Meiosis Sex Cell Formation

Mitosis Versus Meiosis We have learned that MITOSIS is the DIVISION of SOMATIC cells (Body cells) Purpose is for organism GROWTH Retain the same number of chromosomes (2n) through out all divisions In order to produce GAMETES (sex cells), we must go through a different process MEIOSIS Sometimes thought of as REDUCTION division Why? MITOSIS- produces two IDENTICAL daughter cells, each also identical to the original parent cell

Homologous Chromosomes We inherit half of our DNA from each parent The combination of our father’s and mother’s genes give us our own unique set of characteristics Somatic cells contain genetic material that is half from mom and half from dad (2n) HOMOLOGOUS chromosomes are two sets of chromosomes that control the same genes One homolog from mom, one from dad

Meiosis and Chromosome Number The number of chromosomes in any given organism’s cells is defined by the chromosome number “N” Diploid number (2n): cells that contain both “sets” of homologous chromosomes Aka: ALL somatic body cells Haploid number (n): cells that contain 1 “set” of inherited DNA AKA: ALL Sex cells (gametes)

Karyotype: display of chromosomes This is a HUMAN karyotype. There are 23 pairs of DNA Each pair consists of one version from mom, and one version from dad (total=46 chromosomes)

Meiosis Sex Chromosomes – the 23rd chromosome, determines gender Two Forms: X – Females have XX Y – Males have XY

Meiosis- reduction of chromosome number MEIOSIS is the formation of gametes by reducing a diploid cell (2n) into FOUR genetically different haploid cells (n) *Remember- mitosis divided one cell into 2 identical cells… this this is quite different In males: formation of haploid sperm cells In females: formation of haploid egg cells

Meiosis- 2 sets of division Meiosis I: separation of homologous chromosomes Each homolog contains an attached copy (sister chromatids) Meiosis II: separation of sister chromatids This stage looks identical to Mitosis

Meiosis I Prophase I Tetrads – the paired chromosomes Now 4 chromatids Protein cause homologous chromosomes to stick together along with their length Tetrads – the paired chromosomes Now 4 chromatids Chromosome #: 2n 1 set from mom 1 set from dad 1(mom)+1(dad)= 2n

Meiosis I – Prophase I Crossing Over – 2nd “new” step; tetrads exchange genetic material This process introduces unique, new traits. This is one way, other than mutations, organisms can acquire NEW traits

Meiosis I Metaphase I Tetrads move to the middle of the cell and line up across the spindle. Notice now chromosomes line up NEXT to their homologous pair (2 lanes)

Meiosis I Anaphase I Homologous chromosomes separate and migrate to opposite poles Sister chromatids migrate together Each chromosome is made up of two copies

Meiosis I Telophase I The chromosomes arrive at poles Each pole has a haploid daughter nucleus because it only has one set of chromosomes

Meiosis I Cytokinesis Form two daughter cells Chromosomes in each daughter cell are still duplicated (double in number) Chromosome #: n Because mom’s set and dads set were separated, now you have haploid number

Meiosis II Prophase II In each haploid daughter cell, spindle forms Nuclear envelope disappears

Meiosis II Metaphase II Chromosomes line-up in the middle of cell Spindle attaches to centromeres

Meiosis II Anaphase II Sister chromatids separate and move to opposite poles

Meiosis II Telophase II & Cytokinesis Chromatids arrive at poles Now individual chromosomes Nuclear envelope reforms Cytokinesis splits cells

Produced four DIFFERENT haploid daughter cells Meiosis Finished Produced four DIFFERENT haploid daughter cells

Genetic Variation How chromosomes line-up and separate at is a matter of chance So the chromosomes that end up in the resulting cells occur randomly Four combinations possible

Closure- Did you get it?? What are two sets of chromosomes that control the same genes called? Homologous chromosomes What are homologous chromosomes doing when they are “crossing over”? Exchanging genetic material What is the chromosome number at the beginning of meiosis I? at the END of meiosis I? At the end of Meiosis II? 2n (diploid), n (haploid), n (haploid)

Bellringer- Name that phase Name each phase that is either being described or the picture depicts Ready??

Metaphase I

Prophase I Homologous chromosomes exchange genetic material by crossing over and tetrads form

What is separating here??? Anaphase II What is separating here??? Sister Chromatids

Telophase I Two nuclei form, each containing a haploid set of replicated chromosomes

Homologous pairs separate and migrate to opposite ends of the cell Anaphase I Homologous pairs separate and migrate to opposite ends of the cell

Comparing mitosis and Meiosis http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__comparison_of_meiosis_and_mitosis__quiz_1_.html

Mitosis vs Meiosis: Cell division processes Occurs in all growing tissue of organisms Purpose is for organism GROWTH Division of BODY cells (Somatic cells) Examples: hair, blood, muscle, skin, etc. Occurs in the Reproductive Organs of organisms Purpose is to create GAMETES (sex cells) for sexual reproduction Examples: Egg cells Sperm cells

Prophase vs Prophase I Mitosis Meiosis Spindle forms Nuclear membrane breaks down Chromosomes condense Chromosome #: 2n (diploid) Spindle forms Nuclear membrane breaks down Homologous chromosomes cross over and form tetrads Chromosome #: 2n (diploid)

Metaphase vs Metaphase I Mitosis Meiosis Chromosomes line up single file Spindle attaches to centromeres Chromosome #: 2n (diploid) Homologous pairs line up side-by-side Spindle attach to centromeres Chromosome #: 2n (diploid)

Anaphase vs. Anaphase I Mitosis Meiosis Sister Chromatids are pulled apart Cell elongates Chromosome #: 2n (diploid) Homologous pairs are pulled apart Cell elongates Chromosome #: 2n (diploid)

Telophase vs Telophase I (And Cytokinesis) Mitosis Meiosis Cytoplasm divides Nucleus forms Two genetically IDENTICAL daughter cells form Chromosome #: 2n (diploid) Cytoplasm divides Nucleus forms 2 different daughter cells form Nuclei is still REPLICATED Chromosome #: n (haploid)

Meiosis II Mitosis Meiosis DONE Meiosis II begins

Prophase II Nuclear Envelope breaks down Spindle Forms

Metaphase II Sister chromatids Line up single file Spindle attaches to centromeres

Anaphase II Sister Chromatids Separate Cell Elongates

Telophase II Nuclear Membrane reforms Spindle disappears Cytoplasm begins to divide via cytokinesis

Notice how none are the same END of Meiosis Cytokinesis divides both cells Now, 4 genetically different daughter cells All reduced to haploid (n # of chromosomes) Notice how none are the same

We Do Activity Visualizing meiosis Take out a piece of paper and number it 1-8 Take out 3 sheets of blank paper Using scrap paper, cut 8 small strips of paper Color 4 strips one color Color 4 another color Follow Ms. Hamadeh’s directions We Do Activity With your table partner, take out 2 different colored highlighters

Human chromosomes There are 46 chromosomes (23 homologous pairs) in each somatic cell 22 pairs of autosomes 1 pair of sex chromosomes XX = Female, XY = Male Karyotype - chromosomes are arranged according to shape and size

Nondisjunction and chromosomal disorders Nondisjunction – failure of chromosomes to separate and segregate into daughter cells Nondisjunction may occur during meiosis 1 or meiosis 2 Abnormal number of chromosomes may result

Results of crossing-over not shown Normal meiosis MEIOSIS I Replicate DNA Results of crossing-over not shown MEIOSIS II Normal monosomic gametes

Nondisjunction during meiosis I Replicate DNA Non-disjunction MEIOSIS II Disomic gametes Nullisomic gametes

Nondisjunction during meiosis II Replicate DNA MEIOSIS II Non-disjunction Disomic Nullisomic Monosomic Monosomic gametes

EXAMPLES OF NON-DISJUNCTION

AN EXTRA COPY OF CHROMOSOME 21 CAUSES DOWN SYNDROME This condition is called trisomy 21. Person with this condition suffers from the Down syndrome. Characteristic facial features: Round face Flattened nose bridge Small, irregular teeth Short stature Heart defects

Klinefelter's syndrome, 47, XXY It is the most common sex chromosome disorder and the second most common condition caused by the presence of extra chromosomes Symptoms: Language impairment Lanky, youthful build or rounded body type Low levels of Testosterone and small testicles / Infertile

Turner’s Syndrome (X) Common symptoms: Short stature swelling of the hands and feet Broad chest and widely spaced nipples Low hairline Low-set ears Reproductive sterility Increased weight, obesity Small fingernails Characteristic facial features Webbed neck

What genetic disorder is this? They may have never known…

Down Syndrome 47, XY, +21

Down Syndrome 1 in 1,250 births 47 chromosomes XY or XX #21 Trisomy Nondisjunction