Cell Reproduction
Mitosis and Meiosis
A. Mitosis
1. Produces two cells with identical chromosomes (same genes)
2. Unicellular reproduction, embryo development, growth and cell
replacement
B. Meiosis
1. Produces nuclei with one half the chromosomes and new genetic
combinations (avoids doubling of chromosome number with each
generation)
2. Produces gametes only
C. Sexual reproduction
1. Always involves both meiosis and fertilization
Eukaryotic Chromosomes
A. Chromatin (diffuse) condenses prior to division to visible chromosomes
1. Already duplicated when condense
a. Sister chromatids held together by centromere
B. Haploid and diploid
1. Chromosomes found in homologous pairs
a. identical but may have varying gene messages
b. Humans have 46 (23 pairs)
1. 22 homologous pairs (autosomes)
2. X and Y (23rd pair) sex chromosomes
3. Females (XX); Males (XY)
c. Pairs separate during reproduction
1. One of each pair into gamete
2. Offspring receives one of each kind from each parent
d. Capable of exchanging genetic material during crossing over
(Prophase I)
e. Resemble each other in size, shape, and hereditary information
f. Separated during meiosis (Anaphase I)
g. Two paired homologus chromosomes - tetrad
Eukaryotic Chromosomes
2. Diploid (2N) is chromosomes in pairs
a. Fertilized egg and all that arise from it are diploid
(somatic cells)
b. Most normal zygotes
c. Alternation of generation in plants: the recognizable
fern is the diploid sporophyte
3. Haploid (N) is one of each chromosome pair
a. Cells have undergone meiosis
b. Germ cells become gametes
c. Many lower organisms are normally haploid (some
tetraploid)
Fertilization and meiosis
Meiosis
Purpose: production of sex cells (gametes)
for the next generation
Result: 2 divisions resulting in 4 daughter
cells with 1/2 (haploid) sets of
chromosomes
Mitosis?
Comparison/contrast with mitosis
• both preceded by replication of chromosomes
• unlike mitosis1.replication is followed by TWO nuclear cell
divisions
2.produces FOUR daughter cells instead of
two
3.the resultant daughter cells have HALF the
number of chromosomes (meiosis is a
REDUCTION division)
• creates genetic variation
Steps of Meiosis
1. Interphase
• chromosomes replicate
• each duplicated chromosome consists of
two identical sister chromatids attached at
their centromeres
• centriole pairs in animal cells also
replicate into two pairs
Meiosis I (reduces the chromosome
number by one-half)
1. Prophase I
• chromosomes condense
• SYNAPSIS occurs. Homologous chromosomes come
together as pairs (forms a tetrad)
• in each tetrad, sister chromatids of the same
chromosome are attached at centromeres...nonsister
chromatids are linked by X-shaped chiasmata (site of
crossing over; genetic variation)
• spindle forms from microtubules
• nuclear envelope/nucleoli disperse
Metaphase I
• each synaptic pair is aligned at equator so
that the centromeres of homologues point
towards opposite poles
• homologues are destined to separate
towards opposite poles
Anaphase I
• sister chromatids remain attached while
homologues move towards the opposite
pole (different than mitosis where sister
chromatids are moved apart)
Telophase I & Cytokinesis
• each pole now has a haploid set of
chromosomes composed of two sister
chromatids attached at the centromere
• cytokinesis occurs producing two daughter
cells (cleavage furrow in animal cells; cell
plate in plant cells)
• NO DNA replication occurs before Meiosis
II
Meiosis II (similar to mitosis)
NO Interphase; NO DNA replication!
1. Prophase II
• spindle apparatus forms and chromosomes move
towards the metaphase II plate
2. Metaphase II
• chromosomes align on metaphase plate
• kinetochores of sister chromatids point towards
opposite poles
3. Anaphase II
• centromeres of sister chromatids separate
• individual chromosomes move toward opposite poles of
the cell
4. Telophase II & Cytokinesis
• nuclei form
• cytokinesis occurs producing 4 haploid daughter cells
Humans
• Males: meiosis by primary spermatocytes; each
secondary spermatocytes then produces
2 sperm
• Females: at the end of meiosis, one ovum and 3
polar bodies are produced
• Abnormalities: nondisjunction (XXY, 21, 21, 21Down syndrome)
Meiosis in Males - spermatogenesis
Meiosis in Females - oogenesis
Nondisjunction and Translocation
A. Too few or too many chromosomes in gametes
1. Translocation: attachment of all or part of
chromosome to another chromosome
2. Nondisjunction: failure of chromosomes to separate
B. Down Syndrome
1. Translocation of chromosome #15 to #21, or
2. Nondisjunction with extra #21
C. Nondisjunction of sex chromosomes
1. XO- Turner Syndrome; sterile female
2. XXY- Kleinfelter Syndrome; sterile male
3. XXX- Metafemale; limited fertility
4. XYY- normal male; somewhat taller
Genetic Variations
• Independent Assortment – each of the 23 pairs of
•
•
•
chromosomes segregates independently – 223
(8,000,000) gametes with different gene
combinations can be produced from 1 original cell
Crossing Over – portions of each chromatid are
shared – occurs in Prophase I
Random Fertilization – sperm? So, possible
combinations is squared (223 x 223 = 64 trillion)
So???? Evolution……….
Sexual vs. Asexual Reproduction
• Asexual – clone…..why? A single parent passes
•
•
copies of its genes to each of its offspring – no
fusion of haploid cells (ex. fission or budding)
Clone – genetically identical to its parent
Sexual – haploid gametes combine to form
diploid offspring; because both parents
contribute DNA, their offspring have traits of
both, but are not exactly like either
Life Cycle
• The entire span in the life of an organism
from one generation to the next
• 3 types:
Haploid – protists, some fungi and algae
Diploid – most animals
Alternation of generations – plants, algae,
and some protists
Mitosis vs. Meiosis
• http://www.pbs.org/wgbh/nova/baby/divi
_flash.html