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CHAPTER 8
Cellular Reproduction: Cells from Cells
Chapter overview • Reproduction
• Cells vs. organisms
• Reproduction and proliferation
• Non-sexual reproduction
• Mitosis and the cell cycle
• Cancer
• Sexual reproduction
• Meiosis and sex
• Chromosomal abnormalities
• The advantages of diploidy and sex
Today • Reproduction
• Cells vs. organisms
• Reproduction and proliferation
• Non-sexual reproduction
• Mitosis and the cell cycle
• Cancer
• Sexual reproduction
• Meiosis and sex
• Chromosomal abnormalities
• The advantages of diploidy and sex
REPRODUCTION
• Is the creation of new organisms by organisms
• Can refer to single cells and multicellular organisms
• Can be asexual and sexual
• Asexual reproduction (reproduction from one parent) is based on “simple” cell division
Reproduction of Organisms
– Single cell organisms (example: amoeba)
– Some multi-cellular organisms – Lower animals (example:
hydra) – Plants can
• Sexual reproduction (reproduction from two parents) is based on a cycle of fertilization & reduction of chromosome number – More complex than asexual reproduction – Second half of this lecture plus next lecture
“SIMPLE” CELL DIVISION AND
THE CELL CYCLE
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Cell division plays a role in
• The replacement of lost or damaged cells
• Wound healing
• Growth
• Reproduction
• Cancer
• Each minute, ~109 cells die in our body • Each day, ~1012 cells are formed in our body
Physical organization of genetic information
• The central event of cell division is the distribution of genetic information between daughter cells
• Cell division requires equal distribution of genetic information
• The complete set of all genetic information in an organism is called genome
• Human genome contains ~20,000 protein-encoding genes
• The genome is physically organized in pieces called chromosomes
This cell is dead, because it was killed by the staining procedure
Chromosomes
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
DNA double helix Histones
“Beads on a string”
Nucleosome
Tight helical fiber Supercoil
Sister chromatids
Centromere
2
1
3
4
The DNA is packed by coiling and folding: • 1st level: beads-on-a-string (nucleosome); each bead consists of DNA wound around a protein core of 8 histones • 2nd level: the beaded string is wrapped into a helical fiber • 3rd level: a thick supercoil. • 4th level: the supercoil is folded (this maximum folding only during cell division)
Structure of chromosomes
• Chromosomes are large continuous pieces of DNA in complex with packaging proteins (DNA + packaging proteins = chromatin)
• DNA in chromosomes is highly compacted
• Completely unfolded human chromosomes would be 16 – 85 mm long
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Chromosomes may be small or large
The number of chromosomes does not inform about the amount of DNA
Number of chromosomes
• The cell cycle produces two daughter cells from a parent cell
• The central event of the cell cycle is the chromosome cycle
• A chromosome is the genetic information that is physically connected into one unit.
• 1 Chromosome consists of one chromatid or two chromatids (sister chromatids) that are connected at the centromere
• When the cell divides, the two sister chromatids separate from each other
Duplication
Sister chromatids
Cell cycle
1 chromosome = 1 chromatid
1 chromosome = 2 chromatids
1 chromosome = 1 chromatid
centromere
Cell division
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
•All eukaryotic cells that divide repeatedly undergo a cell cycle
•The cell cycle consists of two phases:
•Interphase
•Mitotic phase
Interphase (90% of time)
Mitotic phase (M)
(10% of time)
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
S phase
Interphase
Mitotic phase
G1 G2
Interphase
•The interphase consists of three parts:
•G1 (Gap 1): Cell growth
•S phase (DNA synthesis)
•G2 (Gap 2): Cell growth and preparation of mitotic phase
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Mitosis and Cytokinesis
• Mitosis = division of nucleus
• Cytokinesis = division of the cytoplasm
• Mitosis and cytokinesis are different processes, but occur in a coordinated manner
• Cytokinesis starts in the final subphase of mitosis.
Interphase
Cytokinesis Mitosis
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
The four stages of mitosis
1.Prophase
• Condensation of the chromatin sister chromatids become visible in light microscope
• Centrosomes double, move apart, and produce spindle fibers (microtubules)
• Nuclear envelope dissolves Early mitotic spindle
Centrosome Centromere
Chromosome, consisting of two sister chromatids
Spindle microtubules
Fragments of nuclear envelope
Prophase
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Metaphase
Spindle
Prophase 2. Metaphase
• Chromosomes convene on an imaginary plate equidistant from the two poles of the spindle
• The centromeres of all chromosomes are lined up at this plate.
• Spindle fibers from both poles attach to a centromere of each chromosome
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
3. Anaphase Anaphase
Daughter chromosomes
• Each chromatid is now an independent daughter chromosome.
• Motor proteins (= kinetochore) at the centromeres “walk” the daughter chromosomes along their microtubules toward opposite poles of the cell
• Meanwhile, these microtubules shorten
• The microtubules that are not attached to chromosomes lengthen, pushing the poles farther apart
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
4. Telophase
• Produces two daughter cells
• The reverse of prophase
• Nuclear envelopes form
• The chromosomes uncoil
• Nucleoli reappear
• Spindle disappears
Cleavage furrow
Nuclear envelope forming
Nucleolus forming
Telophase and Cytokinesis
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Cytokinesis (= division of cytoplasm) • In animals, cytokinesis is caused by a ring of cytoskeletal proteins (microfilaments)
• In plants, membrane vesicles that contain cell wall material accumulate in the middle of the cell and gradually fuse
Cell wall
Vesicles containing cell wall material
Cell plate New cell wall
Cleavage furrow Contracting ring of microfilaments
Mitosis animations: http://www.maxanim.com/genetics/Mitosis/Mitosis.htm http://www.johnkyrk.com/mitosis.html Mitosis movies: http://www.youtube.com/watch?v=aDAw2Zg4IgE http://www.youtube.com/watch?v=DD3IQknCEdc
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
•This is actually not very precise: In the human body, 3x109 cells form every minute 30,000 errors occur each minute.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Normal plant and animal cells have a cell cycle control system that consists of special proteins
• These proteins generate “stop” and “go ahead” signals at certain key points during the cell cycle • The cell cycle normally halts at the G1 phase unless the cell receives a go-ahead signal • If that signal never arrives, the cell will switch into a permanently non-dividing state called G0
(for example, nerve and muscle cells) • If the go-ahead signal is received and the G1 checkpoint is passed, the cell will usually complete
the rest of cycle S phase
Interphase G1
Mitotic phase
(M)
G2
Go
G1 checkpoint
Muscle and nerve cells
•More than 500,000 people die of cancer in the USA each year •1/3 of all cancers in men are prostate cancer •¼ of all cancers in women are breast cancer
In Cancer Cells the Cell Cycle is Out of Control
Role
• Enables the continued existence of many species (single cells and multicellular organisms).
• Enables the maintenance and repair of organs.
• Loss of cell cycle regulation can lead to cancer.
Summary – Nonsexual reproduction Nature of nonsexual reproduction
• Creation of a new organism from a single parent, and the offspring is genetically identical to the parent.
Mechanism
• “Simple” cell division (mitosis). Repeatedly dividing cells go through cell cycles. Each cycle produces an exact copy of the genetic information of the parent cell.
Most important words to know
• Genome, chromosome, chromatid, chromatin
• Cell cycle, mitosis, interphase, cytokinesis
요약- 무성생식
무성생식의 특징
• 단일 부모로부터 새로운 유기체를 생산하며, 그 자손은 부모와 유전적으로 동일하다.
기작
• “단순한” 세포분열 (체세포분열, mitosis). 세포주기를 통해 반복적으로 세포가 분열함. 각 주기는 부모 세포의 유전정보를 정확하게 복사한다.
역할
• 많은 종 (단세포와 다세포 유기체)들의 존재를 지속시킨다.
• 장기의 유지와 보수를 가능하게 한다.
• 세포주기를 조절할 수 없는 세포는 암세포가 될 수 있다.
가장 중요한 단어들
• Genome, chromosome, chromatid, chromatin
• Cell cycle, mitosis, interphase, cytokinesis
SEXUAL REPRODUCTION AND MEIOSIS (Introduction)
– A new cell is created by “mixing” the chromosomes of two parent
cells number of chromosomes is doubled
– The cell with the doubled chromosome set can exist a long time and
even divide and create a multicellular organism
– Eventually, a special cell division reduces the chromosome number
back to the original level.
Sexual reproduction
– The mixing step that doubles chromosome number is called fertilization – The division that reduces the chromosome number is called meiosis. – Therefore sexual reproduction involves a cycle of fertilization & meiosis
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
• The human karyotype – 1 haploid set = 22 autosomes + X or Y – 1 diploid set = 22 pairs of autosomes + XX (♀) or XY (♂)
• The karyotype
– is the ordered set of chromosomes characteristic for each eukaryotic species
– The simple set of chromosomes (before fertilization) is called haploid
– The karyotype resulting from fertilization is diploid: The chromosomes exist in two copies
– The two copies are called homologous chromosomes
– The homologous chromosomes are almost identical: Same order of genes
The sexual life style is usually a cycle between a haploid and a diploid karyotype
http://en.wikipedia.org/wiki/Sex
Diploid stage (2n) of life cycle
Haploid stage (1n) of life cycle
In some organisms both the haploid (1n) and the diploid (2n) stages can multiply
http://en.wikipedia.org/wiki/Yeast
Life cycles of yeast
In multicellular organisms, either the haploid or the diploid stage dominates
1 n Main stage in lower plants (mosses)
2 n Main stage in •Higher plants (flowering plants) – only pollen and eggs are haploid; •Animals – only sperm and eggs are haploid
• In higher multicellular sexual organisms (higher plants and animals):
– Somatic cells do not transmit their genetic material to the next generation are mortal
(Somatic cells are diploid)
– Germ cells (egg, sperm) transmit their genetic material to the next generation are immortal
(Germ cells are haploid and generated by meiosis)
Multicellular sexual organisms have two principally different kinds of cells
In this chicken, the eggs and their precursors in the ovary
are the germ line cells - All other cells are somatic cells
• Hence, sex is the reason that we are mortal
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Gametes and the life cycle of humans