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