Lecture 11 - Meiosis

heredity

transmission of traits from one generation to the next

variation

differences between individuals

genetics

the study of heredity and hereditary variations

gametes

reproductive cells that transmit genes from one generation to the next

asexual reproduction

• single parent produces offspring

  • unicellular → divide

  • multicellular → buddy or fragmentation

• in eukaryotes:

  • 1 diploid parent → 2 diploid offspring

  • 1 haploid parent → 2 haploid

• clone production

• advantages:

  • fast

  • less E required

  • safe

  • lots of offspring,

  • keeps adaptations (no genome dilution)

sexual reproduction

• fusion of 2 gametes from different parents to form a zygote(diploid cell resulting from fertilization)

  • gamete(n) + gamete(n) → fertilization → zygote (2n)

• not genetically identical to parents

• advantages:

  • genetic variation; offsprings are combinations of parents’ genes and more likely to survive environmental change or stress

• disadvantages:

  • slower

  • higher E requirement

  • dangerous

  • fewer offspring

  • genome dilution

meiosis

• reduction division

• 4 stages, 2 cell division:

  • interphase

  • meiosis I (1st cell division)

  • interkinesis

  • meiosis II (2nd cell division

interphase

• same as mitosis

  • cell is highly active - growth, synthesis, metabolic activity

  • DNA is chromatin throughout

  • three phases: G1, S, G2

• ADDED chromosomes are duplicated

• ex. in humans: 2n=46, so 46 chromosomes and 92 chromatids enter meiosis

meiosis I

• first meiotic division

• homologous chromosomes separate & ploidy reduced

• summary:

  • start with 1 diploid cell with duplicated chromosomes

  • end with 2 haploid cells with duplicated chromosomes

  • crossing over in prophase I

  • homologous pairs separate in anaphase I

  • sister chromatids do not separate

  • ploidy is reduced

prophase I

• same as mitosis

  • chromatin condenses into chromosomes

  • nucleus & nuclear envelope breaks down

  • mitotic spindle forms

• ADDED crossing over & synapsis occurs

  • homologous chromosomes pair up which result in a tetrad (2 homologous chromosomes & 4 chromatids)

  • homologous recombination where alleles between non-sister chromatids in tetrad exchange; result in new combinations of genes and diversity

• ex. in humans: 46 chromosomes, 23 tetrad, 92 chromatids

metaphase I

• same as mitosis

  • tetrad(instead of chromosomes) align at the metaphase plate

• ADDED homologous chromosomes orient towards opposite poles

anaphase I

homologous chromosomes separate (disjunction)

telophase I + cytokinesis

• might go straight to cytokinesis

• results in 2 haploid cells with duplicated chromosomes

• ex. in human: 23 chromosomes in each cell, 46 chromatids, and 0 tetrad (haploid)

interkinesis

• time between 1st and 2nd meiotic divisions

• interphase-like stage

• usually short

• no S-phase

• no DNA replication

meiosis II

• 2nd meiotic division

• chromatids separate into daughter cells

• basically mitosis