MeiosisMeiosis

Meiosis Meiosis Meiosis is cell division that produces gametes.Meiosis is cell division that produces gametes.

Gametes are sex cells (sperm and eggs).Gametes are sex cells (sperm and eggs).

During meiosis, a cell divides twice after During meiosis, a cell divides twice after copying its DNA.copying its DNA.

This reduces the number of chromosomes in This reduces the number of chromosomes in each ‘daughter’ cell to half the original.each ‘daughter’ cell to half the original.

Meiosis is a part of Meiosis is a part of Sexual ReproductionSexual Reproduction

Meiosis produces gametes.Meiosis produces gametes. In males, 4 sperm cells are produced.In males, 4 sperm cells are produced.

They are equally small in size.They are equally small in size. In females, 1 egg cell & 3 ‘polar In females, 1 egg cell & 3 ‘polar

bodies’ are produced.bodies’ are produced. The egg is larger in size. The egg is larger in size.

Human gametes have 23 Human gametes have 23 chromosomes.chromosomes.

Oogenesis: the Oogenesis: the making of an making of an eggegg

polar bodies

egg cell

SpermatogenesiSpermatogenesis: the making of s: the making of sperm cells sperm cells

The phases of meiosisThe phases of meiosis

Meiosis is a “reduction-division” Meiosis is a “reduction-division” process.process. 1 ‘Diploid’ ends up as 4 ‘Haploid’1 ‘Diploid’ ends up as 4 ‘Haploid’ The number of chromosomes per cell is cut The number of chromosomes per cell is cut

in half.in half. Homologous chromosomes separate.Homologous chromosomes separate.

There are two parts to meiosis:There are two parts to meiosis: Meiosis I (similar to Mitosis)Meiosis I (similar to Mitosis) Meiosis IIMeiosis II

Remember…Remember…

Cell division is a Cell division is a very small part very small part of a cell’s life of a cell’s life cyclecycle

Copy the text part of this diagram onto the drawing found on the back of your

worksheet.

Remember… Remember… Why cells are smallWhy cells are small

Diffusion limits cell sizeDiffusion limits cell size Essential substances (nutrients, water, enzymes, wastes, etc.) Essential substances (nutrients, water, enzymes, wastes, etc.)

must diffuse into and out of the cell.must diffuse into and out of the cell. Surface area to volume ratioSurface area to volume ratio

The volume increases faster than the surface areaThe volume increases faster than the surface area If size of the cell doubles, the cell would require 8 times more If size of the cell doubles, the cell would require 8 times more

nutrientsnutrients A small cell size insures that materials A small cell size insures that materials

can get in and out quickly.can get in and out quickly.

Remember…Remember…

DNA is first copied DNA is first copied during during InterphaseInterphase

before either Mitosis or before either Mitosis or Meiosis occurs.Meiosis occurs.

Remember… Remember… Complete the drawing Complete the drawing on your worksheet with the terms belowon your worksheet with the terms below

Mitosis

centrioles

centriolesnuclear envelope

nuclear envelope

spindle fibers

spindle fibers

centrioles

centromeresister chromatids

sisterchromatids

Daughter cells

chromatin

Prophase

Metaphase

Anaphase

Telophase

Meiosis IMeiosis I

Meiosis I has four phases. They Meiosis I has four phases. They are:are:

1.1. Prophase IProphase I

2.2. Metaphase IMetaphase I

3.3. Anaphase IAnaphase I

4.4. Telophase I and cytokinesisTelophase I and cytokinesis

Prophase IProphase I

Each chromosome pairs with its Each chromosome pairs with its corresponding homologous chromosome to corresponding homologous chromosome to form a tetrad (4 chromosomes).form a tetrad (4 chromosomes).

This is the phase when crossing over This is the phase when crossing over occurs.occurs. Crossing over is when tetrads exchange portions Crossing over is when tetrads exchange portions

of their chromatids.of their chromatids. This creates new, unique chromosome This creates new, unique chromosome

combinationscombinations

Remember…Remember…

Chromosomes are made up of sequences of Chromosomes are made up of sequences of genesgenes

Homologous chromosomes have the ‘same’ Homologous chromosomes have the ‘same’ gene sequences from ‘mom’ & ‘dad’gene sequences from ‘mom’ & ‘dad’

Remember… Remember…

These ‘same’ genes are called These ‘same’ genes are called allelesalleles Some may be Some may be chemically chemically stronger and are stronger and are called called ‘dominant’.‘dominant’.

Those not as Those not as strong are called strong are called ‘recessive’‘recessive’

Crossing over during Crossing over during Prophase I: Prophase I: Complete the drawing on your Complete the drawing on your

worksheet using the labels shown below.worksheet using the labels shown below.

homologous chromosomes

in a tetrad

crossing over (gene jumping)

later separation into gamete cells

2 sets of sister

chromosomes from DNA replication

from ‘mom’

from ‘dad’

a little bit of ‘mom’ & ‘dad’ end up in all cells

Metaphase IMetaphase I

During metaphase I, the spindle During metaphase I, the spindle fibers attach to the chromosomes.fibers attach to the chromosomes.

Prophase I Metaphase I

Anaphase IAnaphase I

During anaphase During anaphase I, the fibers pull I, the fibers pull the homologous the homologous chromosomes chromosomes toward opposite toward opposite ends of the cell.ends of the cell.

Telophase I and Telophase I and CytokinesisCytokinesis

During this phase, During this phase, the nuclear the nuclear membranes reform membranes reform and the cell divides.and the cell divides.

After cytokinesis, After cytokinesis, meiosis II occurs.meiosis II occurs.

Result of Meiosis IResult of Meiosis I

Two daughter cells with ‘half’ the Two daughter cells with ‘half’ the number of chromosomes (‘N’) as the number of chromosomes (‘N’) as the original cell (‘2N’).original cell (‘2N’).

Homologous chromosomes, with their Homologous chromosomes, with their ‘crossed over’ genes in each cell‘crossed over’ genes in each cell

Each homologous chromosome still Each homologous chromosome still has the ‘sister’ formed when DNA was has the ‘sister’ formed when DNA was replicated (also possibly ‘crossed replicated (also possibly ‘crossed over’).over’).

Meiosis IIMeiosis II

The four phases in Meiosis II are:The four phases in Meiosis II are:1.1. Prophase IIProphase II

2.2. Metaphase IIMetaphase II

3.3. Anaphase IIAnaphase II

4.4. Telophase IITelophase II

Prophase IIProphase II

After a brief resting After a brief resting period, the nuclear period, the nuclear envelope breaks envelope breaks downdown

chromatin again chromatin again condenses into condenses into chromosomeschromosomes

Spindle fibers reformSpindle fibers reform

daughter cells from meiosis I

Metaphase IIMetaphase II

The chromosomes line The chromosomes line up along the middle up along the middle (equator) of the cell(equator) of the cell

Spindles attach to sister Spindles attach to sister chromatidschromatids

daughter cells from meiosis I

Anaphase IIAnaphase II

The sister The sister chromatids separate chromatids separate and are pulled and are pulled toward opposite toward opposite ends of the cellends of the cell

daughter cells from meiosis I

Telophase II and Telophase II and cytokinesiscytokinesis

During this phase, During this phase, the nuclear the nuclear membranes reform membranes reform and the cell divides.and the cell divides.

After cytokinesis, After cytokinesis, there are 4 haploid there are 4 haploid sex cells (gametes)sex cells (gametes)

The difference between The difference between mitosis and meiosismitosis and meiosis

Mitosis results in two genetically Mitosis results in two genetically identical diploid cells (2N)identical diploid cells (2N)

Meiosis produces four genetically Meiosis produces four genetically different haploid (N) cells.different haploid (N) cells.

The difference between The difference between mitosis and meiosismitosis and meiosis

Mitosis creates ‘clones’Mitosis creates ‘clones’ necessary for growth and repair of necessary for growth and repair of

tissues in multicellular creaturestissues in multicellular creatures Prokaryotes (e.g. bacteria) reproduce Prokaryotes (e.g. bacteria) reproduce

this waythis way Meiosis produces ‘gametes’Meiosis produces ‘gametes’

leads to greater diversity and chance leads to greater diversity and chance for favorable mutationsfor favorable mutations

Complete the

drawing on your

worksheet by

copying the

circled terms shown here

Diploid cells (2N)

Haploid cells (N)

FertilizationFertilization

Fertilization is the union of 2 gametes Fertilization is the union of 2 gametes (sperm cell + egg cell).(sperm cell + egg cell).

‘‘Haploid’ + ‘Haploid’ = ‘Diploid’Haploid’ + ‘Haploid’ = ‘Diploid’ The zygote is the fertilized egg. The zygote is the fertilized egg.

It is the first cell of a new organism.It is the first cell of a new organism. In humans, 23 + 23 = 46 chromosomesIn humans, 23 + 23 = 46 chromosomes

homologous chromosomes come from homologous chromosomes come from each parenteach parent

FertilizationFertilization

Sperm (N) + Egg (N) Sperm (N) + Egg (N) Zygote Zygote (2N)(2N)

Finire! (Finished!)Finire! (Finished!)

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Double-check you answered the Essential Double-check you answered the Essential Question (2-word answer from notes)Question (2-word answer from notes)

Place worksheet in notebook with Place worksheet in notebook with “Genetics & Heredity” notes. Your next “Genetics & Heredity” notes. Your next test will be over these concepts.test will be over these concepts.