bio lec 6

+sexual

better for an UNSTABLE environment, generations will hopefully adapt and create stability within a population

• more genetic variation

-sexual

• slower

• requires finding a mate

• takes more time and energy

• produces fewer offspring at once

+asexual

better for a STABLE environment, individuals within a species are near perfect, want to duplicate that perfection, NOT change it

• better food quality

• less stress

• fast

• produce many offspring quickly

-asexual

• little to no genetic variation

• individuals affected similarly to disasters

• less ability to adapt to new conditions

fertilization and meiosis in sexual life cycles 1 meiosis

take diploid (2n) cell and produces haploid (n) cells (gametes)

fertilization and meiosis in sexual life cycles 2 fertilization

two haploid gametes combine, creating a diploid zygote

n+n=2n

fertilization and meiosis in sexual life cycles 3 growth

diploid zygote grows into multicellular organism

fertilization and meiosis in sexual life cycles 4 meiosis pt2

when its time to make gametes, meiosis reduces the chromosome number back to haploid

2n—meiosis—n—fertilization—2n

gonads

testes (produce sperm, testosterone)

ovaries (produce eggs, estrogen)

somatic cells

any cells in a multicellular organism that form the body EXCLUDING reproductive cells (gametes)

mitosis

somatic cells, EXACT COPIES. goes through TWO phases of PAMTC

meiosis

gonads, GENETIC DIVERSITY. goes through one phase of PAMTC

Explain how meiosis reduces the number of chromosome pairs from diploid (2n) to haploid (n)

• diploid cell (2n) has two copies of each chromosome (ONE FROM EACH PARENT)

• results in 46 chromosomes=23 pairs

• meiosis i REDUCES it by separating the matching chromosome pairs so that each cells gets only one chromosome from each pair (2n—n)

• meiosis ii seperates the sister chromatids and produces 4 haploid cells

homologous chromosomes

sister chromatids

nonsister chromatids

nonhomologus chromosomes

meiosis i

seperates homologus chromosomes

meiosis i: interphase

dna copies once before meiosis while chromatin stays uncondensed

meiosis i: prophase i

chromosomes condense as homologus pairs from tetrads and exchange segments

meiosis i: metaphase i

homologus chromosome pairs align together at center

meiosis i: anaphase i

homologus chromosomes seperate to opposite poles while sister chromatids stay together

meiosis i: telophase i+cytokinesis

the cell divides into TWO HAPLOID cells that still contain duplicated chromosomes

meiosis ii

separates the sister chromatids, producing FOUR HAPLOID CELLS

meiosis ii: prophase ii

a new spindle forms in each haploid cell around the duplicated chromosomes

meiosis ii: metaphase ii

chromosomes line up individually at the center of each haploid cell

meiosis ii: anaphase ii

sister chromatids separate and move toward opposite poles in each cell

meiosis ii: telophase ii+cytokinesis

nuclei reform and cytokinesis produces four haploid daughter cells

genetic variation: independent assortment

random distribution of homologous chromosomes during meiosis

(with 23 chromosomes assorting independently there are 2²³—8mil possible assortments of chromosomes inherited for every cell

genetic variation: crossing over

homologous chromosome exchange reciprocal portions of themselves

genetic variation: random fertilization

in humans: the ovum has 8mil possible chromosome combinations, so does the sperm cell. 8milx8mil=64 trillion possible diploid combinations

compare meiosis and mitosis

• start with once cell

• start with copied dna/chromosomes

• PMATC

• use spindle fibers to move chromosomes

• end with new daughter calles

• helps cells divide and reproduce

contrast meiosis and mitosis

• mitosis conserves # of chromosome sets, producing 2 cells that are genetically identical to the parent cell; meiosis reduces # of chromosome sets from 2 (diploid) to 1 (haploid) producing 4 genetically different cells

• happen in meiosis but not mitosis=synapsis and crossing over during P1, alignment of homologus pairs at metaphase plate during M1, seperation of homologs during anaphase

• mitosis=cohesins cleave at end of metaphase; meiosis=cohesins cleave along chromosome arms in A1 (homologs seperate) and at centromeres in A2 (seperation of sister chromatids)