Cell ReproductionCell ReproductionMitosis and MeiosisMitosis and MeiosisA. MitosisA. Mitosis

1. Produces two cells with identical chromosomes (same 1. Produces two cells with identical chromosomes (same genes)genes)2. Unicellular reproduction, embryo development, growth 2. Unicellular reproduction, embryo development, growth and cell replacementand cell replacement

B. MeiosisB. Meiosis1. Produces nuclei with one half the chromosomes and new 1. Produces nuclei with one half the chromosomes and new genetic combinations (avoids doubling of chromosome genetic combinations (avoids doubling of chromosome number with each generation)number with each generation)2. Produces gametes only2. Produces gametes only

C. Sexual reproductionC. Sexual reproduction1. Always involves both meiosis and fertilization1. Always involves both meiosis and fertilization

Eukaryotic ChromosomesEukaryotic ChromosomesA. Chromatin (diffuse) condenses prior to division to visible A. Chromatin (diffuse) condenses prior to division to visible

chromosomeschromosomes1. Already duplicated when condense1. Already duplicated when condense

a. Sister chromatids held together by centromerea. Sister chromatids held together by centromereB. Haploid and diploidB. Haploid and diploid

1. Chromosomes found in homologous pairs1. Chromosomes found in homologous pairsa. identical but may have varying gene messagesa. identical but may have varying gene messagesb. Humans have 46 (23 pairs)b. Humans have 46 (23 pairs)

1. 22 homologous pairs (autosomes)1. 22 homologous pairs (autosomes)2. X and Y (23rd pair) sex chromosomes2. X and Y (23rd pair) sex chromosomes3. Females (XX); Males (XY)3. Females (XX); Males (XY)

c. Pairs separate during reproductionc. Pairs separate during reproduction1. One of each pair into gamete1. One of each pair into gamete2. Offspring receives one of each kind from each 2. Offspring receives one of each kind from each

parentparentd. Capable of exchanging genetic material during crossing d. Capable of exchanging genetic material during crossing

over over (Prophase I)(Prophase I)e. Resemble each other in size, shape, and hereditary e. Resemble each other in size, shape, and hereditary

informationinformationf. Separated during meiosis (Anaphase I)f. Separated during meiosis (Anaphase I)g. Two paired homologus chromosomes - tetradg. Two paired homologus chromosomes - tetrad

Eukaryotic ChromosomesEukaryotic Chromosomes

2. Diploid (2N) is chromosomes in pairs2. Diploid (2N) is chromosomes in pairsa. Fertilized egg and all that arise from it are a. Fertilized egg and all that arise from it are diploid (somatic cells)diploid (somatic cells)b. Most normal zygotesb. Most normal zygotesc. Alternation of generation in plants: the c. Alternation of generation in plants: the recognizable fern is the diploid sporophyterecognizable fern is the diploid sporophyte

3. Haploid (N) is one of each chromosome pair3. Haploid (N) is one of each chromosome paira. Cells have undergone meiosisa. Cells have undergone meiosisb. Germ cells become gametesb. Germ cells become gametesc. Many lower organisms are normally haploid c. Many lower organisms are normally haploid (some tetraploid) (some tetraploid)

Fertilization and meiosisFertilization and meiosis

MeiosisMeiosis

Purpose: production of sex cells Purpose: production of sex cells (gametes) for the next generation(gametes) for the next generation

Result: 2 divisions resulting in 4 Result: 2 divisions resulting in 4 daughter cells with 1/2 (haploid) sets daughter cells with 1/2 (haploid) sets of chromosomesof chromosomes

Mitosis?Mitosis?

Comparison/contrast with Comparison/contrast with mitosismitosis

• • both preceded by replication of both preceded by replication of chromosomeschromosomes

• • unlike mitosis-unlike mitosis-1.replication is followed by TWO 1.replication is followed by TWO

nuclear cell nuclear cell divisionsdivisions 2.produces FOUR daughter cells 2.produces FOUR daughter cells

instead of instead of twotwo3.the resultant daughter cells have 3.the resultant daughter cells have

HALF the HALF the number of chromosomes (meiosis number of chromosomes (meiosis is a is a REDUCTION division)REDUCTION division)

• • creates genetic variation creates genetic variation

Steps of MeiosisSteps of Meiosis

1. Interphase1. Interphase• • chromosomes replicatechromosomes replicate• • each duplicated chromosome consists each duplicated chromosome consists of two identical sister chromatids of two identical sister chromatids attached at their centromeresattached at their centromeres• • centriole pairs in animal cells also centriole pairs in animal cells also replicate into two pairsreplicate into two pairs

Meiosis I (reduces the Meiosis I (reduces the chromosome number by one-half)chromosome number by one-half)

1. Prophase I1. Prophase I• • chromosomes condensechromosomes condense• • SYNAPSIS occurs. Homologous chromosomes SYNAPSIS occurs. Homologous chromosomes come together as pairs (forms a tetrad)come together as pairs (forms a tetrad)• • in each tetrad, sister chromatids of the same in each tetrad, sister chromatids of the same chromosome are attached at chromosome are attached at centromeres...nonsister chromatids are linked by centromeres...nonsister chromatids are linked by X-shaped chiasmata (site of crossing over; X-shaped chiasmata (site of crossing over; genetic variation)genetic variation)• • spindle forms from microtubulesspindle forms from microtubules• • nuclear envelope/nucleoli dispersenuclear envelope/nucleoli disperse

Metaphase IMetaphase I

• each synaptic pair is aligned at each synaptic pair is aligned at equator so that the centromeres of equator so that the centromeres of homologues point towards opposite homologues point towards opposite polespoles

• • homologues are destined to separate homologues are destined to separate towards opposite polestowards opposite poles

Anaphase IAnaphase I

• • sister chromatids remain attached sister chromatids remain attached while homologues move towards the while homologues move towards the opposite pole (different than mitosis opposite pole (different than mitosis where sister chromatids are moved where sister chromatids are moved apart)apart)

Telophase I & CytokinesisTelophase I & Cytokinesis

• • each pole now has a haploid set of each pole now has a haploid set of chromosomes composed of two chromosomes composed of two sister chromatids attached at the sister chromatids attached at the centromerecentromere

• • cytokinesis occurs producing two cytokinesis occurs producing two daughter cells (cleavage furrow in daughter cells (cleavage furrow in animal cells; cell plate in plant cells)animal cells; cell plate in plant cells)

• • NO DNA replication occurs before NO DNA replication occurs before Meiosis IIMeiosis II

Meiosis II (similar to Meiosis II (similar to mitosis)mitosis)NO Interphase; NO DNA replication!NO Interphase; NO DNA replication!1. Prophase II1. Prophase II

• • spindle apparatus forms and chromosomes move spindle apparatus forms and chromosomes move towards the metaphase II plate towards the metaphase II plate

2. Metaphase II2. Metaphase II• • chromosomes align on metaphase platechromosomes align on metaphase plate• • kinetochores of sister chromatids point towards kinetochores of sister chromatids point towards opposite poles opposite poles

3. Anaphase II3. Anaphase II• • centromeres of sister chromatids separatecentromeres of sister chromatids separate• • individual chromosomes move toward opposite individual chromosomes move toward opposite poles of the cellpoles of the cell

4. Telophase II & Cytokinesis4. Telophase II & Cytokinesis• • nuclei formnuclei form• • cytokinesis occurs producing 4 haploid daughter cytokinesis occurs producing 4 haploid daughter cellscells

HumansHumans

• • Males: meiosis by primary spermatocytes; Males: meiosis by primary spermatocytes; each secondary spermatocytes then each secondary spermatocytes then producesproduces

2 sperm2 sperm

• • Females: at the end of meiosis, one ovum Females: at the end of meiosis, one ovum and 3 polar bodies are producedand 3 polar bodies are produced

• • Abnormalities: nondisjunction (XXY, 21, 21, Abnormalities: nondisjunction (XXY, 21, 21, 21-Down syndrome)21-Down syndrome)

Meiosis in Males - spermatogenesis

Meiosis in Females - oogenesis

Nondisjunction and Nondisjunction and TranslocationTranslocationA. Too few or too many chromosomes in gametesA. Too few or too many chromosomes in gametes

1. Translocation: attachment of all or part of 1. Translocation: attachment of all or part of chromosome to another chromosomechromosome to another chromosome2. Nondisjunction: failure of chromosomes to 2. Nondisjunction: failure of chromosomes to separateseparate

B. Down SyndromeB. Down Syndrome1. Translocation of chromosome #15 to #21, or1. Translocation of chromosome #15 to #21, or2. Nondisjunction with extra #212. Nondisjunction with extra #21

C. Nondisjunction of sex chromosomesC. Nondisjunction of sex chromosomes1. XO- Turner Syndrome; sterile female1. XO- Turner Syndrome; sterile female2. XXY- Kleinfelter Syndrome; sterile male2. XXY- Kleinfelter Syndrome; sterile male3. XXX- Metafemale; limited fertility3. XXX- Metafemale; limited fertility4. XYY- normal male; somewhat taller4. XYY- normal male; somewhat taller

Genetic VariationsGenetic Variations• Independent AssortmentIndependent Assortment – each of the 23 pairs – each of the 23 pairs

of chromosomes segregates independently – of chromosomes segregates independently – 222323 (8,000,000) gametes with different gene (8,000,000) gametes with different gene combinations can be produced from 1 original combinations can be produced from 1 original cell cell

• Crossing OverCrossing Over – portions of each chromatid – portions of each chromatid are shared – occurs in Prophase Iare shared – occurs in Prophase I

• Random FertilizationRandom Fertilization – sperm? So, possible – sperm? So, possible combinations is squared (2combinations is squared (22323 x 2 x 22323 = 64 trillion) = 64 trillion)

• So???? Evolution……….So???? Evolution……….

Sexual vs. Asexual Sexual vs. Asexual ReproductionReproduction• AsexualAsexual – clone…..why? A single parent – clone…..why? A single parent

passes copies of its genes to each of its passes copies of its genes to each of its offspring – no fusion of haploid cells (ex. offspring – no fusion of haploid cells (ex. fission or budding)fission or budding)

• CloneClone – genetically identical to its parent – genetically identical to its parent

• SexualSexual – haploid gametes combine to form – haploid gametes combine to form diploid offspring; because both parents diploid offspring; because both parents contribute DNA, their offspring have traits contribute DNA, their offspring have traits of both, but are not of both, but are not exactlyexactly like either like either

Life CycleLife Cycle

• The entire span in the life of an organism The entire span in the life of an organism from one generation to the nextfrom one generation to the next

• 3 types:3 types:HaploidHaploid – protists, some fungi and algae – protists, some fungi and algaeDiploidDiploid – most animals – most animalsAlternation of generationsAlternation of generations – plants, algae, – plants, algae,

and some protistsand some protists

Mitosis vs. MeiosisMitosis vs. Meiosis

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