the separate and the zygote gets 3 or zygote gets 3 or more sets of more sets of chromosomeschromosomes

EX: instead of 23 EX: instead of 23 pairs, you get 46 pairs, you get 46 pairs of pairs of chromosomes…this chromosomes…this means 92 means 92 chromosomes!!!chromosomes!!!

How many chromosomes How many chromosomes does a human have?does a human have?

46 (23 pairs)46 (23 pairs) What are pairs 1-22 called?What are pairs 1-22 called? AutosomesAutosomes What is the 23rd pair called?What is the 23rd pair called? Sex chromosomesSex chromosomes Male? Female?Male? Female? XYXY XX XX

Normal Karyotype

Monosomy

Trisomy

Trisomy

Trisomy

Can you find the Can you find the mistake?mistake?




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Meiosis A reduction division. Meiosis Multi-celled organisms Multi-celled organisms produce gametes produce gametes (cells) through the process (cells)