Chapter 11 Meiosis and Sexual Reproduction

heredity

transmission of traits from one generation to the next

variation

shows that offspring differ somewhat in appearance from parents and siblings

genetics

scientific study of heredity and hereditary variation

genes

units of heredity and are segments of DNA

chromosome inheritance

how do offspring acquire genes from parents

locus

gene location on a certain chromosome

asexual reproduction

one parents produces genetically identical offspring via mitosis

sexual reproduction

two parents give rise to offspring that have unique combinations of genes inherited from the two parents

unique combination of alleles

46 chromosomes

how many chromosomes are in human somatic cells

23 chromosomes

how many chromosomes are in 1 set of a somatic cell

reduction of chromosomes

important event that must occur during chromosome inheritance

karyotype

ordered, visual representation of the chromosomes in a cell

metaphase (most compact)

what stage cells are used for karyotyping

homologous chromosomes

2 chromosomes composing a pair (mom and dad)

have the same characteristics (genes)

also called autosomes

sex chromosomes

distinct from each other in their characteristics

represented as X and Y

determine the sex of the individual

XX

what is the sex chromosome combination for female

XY

what is the sex chromosome combination for male

ploidy

measure of the number of sets of chromosomes

diploid cell

cell that has 2 sets of each of its chromosomes

after DNA synthesis

all chromosomes are duplicated and each consists of 2 identical sister chromatids

gametes

haploid cells containing only 1 set of chromosomes

23

how many chromosomes in a human gamete

life cycle

generation to generation sequence of stages in the reproductive history of an organism

ferilization and meiosis

what stages alternate in sexual life cycles

sexual maturity

point where the ovaries and testes produce haploid gametes via meiosis

fertilization

where gametes (sperm and ovum) fuse to form a diploid zygote

zygote

diploid cell that develops into an adult organism

diploid dominant, haploid dominant, alternation of generations

3 categories of life cycles in multicellular organisms

diploid dominant

in animals

meiosis occurs during gamete formation

gametes are the only haploid cells

haploid dominant

most fungi and some protists

meiosis produces haploid cells that give rise to a haploid multicellular adult organism

the haploid adult carries out mitosis producing cells that will become gametes

alternation of generations

plants and some algae

life cycle includes both diploid and haploid multicellular stages

meiosis

reduces the number of chromosomes sets from diploid to haploid

takes place in two sets of divisions, meiosis I and meiosis II

meiosis I

meiosis division set that reduces the number of chromosomes from diploid to haploid

meiosis II

meiosis division set that takes the products of meiosis I and produces four haploid daughter cells

S phase

chromosomes are duplicated during interphase

resulting sister chromatids are held together at the centromere

centrosomes also duplicated

prophase I

chromosomes condense and the nuclear envelope fragments. homologous chromosomes bind firmly together along their length forming a tetrad. chiasmata form between non sister chromatids. crossing over occurs at the chiasmata

sprindle fibers emerge from the centrosomes

prometaphase I

homologous chromosomes are attached to spindle microtubules at the fused kinetochore shared by the sister chromatids

chromosomes continue to condense and the nuclear envelope disappears completely

metaphase I

homologous chromosomes randomly assemble at the metaphase plate where they are maneuvered into place by the microtubules

anaphase I

spindle fibers pull the homologous chromsomes apart. the sister chromatids are still attached at the centromere

telophase I and cytokinesis

sister chromatids arrive at the poles of the cell and begin to decondense

nuclear envelope forms around each nucleus and cytoplasm divided by a cleavage furrow

result is 2 haploid cells with each cell containing one duplicated copy of each homologoous chromosome pair

prophase II

sister chromatids condense

new spindel begins to form

nuclear envelope starts to fragment

prometaphase II

nuclear envelope disappears and the spindle fibers engage the individual kinetochores on the sister chromatids

metaphase II

sister chromatids line up at the metaphase plate

anaphase II

sister chromatids are pulled apart by the shortening of the kinetochore microtubules

nonkinetochore microtubules lengthen the cell

telophase II and cytokinesis

chromosomes arrive at the poles of the cell and decondense

nuclear envelope surround the four nuclei

cleavage furrow divide the two cells into four haploid cells

synapsis and crossing over, tetrads at the metaphase plate, separation of homologues

3 events in meiosis I that help distinguish between meiosis and mitosis

synapsis and crossing over

homologous chromosomes physically connect and exchange genetic information

occur in Prophase I

chiasmata

x shaped structures formed during meiosis where homologous chromsomes exchange genetic material

tetrads on the metaphase plate

at metaphase I of meiosis, paired homologous chromsomes (tetrads) are positioned on the metaphase plates

separation of homologues

at anaphase I of meiosis, homologous pairs move toward opposite poles of the cell

in anaphase II of meiosis, sister chromatids separate

independent assortment

each pair of chromosomes sorts its maternal and paternal homologues into daughter cells independently of the other pairs

crossing over

what produces recombinant chromosomes that carry genes derives from two different parents

produces genetic variation

what is the purpose of reshuffling genetic material in meiosis

behavior of chromosomes during meiosis and fertilization

what makes up for most of the variation that arise in each sexually reproducing generation

2^(23) or 8.4 million

how many combinations are there for each gamete in humans

random fertilization

the fusion of gametes that produces a zygote with any of the 70 trillion diploid combinations (in humans)

genetic variation

raw material for evolution by natural selection

mutations

original source of genetic variation

sexual reproduction

produces new combinations of variant genes, adding more genetic diversity