Transcript 10.1 MEIOSIS

10.2 MEIOSIS
Martin 2022
GENES, CHROMOSOMES, AND
NUMBERS
• A. Dipolid and Haploid Cells:
▫ Diploid Cells: cell that have two of each kind of
chromosome
 2n
 Found in all “body” cells
▫ Haploid Cells: only have 1 of each kind of
chromosome
n
 Found ONLY IN SEX CELLS (sperm and egg)
 Sex cells also known as gametes
GENES, CHROMOSOMES, AND
NUMBERS
• B. Homologous Chromosomes:
▫ Two chromosomes in each pair of a diploid
cell
▫ Each pair has genes for the same trait
 Genes arranged in the same order, but may
be different in their alleles (dominant or
recessive)
GENES, CHROMOSOMES, AND
NUMBERS
• Why Meiosis?:
▫ Meiosis: a type of cell division that allows
gametes (sex cells) to form, so that after
fertilization, offspring have the same
number of chromosomes as their parents
 Sperm: male gametes
 Egg: female gametes
GENES, CHROMOSOMES, AND
NUMBERS
• Meiosis is used to create cells for sexual
reproduction:
▫ Male gamete (23) fertilizes female gamete
(23) and produces a zygote (offspring with
46 chromosomes)
• Remember, this is different from mitosis
▫ Mitosis = 2n  2n
▫ Meiosis = 2n  n
THE PHASES OF MEIOSIS
• PMAT I, then PMAT II
▫ Has 2 cell divisions that lead to 4 haploid cells
• During PROPHASE I
▫ 2 homologous chromosomes pair so tightly that
crossing over can occur
 Where non-sister chromatids can break off and
exchange genetic information
 Can occur at ANY LOCATION on a chromosome, at
several locations, and at the same time
Crossing Over
THE PHASES OF MEIOSIS
• METAPHASE I
▫ Homologous chromosomes line up along the
midline/equator as a pair (tetrads)
• ANAPHASE I
▫ Homologous chromosomes separate
• METAPHASE II
▫ Chromosomes line up independently/randomly
along equator
• ANAPHASE II
▫ Sister chromatids are separated
MEIOSIS PROVIDES FOR GENETIC
VARIATION
• This variation can be because of crossing over
during metaphase I
• Can also be due to genetic recombination
▫ the reassortment of chromosomes and the genetic
information they carry by crossing over or
independent segregation of homologous
chromsomes
NONDISJUNCTION
• Sometimes there’s an error in meiosis
• Nondisjunction:
▫ The failure of homologous chromsomes to
separate during meiosis
▫ Leaves 1 gamete with an extra chromosome, and 1
gamete with 1 missing chromosome (22, 24)
▫ Trisomy 21 = Down Syndrome