Unit 3Chapter 9: Meiosis and Sexual

Reproduction

Impact, Issues: Why Sex?Asexual Reproduction: quick and efficient

Does not require the participation of a partner

BUT the offspring are all clones – no variation

Sexual Reproduction: most costly, but also most responsive to changing conditionsMale and female partners must find each

other and exchange genetic materialThe variation introduced by sex has

selective advantages

Asexual ReproductionOne parent passes a duplicate of its

genes (stored in DNA molecules) to its offspring

Offspring can only be genetically identical __________ of the parent

Bacteria:

Sexual ReproductionTwo ParentsEach parent contributes one gene for each

traitOffspring has a pair of genes on a pair of

chromosomesOne chromosome of a pair is maternal, and the

other is paternalOffspring differ from its parents and each

otherSexual reproduction includes: meiosis,

formation of mature reproductive cells called gametes, and fertilization

AllelesGenes for each trait come in slightly

different forms called ___________Alleles are unique molecular forms of the

same gene; they specify different versions of a trait

Originally produced by mutationsExample: gene for eye color

Different alleles would include:

Sexual ReproductionMeiosis shuffles the alleles during gamete

formationFertilization produces offspring with

unique combination of allelesThe variation generated by sexual

reproduction allows for natural selection to occur and is the basis for evolutionary change

MeiosisMeiosis is a nuclear division process that divides the

parental chromosome number in halfIn animals: gametes form by meiosis of germ cellsPlants: spores

Begin with diploid (2n) germ cells and produces ____________ gametes (n)Humans:

In 2n cells, there are TWO chromosomes of each type, which are called _______________ chromosomesHomologous chromosomes line up during meiosis,

including sex chromosomesEach gamete produced by meiosis has one of each

pair of homologous chromosomes

Homologous Chromosomes

The same length, size, and genes except for the non-identical sex chromosomes (X and Y)

Where Gametes Form

Figure 9.3, pg 140

______________In most multicelled species, gametes

form from cells in reproductive structures or organs

Fertilization is the fusion of two gamete nucleiRestores the parental chromosome

numberForms a ____________ , the first cell of a

new individual

Two Divisions, Not One!Mitosis and meiosis are similar, but differentSimilarities between meiosis and mitosis:

Chromosomes are duplicated during interphase to form sister chromatids held together at the centromere

Chromosomes are moved by spindle fibersMeiosis is different in that it has two series of

divisions: ______________ and _______________

Meiosis IMeiosis I is the ______________________________Each duplicated chromosome lines up with its

homologous partnerHomologous chromosomes are separated

The two homologous chromosomes move apart toward opposite spindle poles

Each of the two daughter cells receives a haploid number of chromosomes

After meiosis I, each chromosome is ________ ____________________

Meiosis IIThe _______________________________In meiosis II, the sister chromatids of each

chromosome separate and the cytoplasm divides againOnce pulled away from each other, each

sister chromtid is now an individual chromosome

Results in FOUR _________ cells (n)Each have one unduplicated chromosome

Meiosis I and Meiosis II

Figure 9.12, pg 150

Prophase IChromosomes condense and align tightly with

their homologues Each homologous pair undergoes ____________

_____Microtubules form the bipolar spindleOne pair of centrioles moves to the other side of

the nucleus Nuclear envelope breaks up

Microtubules growing from each spindle pole enter the nuclear region

Microtubules tether one or the other chromosome of each homologous pair

Prophase I

Figure 9.5, pg 142

Metaphase IMicrotubules from

both poles position all pairs of homologous chromosomes at the spindle equator

Figure 9.5, pg 142

Anaphase IMicrotubules separate each

chromosome from its homologue

As in mitosis, other microtubules that overlap at the equator slide past each other to push the poles farther apart

At the end of anaphase I, one set of duplicated chromosomes nears each spindle pole

Figure 9.5, pg 142

Telophase ITwo nuclei formIn most species, the

cytoplasm divides

Each chromosome still consists of TWO _________ ______________(remains duplicated)

Figure 9.5, pg 142

Prophase IIThere is no DNA replication

between the two nuclear division

Prophase II is the beginning of meiosis II

One of the two centrioles moves to the opposite side of the cell

Figure 9.5, pg 143

Metaphase IIChromosomes are aligned at

the equatorSister chromatids are attached

to spindle fibers from opposite poles

Figure 9.5, pg 143

Anaphase IIOne chromosome of each

type is moved toward opposite spindle poles

Figure 9.5, pg 143

Telophase IIEach step of Meiosis II

occurs in BOTH nuclei formed in meiosis I

By the end of telophase II, there are four haploid nuclei, each with unduplicated chromosomes

Figure 9.5, pg 143

Haploid Daughter CellsOnce the cytoplasm divides after meiosis,

One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies