Chapter 8 – Cellular Reproduction
Jan 16, 2016
Chapter 8 – Cellular Reproduction
In order for organisms to grow and reproduce, cells must divide.
Cell Division (8.1)
Cell division is part of asexual reproduction
• Single-celled organisms reproduce by simple cell division
• There is no fertilization of an egg by a sperm
Sexual reproduction
• Requires fertilization of an egg by a sperm using a special type of cell division called meiosis.
• Thus, sexually reproducing organisms use:– Meiosis for reproduction – Mitosis for growth and maintenance
Chromosomes Contain Our Genes
• Long stretches of DNA and proteins• Humans have 23 different chromosomes• Diploid – 2 of each chromosome (our body
cells)• Haploid – 1 of each chromosome (our sex
cells)
Duplicated chromosomes(sister chromatids) T
EM
Tight helical fiber
Looped domains
TE
M
Centromere
Nucleosome
“Beads ona string”
Histones
DNA double helix
Figure 8.4
Chromosomes 8.2
Cell Cycle
1. Cells grow and do “cell” things2. Cells copy DNA3. Each copy moves to opposite ends of cell.4. Cells divide; each new cell has a copy of DNA
Chromosomes are duplicated before cell division
Cell Cycle (8.6)
Eukaryote cells divide by mitosis or meiosis
• Mitosis– One cell divides producing two identical nuclei
followed by cell division– Almost all cell reproduction
• Meiosis (only in testes and ovaries)– One cell divides twice to produce four genetically
different cells– Produces egg and sperm cells (gametes)
A Cell’s life cycle is divided into phases.
• Most of the time a cell is in Interphase• During interphase, a cell:
– Performs its normal functions– Doubles everything in its cytoplasm– Grows in size
Interphase and Prophase (8.7)
Prophase
• Chromosomes condense and are visible under light microscope
• Nuclear membrane breaks down • Spindle microtubules grow from two
centrosomes, clouds of cytoplasmic material that in animal cells contain centrioles.
Metaphase and Anaphase• Metaphase• Mitotic spindle guides the separation of two sets of
daughter chromosomes.• Chromosomes line up along middle axis of cell
(metaphase plate)• Anaphase• Spindle fibers pull the chromosomes away from center• Centromeres break and each sister chromosome goes to
opposite poles of the cell.
Telophase and Cytokinesis
• Chromosomes unwind at the poles • New nuclear membrane forms• Cytokinesis typically:
–Occurs during telophase–Divides the cytoplasm–Is different in plant and animal cells
Cytokinesis (8.8)
Cytokinesis (8.8)
Meiosis
• Two cell divisions (Meiosis I and II)• Four daughter cells result each containing half
of the chromosomes as the parent cell• In males meiosis results in the production of 4
sperm cells• In females meiosis results in the production of
1 large egg cell and 3 small polar bodies
Multicellulardiploid adults(2n 46)
MEIOSIS FERTILIZATION
MITOSIS
2n
and development Key
Sperm cell
n
n
Diploidzygote(2n 46)
Diploid (2n)
Haploid (n)
Egg cell
Haploid gametes (n 23)
Figure 8.12
Meiosis has two stages
• Meiosis I• Meiosis II
Meiosis I
• Similar to mitosis except during first metaphase chromosome pair up side by side
• During first anaphase one set of chromosomes move to either pole of the cell.
• Sister chromotids stay joined in the new daughter cells
Meiosis I (8.14)
Meiosis II
• Anaphase II sister chromotids separate• Daughter cells have only one of each
chromosome (Haploid)
Meiosis II (8.14)
Crossing Over Increases Genetic Variation
The number of times a cell can divide is limited.
Cloning (NIB)
Dolly was euthanized
• When chromosomes replicate they lose part of their ends (telomeres)
• Linked to aging• Dolly’s chromosomes came from an adult
sheep with already shortened telomeres• Dolly developed age-related illnesses
Dolly taught us a lot about cellular aging.
Mitosis and Meiosis