gen bio - lecture 11: meiosis

heredity

transmission of traits from one generation to the next (aka inheritance)

variation

differences between individuals

genetics

the study of heredity and hereditary variation

gametes

reproductive cells that transmit genes from one generation to the next

definition + advantages of asexual reproduction

single parent produces offspring

• fast

• low energy

• safe

• lots of offspring

  • minimal genetic diversity = well adapted

in eukaryotes mitotic division creates… (also explain results of 1 diploid parent vs 1 haploid parent)

… genetically identical offspring/clones

• 1 diploid parent = 2 diploid offspring

• 1 haploid parent = 2 haploid offspring

definition of sexual reproduction

fusion of two gametes to form a zygote

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

• gametes are usually from different parents

• offspring = NOT GENETICALLY IDENTICAL to parents or other offspring

negatives to sexual reproduction

• slow

• high energy

• dangerous (predation, disease)

• fewer offspring

• genome dilution (offspring get ½ your genes)

meiosis OVERVIEW

• cell DIVIDES TWICE

  • 1 diploid (2n) cell to 4 haploid (n) cells

• 4 stages and 2 divisions:

  • interphase, meiosis 1, interkinesis, meiosis 2

interphase

• chromosomes duplicate

  • each chromosome → 2 sister chromatids (BUT STILL CHROMATIN FORM)

meiosis 1 + cytokinesis

• first meiotic division → homologous chromosomes separate

• PLOIDY REDUCED!!!!!!

  • P1, M1, A1, T1 (same general steps as mitosis but with homologous chromosomes!)

prophase 1

• same 3 things as mitosis (chromosomes condense, nucleus breaks down, mitotic spindles/kinetochores form)

• synapsis: homologous chromosomes PAIR UP (forms tetrad = 2 homologous chromosomes held together by proteins)

  • genes in the chromosomes align → CROSSING OVER

    • exchanges between non-sister chromatids in tetrad

    • results in new combinations of genes

    • IMPORTANT SOURCE OF GENETIC DIVERSITY

metaphase 1

• tetrads align @ metaphase plate

• homologous chromosomes orient towards opposite poles randomly (LAW OF INDEPENDENT ASSORTMENT)

anaphase 1

• disjunction/LAW OF SEGREGATION: homologous chromosomes separate

  • sister chromatids are still connected!

  • chromosomes act independently and direction of separation depends on orientation of tetrad

PLOIDY SPECIFICALLY REDUCED HERE!

telophase 1 + cytokinesis

• chromosomes MAY decondense and nuclear envelope MAY reform

• cytokinesis occurs

• result? 2 HAPLOID cells with duplicated chromosomes (sister chromatids still together)

interkinesis

• time between 1st and 2nd meiotic divisions

  • usually short, interphase-like stage but NO DNA REPLICATION OCCURS

meiosis 2

• 2nd meiotic division (P2, M2, A2, T2)

• chromatids separate into daughter cells (basically mitosis)

STATS: human numbers @ end of prophase 1

2n = 46

46 chromosomes

92 sister chromatids

23 tetrads

STATS: human numbers @ end of telophase 1

2n = 46

23 chromosomes in each cell

46 sister chromatids in each cell

ZERO tetrads

STATS: human numbers @ end of telophase 2

2n = 46

23 chromosomes in each cell

ZERO sister chromatids in each cell

ZERO tetrads