Meiosis and Sexual Life Cycles (CHAPTER 10)
[Offspring acquire genes from parents by inheriting chromosomes ]
Genes = segments of DNA that code for the basic units of heredity that are transmitted from one
generation to the next that are called gametes
Locus = location of a gene on a chromosome
Asexual Reproduction = single and passes copies of all its genes to its offspring (MITOSIS)
Clone = a group of genetically identical individuals
Sexual Reproduction = two individuals contributes genes to the offsprings (greater genetic
variation)
Chiasmata = regions of chromosomes where nonsister chromatids cross over
[Fertilization and meiosis alternate in sexual life cycles]
Life Cycle = generation to generation sequence of stages
Somatic Cells = (NOT GAMETES) each somatic cell in humans has 46 chromosomes
Karyotype = picture of complete set of chromosomes (arranged in pairs of homologous
chromosomes)
● Homologous chromosomes are similar in length and centromere position
● One homologous chromosomes from each pair is inherited from each parent (in the set of
46 chromosomes; half was inherited from your mother and the other half is inherited
from your father)
Sex Chromosomes = X and Y; Females = XX, Males = XY
Autosomes = NO X and Y
Gametes = sperm and ova (eggs) are HAPLOID cells (half the number of chromosomes of
somatic cells)
● Contain 22 autosomes PLUS a single sex chromosomes (X in females, or X or Y in
male); 23 IN TOTAL
● Haploid number of chromosomes is symbolized by n
Meiosis and fertilization are the key events in sexually reproducing life cycles.
Meiosis and Sexual Life Cycles (CHAPTER 10)
Fertilization = (combination of sperm cell and an egg cell), one haploid gamete from the father
fuses with one haploid gamete from the mother
Zygote = fertilized egg (RESULT OF FERTILIZATION) (DIPLOID) (2n)
Spore = gives rises to a multicellular organism without fusing with another cell
Meiosis is the type of cell division that reduces the numbers of set of choromos from TWO to
ONE
Fertilization RESTORES the diploid number
[Meiosis reduces the number of chromosome sets from diploid to haploid]
Both Meiosis and Mitosis are preceded by the replication of the cell’s DNA, BUT in Meiosis thi
s replication is followed by two stages of cell divisions, Meiosis I and Meiosis II
Daughter Cells = final result of meiosis, EACH of which has half as many chromosomes as the
parent cell
[IPMAT (Interphase, Prophase, Metaphase, Telophase)]
*Interphase = Each chromosome replicates its DNA (doubling the amount of DNA);
centrosome divides
[MEIOSIS I]
PROPHASE I
● Synapsis occurs, forming tetrads
● Crossing over occurs between HOMOLOGOUS chromosomes in the tetrads
● Crossing over increases genetic variation
● Areas of crossing over form CHIASMATA
● The nuclear envelope disintegrates, allowing the spindle to attach to the homologs
METAPHASE I
● Homologous pairs of chromosomes are lined up at the metaphase plate
● INDEPENDENT ASSORTMENT
ANAPHASE I
● SPINDLE APPARATUS move the chromosomes towards opposite ends of the cell
Meiosis and Sexual Life Cycles (CHAPTER 10)
● Sister chromatids stay connected
TELOPHASE I and CYTOKINESIS
● Each pole contains a haploid set of chromosomes, with each chromosome still consisting
of two sister chromatids
● Cytokinesis is the division of the cytoplasm
● Cleavage furrow occurs in animal cells, Cell Plates (new cell wall) occur in plant cells
● Results in formation of two haploid cells
● Sister chromatids are still attached; homologous pairs have seperated
[MEIOSIS II] (Begins with Haploid Cells)
PROPHASE II:
● Spindle Apparatus forms and sister chromatids move toward the metaphase plate
METAPHASE II:
● Haloid number of chromosomes are arranged on the metaphase plate
● Kinetochores of each sister chromatid are attached to microtubules from opposite poles
ANAPHASE II:
● Centromeres of the sister chromatids separate and individual chromosomes move to the
opposite ends of the cell
TELOPHASE II:
● Nuclei reappear
● Cytokinesis occurs
● Four daughter cells has the HAPLOID number
● Genetically different from the other daughter cells and from the parent cell
THREE EVENTS OCCUR DURING MEIOSIS I THAT DO NOT OCCUR DURING
MITOSIS
Meiosis and Sexual Life Cycles (CHAPTER 10)
1. Synapsis and crossing over do not occur during mitosis
2. At Metaphase I, paired homologous chromosomes (tetrads) are positioned on the metaphase
plate, rather than individual replicated chromosomes, asm in mitosis
3. At Anaphase I, duplicated chromosomes of each homologous pair separate, but the sister
chromatids of each duplicated chromosome stay attached. In Mitosis, the chromatids separate.
* Diploid cells can divide by either mitosis or meiosis, but Haploid cells can only divide by
mitosis.