Meiosis & Sexual Reproduction
Genetics - study of the patterns of inheritance

• Gene - unit of hereditary information, sequence of DNA, codes for a trait
• Study transmission, expression & capacity for change in genes
• Reproduction
• Asexual reproduction
• Sexual reproduction

Asexual reproduction

• All genetic information comes from one parent
• All individuals are genetically identical to parents - are clones
• Unicellular organisms -binary fission or mitosis & cell division
• Multicellular organisms - masses of cells resulting from mitosis

Sexual reproduction

• Requires 2 parents
• Offspring are a combination of genes of both parents - genetically different from either parent

Numbers of chromosomes

• Nuclei with 1 set - haploid - 1n
• Nuclei with 2 sets - diploid - 2n
• Human body cell has 46 chromosomes -
• Occur as pairs - 23 pairs (1n = 23)
• One set from father
• One set from mother

Homologous chromosomes

• Pairs of chromosomes, One from each parent
• Look identical, contain genes for same traits
• Occur in diploid nuclei

Sexual reproduction

• Involves two events
• Fertilization - union of two gametes, sperm & egg (each 1n), forms zygote (2n)
• Meiosis - reduction division - separates homologous pairs of chromosomes - reduces number from 2n to 1n

Sexual life cycle

• Fertilization & meiosis separate haploid & diploid phases
• Fertilization starts 2n phase
• Meiosis starts 1n phase
• Organisms differ, may have diploid, haploid or haploid-diploid life cycles

Diploid life cycle, See Figs. 13.3 & 13.4a

• Only gametes are 1n, Body is 2n, Mitosis in 2n cells
• Animals have this type of life cycle

Haploid life cycle, See Fig. 13.4b

• Only the zygote is 2n, Body is 1n, Mitosis in 1n cells
• Some fungi & algae have this type of life cycle

Haploid-Diploid Life Cycle, See Fig. 13.4c

• Produce both 1n & 2n bodies, Mitosis in both 2n & 1n cells
• Plants have this type of life cycle

Meiosis - overall, See Figs. 13.5 & 13.6

• 2 successive nuclear divisions - meiosis I & meiosis II
• Starts with a diploid nucleus & produces 4 haploid nuclei
• 1-2n® meiosis I ® meiosis II ® 4-1n
• Homologous chromosomes separated
• Daughter nuclei genetically different & different from original nucleus

Meiosis

• Will concentrate on critical steps
• In interphase before meiosis, DNA is replicated in S phase

 

Prophase I

• Chromosomes condense - each has 2 chromatids, nucleolus & nuclear envelope disappear, spindle forms
• Synapsis - homologous chromosomes come together & line up side to side - 4 chromatids together - tetrad
• Crossing over - nonsister chromatids exchange pieces - form chiasmata, (chiasma, singular) Exchange of pieces of nonsister chromatids (pieces of father’s & mother’s chromosomes are mixed)

Metaphase I

Homologous chromosomes separate - note that chromosomes not chromatids separate

Telophase I

• 2 nuclei reorganize - now each nucleus has only 1 set of chromosomes (1n)

Prophase II

• Preparatory phase

Metaphase II

• Chromosomes line up in center

• Chromatids separate

Telophase II

• 4 nuclei are reorganized
• Nuclei are not genetically identical

Genetic recombination

• In sexual reproduction, offspring are not identical to parents
• Mixing of genetic information from parents - genetic recombination
• Meiosis
• Independent assortment
• Crossing over
• Fertilization
• Random fusion of gametes

Independent assortment, See Fig. 13.8

• When homologous chromosomes are separated in Anaphase I, each has an equal chance of going one way or the other
• Movement of one set of chromosomes to poles is independent of what occurs with others

Independent assortment - 2n=4, 4 possible combinations of chromosomes

Independent assortment 2n=6, 8 possible combinations of chromosomes

Independent assortment

• Number of possible combinations possible is 2ⁿ where n= haploid no.
• n=2, 2²=4
• n=3, 2³=8
• In humans, n=23, 2²³=~8x10⁶
• During meiosis, 8 million different combinations of mother’s & father’s chromosomes as a result of independent assortment

Crossing over, See Fig. 13.9

• Exchange of pieces of nonsister chromatids in prophase I

• Mixes parents’ genetic information within chromosomes

Which gametes fuse is also random, so large no. of possible combinations

Meiosis

• In humans
• 2n nucleus has 46 chromosomes (23 pairs of homologues), 2 sets of genetic info, one from each parent
• ¯ Meiosis I, II
• 23 chromosomes in 4 - 1n nuclei (no pairs), 1 set of genetic info, mixed from both parents

Mitosis vs Meiosis, See Fig. 13.7