Chapter 8: Cell Reproduction

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Honors BiologyChapter 10Cell Reproduction
Omnis cellula e cellula (Every cell from a
cell) -Virchow
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Students Will Be Able To

• Describe the reasons for and control of cell
  division.
• Distinguish between homologous chromosomes and
  sister chromatids.
• Distinguish between diploid and haploid cells.

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Why do cells divide?

• Its hard! Why bother? What are some reasons
  you can think of?

amoeba
starfish
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Why do cells divide
amoeba
starfish
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Asexual reproduction

• Single-celled eukaryotes
• yeast
• Paramecium
• Amoeba
• Simple multicellular eukaryotes
• Hydra
• budding

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Binary Fission--Prokaryotes

• Binary Fission division of a prokaryotic cell.
• Each new cell is identical
• Like mitosis for cells that dont have nuclei

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Limits to Cell Growth

• Cells can only grow so big for two reasons
• DNA Overload
• Exchanging Material
• DNA Overload
• A single copy of DNA holds information for an
  entire cell.
• As a cell gets larger it places more demands on
  its DNA.
• Example A library has only a certain number of
  books. If a town doubles or triples in population
  there wouldnt be enough books for everyone.

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Limits to Cell Growth

• Exchanging materials
• Food, oxygen, and water enter and waste products
  leave cells through the cell membrane.
• The rate at which materials enter/exit depends on
  the surface area or the amount of membrane of the
  cell
• The rate at which the food and oxygen are used up
  depends on the cells volume or how big it is.
• A small surface area to volume ratio would not
  provide enough area for the amount of nutrients
  required and wastes created.

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Ratio of Surface Area to Volume

• Volume increases faster than surface area in a
  growing cell.
• Area L x W x 6 sides
• Volume L x W x H
• 1 cm 2 cm
  4 cm
• A 6 cm2 A 24 cm2 A 96 cm2
  
• V 1 cm3 V 8 cm3 V 64 cm3
  
• Surface Area/ 6/1 3/1
  1.5/1
• Volume Ratio

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So, lets divide!

• What has to be copied
• DNA
• organelles
• cell membrane
• lots of othermolecules
• enzymes

animal cell
plant cell
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Copying DNA

• A dividing cell duplicates its DNA
• creates 2 copies of all DNA
• separates the 2 copies to opposite ends of the
  cell
• splits into 2 daughter cells

• But the DNA starts loosely wound in the nucleus
• If you tried to divide it like that, it could
  tangle break

DNA
cell
nucleus
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Organizing packaging DNA
DNA
cell
DNA has been wound up
nucleus
DNA in chromosomes ineveryday working cell
cell
nucleus
4 chromosomesin this organism
DNA in chromosomes in cell getting ready to
divide
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Chromosomes

• Chromosome Rod shaped structure of a DNA
  molecule wrapped around proteins (histones) which
  maintain the shape

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Copying packaging DNA

• When cell is ready to divide
• copy DNA first, then
• coil up doubled chromosomes like thread on a
  spool
• now can move DNA around cell without having it
  tangle break

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The Chromosome

• Each half of a copied chromosome a chromatid.
• When DNA copies itself, the result is two sister
  chromatids
• When the cell divides, each of the new cells will
  receive one chromatid from each chromosome.
• The waist of each chromatid, where it is
  attached to its sister chromatid centromere.

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double-strandedhuman chromosomes ready for
mitosis
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Chromosome Numbers

• Each species has a characteristic number of
  chromosomes in each cell
• Sex chromosomes are chromosomes that determine
  the sex of the organism.
• Example humans are either XX or XY.
• All other chromosomes autosomes.

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Chromosome Numbers

• The cell receives one copy of each autosome from
  each parent. The two copies of each autosome are
  called homologous chromosomes.

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Chromosome Numbers
Homologous pairs PRIOR to DNA replication (no
sister chromatids)
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Chromosome Numbers
Homologous pairs AFTER DNA Replication (each
chromosome a pair of sister chromatids)
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Chromosomes of Human Female
46 chromosomes 23 pairs
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Chromosomes of Human Male
46 chromosomes 23 pairs
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Diploid and Haploid Cells

• Diploid cells have both chromosomes for each
  homologous pair.
• All normal human body cells are diploid
• Diploid is abbreviated as 2n.
• Haploid cells lack homologous pairs, have just
  one chromosome of each kind.
• Sperm and egg cells are haploid cells.
• Haploid is abbreviated as 1n.
• When a sperm cell(1n) and an egg cell (1n)
  combine to create the first cell of a new
  organism, the new cell will be diploid (2n).

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The Cell Cycle Division
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Students Will Be Able To

• Describe and model the major stages of the cell
  cycle (G1, S, G2, M), including the stages of
  mitosis (prophase, metaphase, anaphase,
  telophase) and cytokinesis.
• Distinguish between diploid and haploid cells.
• Identify cell organelles involved in cell
  division.

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The Cell Cycle

• Cell Cycle the life cycle of the cell.
• Interphase time between divisions90 of the
  cell cycle.
• G1 cells grow to full size.
• S DNA is copied.
• G2 rapid metabolism, cell prepares for
  division.
• Cell Division cell divides into 2 daughter cells
• M Mitosis nucleus divides
• C Cytokinesis cytoplasm divides

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Cell Cycle
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G0 Phase

• Cells can also enter the G0 stasis or paused
  phase, no division preparation.
• They can later exit G0 or remain there
  perpetually.
• Example human nervous cells stop dividing at
  maturity and normally never divide again.

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Interphase

• G1, S, and G2
• The nucleus is well defined and has nuclear
  envelope with nucleoli
• Centrioles have replicated and are together
• Spindle fibers, a kind of microtubule, begin to
  extend from centrioles
• Chromosomes have been duplicated, but cannot be
  seen because they are in chromatin form

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Mitosis Dividing DNA cells

• Stage 1 cell copies DNA

Copy DNA!
DNA
cell
nucleus
(interphase)
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DNA must be duplicated
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Phases of MitosisProphase

• Chromatin fibers condense into chromosomes
  (tightly coiled).
• Nucleoli disappear
• Mitotic Spindle begins to form (microtubules
  extending from centrosomes)
• Centrosomes move to opposite poles of cell
• Nuclear membrane breaks down

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Mitosis Dividing DNA cells

• Stage 2 DNA winds into chromosomes
• DNA is wound up into chromosomes to keep it
  organized

duplicated chromosomes
Wind up!
cell
nucleus
(prophase)
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Phases of MitosisMetaphase

• Centrosomes are at opposite poles
• Chromosomes line up on Metaphase (Mitotic) Plate,
  an imaginary line across the center of the cell
• Chromosomes are attached to 2 spindle fibers, one
  from each pole

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Mitosis Dividing DNA cells

• Stage 3 Chromosomes line up
• chromosomes line up in middle
• attached to protein cables that will help them
  move

Line up!
duplicated chromosomes lined up in middle of cell
(metaphase)
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Phases of MitosisAnaphase

• Chromatids separate into separate chromosomes
• Chromosomes are dragged, centromere first, toward
  opposite poles
• The cell stretches
• At the end of anaphase, there are identical,
  complete set of chromosomes at each pole

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Mitosis Dividing DNA cells

• Stage 4 Chromosomes separate
• chromosomes split, separating pairs
• start moving to opposite ends

Separate!
chromosomes split move to opposite ends
(anaphase)
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Phases of MitosisTelophase

• Spindle fibers begin to break down
• Daughter nuclei begin to form at the poles
• Nuclear envelopes arise
• Nucleoli form
• Chromatin fibers uncoil
• Mitosis is complete

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Mitosis Dividing DNA cells

• Stage 5 Cell starts to divide
• cells start to divide
• nucleus forms again

Divide!
(telophase)
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Phases of MitosisCytokinesis

• Separation of the cytoplasm cleavage
• Cell pinches in a separates
• 2 new daughter cells are identical

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Cytokinesis in Plants

• Plants cannot pinch in because of cell walls
• Instead, they form a cell plate

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Mitosis Dividing DNA cells

• Stage 6 DNA unwinds again
• cells separate
• now they can do their every day jobs

(cytokinesis)
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Mitosis in whitefish embryo
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Mitosis in plant cell
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Overview of mitosis
Interphase
Prophase
Cytokinesis
Metaphase
Anaphase
Telophase
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Students Will Be Able To

• Describe and model the major stages of the cell
  cycle (G1, S, G2, M), including the stages of
  mitosis (prophase, metaphase, anaphase,
  telophase) and cytokinesis.
• State the difference between cancerous and
  non-cancerous cells.

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Controls on Cell Division

• Normally, cells stop growing when they come in
  contact with other cells.
• If cells are removed or damaged, cells bordering
  the open space will divide until they have filled
  the space and then stop.

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Cell Cycle Regulation

• Cell growth can be turned on and off by proteins,
  called cyclins, within the cell.
• Not all cells move through the cell life cycle at
  the same rate.
• The lining of the digestive tract divides every
  1-3 days
• Nerve and muscles, do not divide once full grown

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Regulating the Cell Cycle
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What if

• Something goes wrong, and a cell stops at a
  checkpoint when it shouldnt? What is the
  result?
• Something goes wrong, and a cell passes
  checkpoints when it shouldnt? What is the
  result?

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Uncontrolled Cell Growth

• is a disorder in which cells lose the
  ability to control their growth and division.
• do not respond normally to cyclins
  and other signals.
• As a result,

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Ch. 11-4 Meiosis Sexual Reproduction
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Students Will Be Able To

• Describe the stages of meiosis, and how meiosis
  maintains consistency (chromosome number) and
  diversity (genetic content) across generations.
• Compare and contrast mitosis and meiosis in terms
  of their purposes, processes, and their products.
• State how gametogenesis varies between males and
  females (in humans).

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How about sexual reproduction?

• What if a complex multicellular organism (like
  us) wants to reproduce?
• joining of egg sperm
• Do we make egg sperm by mitosis?

No!
What if we did, then.

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92
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egg
sperm
zygote
Doesnt work!
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How do we make sperm eggs?
zygote
egg
meiosis
fertilization
sperm
gametes
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Meiosis

• The formation of reproductive cells (gametes)
• Nuclear division that cuts the number of
  chromosomes in half
• Produces 4 genetically different haploid gametes.
  (1n sperm or eggs)

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Meiosis makes sperm eggs

• Chromosome number will be halved by separating
  homologous pairs

egg
meiosis
haploid
diploid
sperm
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Remember from before

• Homologous chromosomes
• Same genes, but not identical!
• control same inherited characters, but have
  different instructions for those characters
• homologous same information

diploid2n 2n 4
eye color (brown?)
eye color (blue?)
homologouschromosomes
double strandedhomologous chromosomes
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What do you think

• If homologous chromosomes are NOT identical
• and we are going to undergo a reduction
  division, in which daughter cells will get only
  HALF the chromosomes of the parent cell
• what is the implication for genetic sameness or
  diversity?

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Stages of Meiosis

• Cells go through normal interphase growing,
  duplicating DNA, and getting ready to divide.

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Stages of Meiosis IProphase I

• DNA coils into chromosomes
• Spindle fibers appear
• Nucleus and nucleolus disassemble
• Homologous chromosomes pair up to form tetrads
• Crossing over can occur

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Crossing Over

• Exchanges DNA between homologous chromosomes in
  tetrads
• Results in genetic recombination or a new mix of
  genes

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Stages of Meiosis IMetaphase I

• Tetrads line up at middle of the cell
• Spindle fibers attach each homologous chromosome
  to the pole

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Stages of Meiosis IAnaphase I

• Spindles pull the tetrads apart, sending the
  homologous chromosomes to opposite ends of the
  cell
• Independent Assortment genetic material is
  pulled apart randomly.
• Its random whether the maternal or paternal
  chromosome goes north while the other goes
  south

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Stages of Meiosis ITelophase I and Cytokinesis I

• Nuclear membranes form.
• The cell separates into two cells.
• New cells are haploid- one of each homologous pair

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Meiosis II

• The two cells produced by Meiosis I now
  immediately enter a second mitotic
  division?Meiosis II
• No interphase, no DNA replication.

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Stages of Meiosis IIProphase II

• The chromatin condenses into chromosomes.
• The centrioles separate to the poles
• The spindle fibers form
• The nuclear membrane breaks down.

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Stages of Meiosis IIMetaphase II

• The chromosome line up across the center of the
  cell.
• Each chromosome is connected to a spindle fiber
  at its centromere.

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Stages of Meiosis IIAnaphase II

• The sister chromatids separate
• The spindle fibers pull chromosomes to poles

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Stages of Meiosis IITelophase II

• A nuclear membrane forms around the chromosomes
  in each of the four new nuclei.
• Cytokinesis II
• Cytoplasm divides
• Results in 4 new cells, each of which contains ½
  of the original cell s number of chromosomes.

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Meiosis mitosis

• Meiosis to make gametes
• sperm egg
• Fertilization two gametes unite to form a
  zygote, new cell with combined DNA
• Mitosis to make copies of cells
• growth, development

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The value of meiosis 1

• Consistency over time
• meiosis keeps chromosome number same from
  generation to generation

from Mom
offspring
from Dad
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The value of meiosis 2

• Change over time
• meiosis introduces genetic variation
• gametes of offspring do not have same genes as
  gametes from parents
• new combinations of traits

from Dad
variation
from Mom
offspring
new gametes made by offspring
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Formation of Gametes

• Males Spermatogenesis the original cell
  produces four sperm cells by meiosis.
• Females Oogenesis the original cell produces
  one egg and three polar bodies by meiosis. Polar
  bodies cant be fertilized. The egg cell receives
  most of the original cells cytoplasm.

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One final, tough question

• (Think of how fertilization works to answer this
  one)
• WHY do you think that spermatogenesis produces
  four gametes, but oogenesis sacrifices three
  potential gametes to produce just one?