Download Chapter 8: Cellular Reproduction

Chapter 8:
Cellular Reproduction
1. The Cell Cycle
2. Mitosis
3. Meiosis
2 Types of Cell Division
2n
1n
Mitosis:
• occurs in somatic cells (almost all cells of the body)
• generates cells identical to original
Meiosis:
• occurs in gamete production (sperm & egg)
• ½ the normal chromosome content (haploid or “1n”)
Chromosome
Content
• somatic cells
are diploid (2n)
• 2 of each
chromosome
• i.e., 1 from each
parent
• gametes are
haploid (1n)
• 1 of each
chromosome
23 pairs of human chromosomes
1. The Cell Cycle
The Cell
Division
Cycle
M
G1
G2
S
• all eukaryotic
cells progress
through the 4
phases of the
cell cycle with
each division
Stages of the Cell Cycle
G1:
• preparation for DNA replication
• non-dividing cells are arrested at this stage of
the cell cycle (referred to as G0)
S phase:
• replication of genetic material (DNA Synthesis)
G2:
• preparation for cell division
M phase:
• cell division (Mitosis or Meiosis)
***G1, S & G2 collectively make up “Interphase”***
2. Mitosis
4 Stages of Mitosis
1) Prophase
2) Metaphase
3) Anaphase
4) Telophase & Cytokinesis
Interphase
G1, S phase & G2
• all events in preparation
for cell division
• DNA replication
• duplication of organelles
• preparation of “mitotic
apparatus”
Prophase
1) duplicated
chromosomes
condense, remain
attached at the
centromere
2) break down of
nuclear envelope
3) centrosomes move
to opposite poles
4) spindle fibers form
Duplicated Chromosomes
• in prophase, condensed duplicated chromosomes
remain attached at the centromere
• each “half” of duplicated chromosome referred to
as sister chromatids
Still considered as
1 chromosome!
Metaphase
• chromosomes
align along the
metaphase plate
of the cell
• mediated by spindle
fibers connecting
each centromere to
a centrosome at ea
“pole” of the cell
Anaphase
• sister chromatids
are pulled to
opposite poles of
the cell
• separate at the
centromere, each
chromatid still
attached to a
spindle fiber
• spindle fibers
shorten, pulling
each chromatid to
opposite poles
Telophase
Opposite of prophase:
• nuclear envelope
reforms around
chromosomes at
each pole
• spindle fibers break
down
• chromosomes
decondense
Cytokinesis begins…
Cytokinesis
Division of the
cytoplasm
completes the
process:
• the 2 daughter cells
are genetically
identical to the
original parent cell
• each daughter cell
is now in G1 (the
beginning of
interphase)
3. Meiosis
Meiosis Produces 1n Gametes
Meiosis requires 2 rounds of cell division:
Meiosis I
• differs from mitosis in prophase, metaphase, anaphase
Meiosis II
• basically the same process as mitosis
1n
(chromosome #) 2n
1n
(DNA content)
4n
2n
Meiosis I
Differs from mitosis in several key ways:
prophase I
metaphase I
anaphase I
Prophase I: homologous chromosomes pair, exchange
genetic material by “crossing over”
Metaphase I: homologous chromosomes align as pairs
Anaphase I: homologous chromosome pairs separate
(sister chromatids are NOT separated)
Coat-color
genes
C
Eye-color
genes
E
Tetrad (homologous pair of
chromosomes in synapsis)
e
c
1
Breakage of homologous chromatids
C
E
c
e
2
Joining of homologous chromatids
E
C
Chiasma
e
c
3
Separation of homologous
chromosomes at anaphase I
C
E
C
e
c
E
c
e
4
Separation of chromatids at anaphase
II and completion of meiosis
C
E
C
e
c
E
c
e
Parental type of chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of chromosome
Gametes of four genetic types
“Crossing Over”
in Prophase I
• an important source of
new genetic combinations
(genetic variability)
Independent Assortment of Chromosomes
due to alignment in Metaphase I
• an additional source of genetic variability
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
2n
possibilities
(n = # of
homologous
pairs, e.g. 23)
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
Meiosis II
prophase II
metaphase II
anaphase II
telophase II/cytokinesis
• essentially the same as mitosis
• results in four haploid (1n) gametes
• each is genetically unique due to 1) “crossing over” and
2) independent assortment of homologous chromosomes
Key Terms for Chapter 8
• cell cycle, mitosis, meiosis
• haploid, diploid, somatic cell, gamete
• interphase, prophase, metaphase, anaphase
telophase, cytokinesis
• centromere, centrosome, spindle fibers
• sister chromatids vs non-sister chromatids
• homologous chromosomes
• crossing over, independent assortment
Relevant Review Questions:
1-6, 8, 9, 13, 17, 20