2.1 - sexual reproduction

sexual reproduction benefits

• mixes genes to produce genetically variable offspring

• more variability = better adapted to survive env. changes

• variability = faster evolution

asexual reproduction

genetically identical offspring

gamete

haploid sex cells (egg + sperm)

• 1 copy of each chromosome

zygote

diploid cell

• 2 copies of each chromosome (1 from each parent)

genetic variability

arises from which homologous chromosome a parent passes on to its offspring

• can be different due to different environmental histories and genetic origins

alleles

corresponding genes on homologous chromosomes

meiosis

producing haploid gametes from diploid cells

• genome is duplicated once, then divided two times

• produces non-identical haploid cells

meiosis process

1. DNA duplicates from 1 → 2 strands

2. homologous chromosomes pair

3. crossing over btw sister chromatids of homologs

4. homologs pulled apart → cells divide

5. each daughter cell only has one set of 2-stranded chromosomes

6. chromosomes line up

7. sister chromatids pulled apt

8. cells divide

mitosis

genome is duplicated once, then divided ONCE

• produces 2 identical diploid cells

meiosis 1 prophase

duplicated M + P homologs must pair before lining up at metaphase plate

• ensures that each haploid cell will have 1 sister chromatid from each chromosome set

mitosis prophase

individual duplicated M + P chromosomes line up independently at metaphase plate

• segregate into daughter nuclei

bivalent

formed during prophase

• 4 sister chromatids stick together until ready to divide

• pairing depends on interactions btw matching M + P DNA

• meiosis 1 = homologs separate; meiosis 2 = sister chromatids separate

homologous recombination

cross-strand exchange w/ non-sister chromatids in each bivalent

• crossing-over = physically swap chromosomal segments at homologous regions

• generate new alleles that contain pieces of M + P DNA

synaptonemal complex

facilitates crossing over

• cohesin, axial core and rod-shaped transverse filaments

chiasma

site of crossing over btw sister chromatids in each bivalent

• most bivalents have 2-3

• help ensure proper segregation of duplicated homologs during the 1st meiotic division

• resists tension + position

anaphase of meiosis 1

1. cohesins along arms break down, allowing homologs to be separated

2. cohesins at centromere cont. to hold the sister chromatids together as homologs are pulled apt by kinetochores

meiosis 2 (mitosis)

kinetochores on each sister chromatid function independently

• 2 sister chromatids pulled to opposite poles

2 kinds of genetic reassortment

independent assortment and homologous recombination

nondisjunction

homolog failure to separate during 1st meiotic division

aneuploidy

abnormal # of chromosomes in a cell

fertilization

reconstituting the diploid genome

• sperm egg transverses zona pellucida to fuse w/ plasma

mendel’s discoveries

• true-breeding plants exp = traits don’t blend, they are inherited as discrete units

• dominance + recessivity

• law of segregation

mendel’s law of segregation

2 alleles for each treat (one from each parent) segregate during gamete formation + unite at random during fertilization

• applies to all sexually reproducing organisms

law of independent assortment

alleles that segregate independently are packaged into gametes in all possible combinations

• if genes are far enough, 2 genes on the same chromosome can independently assort