Mitosis & Meiosis

Not Happy with your grade?
Not understanding the material?
Remember that the TLCC has
Free Biology Tutoring
The authors of thr book
Biology for a
Changing World
Probably dislike me lumping
CHAPTERS 9 & part of 11
Mitosis & Meiosis
Stuff you’ll probably want to know
Stages of mitosis and when it happens
What is cancer?
How is making gametes (sex cells) different from
making normal cells?
How is sex determined in humans? In other animals
Cell Division – 2 ½ types
1. Meiosis – eukaryotes make cells for sex
In humans, “eggs” or “sperm” will discuss in detail later
Half the DNA of normal cells
2. Mitosis – eukaryotes make any other cells
½ “binary fission” – prokaryote division
Basically like mitosis, but we gave it special name
Mitosis & Binary Fission
“Mitosis” – how eukaryotes make normal cells
New cells for growth
New cells for replacement of old/damaged cells
Asexual reproduction
Term often misused, even by biologists!!!
mitosis = nucleus
cytokinesis=division of rest of cell
“Binary Fission” – how prokaryotes reproduce
Cell Division - Terms to know
• “parent cell” – one that will divide
• “daughter cell” – created by cell
• One parent forms two daughters
Equal division (daughters are identical)
1 Parent  2 Daughter
Mitosis – eukaryote’s normal cell division
• Eukaryotes – equal DNA in each daughter
• One normal cell  adult human
(or any other organism)
• Replacement cells
RBC wear out
Stomach cells eaten by acid
Skin cells worn away
Mitosis: 3 reasons
Cell replacement
Wound healing
Mitosis – asexual reproduction
• Many single-celled eukaryotes
• Some multicellular eukaryotes
Example: fungi, seaweed, and sponges
• DO NOT ASK (yet)
Eukaryotes – “mitotic division”
• Mitotic division – 1 parent  2 daughter cells
Each daughter identical to parent
Same DNA as parent
• Two processes in mitotic division
Mitosis – dividing the nucleus
Cytokinesis – dividing the rest of the cell
Prokaryotes – binary fission
• Prokaryote cell repro
“binary fission”
• Same idea as mitosis
• No nucleus to divide
Mitosis – DNA In Daughter Cells
• Same DNA as each other
• Same amount as parent originally had
• How?
– Double the DNA, then Divide
• When?
– Before mitosis
Both mitosis and binary fission
• Mitosis - “Double the DNA, then divide”
• Parent starts with normal
• Parent DOUBLES the dna
• Parent divides
1 parent  2 daughter cells
• Each daughter has normal amount of DNA
How do cells divide?
• cell does not simply split in half
• pass through a series of phases
Cell cycle
• ordered sequence of stages
• preparatory and division phases
• one cell to two identical cells
Interphase – when we double
• “Interphase” = time between divisions
most of the time is spent in
• Doubling of DNA happens during interphase
• More details later
How do cells divide?
Division phase
• mitosis
• cytokinesis
The Cell Cycle
• The cell cycle = new cell  dividing cell
• 90 minutes to 24 hours, varies with cell type
• Two main stages
Mitosis (cell division)
Cell Cycle – 2 main stages
• Interphase:
– Does normal cell stuff
• Nutrients in, makes proteins, special functions
– Grows
– Gets ready to divide (includes “double the DNA”)
• Mitosis (divides):
– Physical division of the cell
– Production of two daughter cells
How do cells divide?
Preparatory phase
• copy cellular
• duplicate
organelles, DNA,
and cytoplasm
• each new cell has
the same amount
as original cell
• interphase
• G1, S, G2
Interphase – three parts
• Interphase = G1, S, and G2
– G1 = Grow
– S = “Synthesis”
– G2 = Grow some more
How do cells divide? Interphase
G1 phase
• cell grows, makes cytoplasm
S phase
• DNA replication, chromosomes
• identical sister chromatids
G2 phase
• cell prepares for division
Most Cells in Adult Don’t Divide
• Most adult cells enter a nondividing phase
called G0
• The G0 phase can last from a few days to the
lifetime of the organism
Proteins regulate growth & division
• Cells divide when signaled
(internal or external)
• Humans have cell cycle
regulatory proteins
• Can start division or
stop/pause it
Checkpoint = regulatory protein
Cell division is tightly regulated
Mutation occurs
Cell cycle checkpoints
• either repair damage
• direct cell to commit
suicide (apoptosis)
Cancer: when checkpoints fail
If there’s enough DNA damage to interfere with multiple
•cancer: cells divide uncontrollably
•tumor may form
Kills by
– crowding out normal cells
– invading other organs
– secreting poisonous chemicals
OK, back to happy topics
Doubling the DNA (S part of interphase)
• Chromosomes
• Doubled chromosomes
• chromatids
“chromatin” = DNA wrapped around histone proteins
Chromatin is packed into
Normal  doubled
Normal chromosomes have
one strand
Doubled chromosomes have
two strands
• DNA is doubled during S phase
– Two identical copies, hooked together at “centroMere”
– “sister chromatids”
– Will separate and put one chromatid in each daughter cell
Do not confuse with
Centrosome or centriole
In the cytoplasm
Replicated chromosomes & chromatids
Mitosis = dividing the DNA
• chromosomes are
evenly divided
• sister chromatids are
• Centromere
• One doubled
chromosome (two
chromatids) becomes
two chromosomes
Chromosomes & chromatids
Before division:
• doubled chromosome
– Each ½ is a “chromatid”
– Each ½ of doubled has normal amount of DNA
• Anaphase:
pulled apart
Doubled chromosome  2 normal chromosomes
(one for each daughter cell)
After Division
• “Chromosome” = normal chromosome
– What used to be called “chromatid”
Most Human Cells Have Two Copies of
Each Type of Chromosome
• two copies of each chromosome
– homologous pairs
• Humans: 46 chromosomes (23 pairs)
• One set of chromosomes, called sex
chromosomes, determines the sex of an
individual animal
Karyotype – picture of chromosomes
• Different species have
chromosome #
• “karyotype” = Picture
of all the
chromosomes in
normal cell
• “Homologous” pairs
Cell Cycle
• Interphase – growing cell and doubling DNA
• Mitosis – dividing the nucleus and
• Stages of mitosis
– PMAT (nonsense word)
• Cytokinesis
Mitosis = separating chromatids
• four main phases of mitosis:
• Mitosis = separating DNA and putting a copy
in each daughter cell
The mitotic spindle
The structure that separates sister chromatids during mitosis
Made of microtubules
• hollow protein fibers
• key components of cytoskeleton
• attach to centromere on chromosome via kinetochore
For each stage, you should know
• What happens to the nuclear envelope
• What happens to the chromosomes
• What happen to the spindle fibers
Up close: cell cycle and mitosis
• G1 = Growth
• S = Double the DNA
• G2 = Growth
• Spindle forms
• Chromatin packaged as
• Nuclear envelope
breaks down
Early Prophase
• Chromatin packaged into chromosomes
• Two cytoskeletal structures called centrosomes begin
to move toward opposite ends of the cell
• mitotic spindle forms
centrosomes move towards pole
microtubules grow
Late Prophase
• nuclear envelope breaks down
• Spindle fibers attached to centromeres
• Chromosomes line up in
the middle
• Why the middle?
Spindle fibers pulling
• Chromosomes are torn
• Spindle fibers shorten
(pull chromosomes
Anaphase – Chromosome pulled Apart
• sister chromatids are
Why? Microtubules get shorter
• Once separated, each
“chromatid” is considered
a new “chromosome”
• Chromosomes are
• “Tel” = distant
• Nucleus reforms
• Cytokinesis continues
• Chromosomes are
• “Tel” = distant
• Nucleus reforms
• Cytokinesis continues
Telophase - New Nuclei
• nuclear envelopes for each set of
• chromosomes begin to unwind
(become less visible)
Mitosis: the quick review
• Prophase: nucleus breaks down, chromosomes
condense, spindle fibers form as centrioles move to
• Metaphase: no nucleus, chromosomes line up at the
middle as spindle fibers pull on them
• anaphase: no nucleus, chromosomes torn apart by
shortening spindle fibers
• telophase: nucleus reforms, chromosomes begin to
unwind, spindle fibers break down
starts in telophase
How do cells divide? Cytokinesis
“Cyto”(cell) + “kinesis”(movement)
• enlarged cell  2 daughter cells
• each has full complement of DNA
• one parent cell, into two daughter cells
Cytokinesis – different in plant & animal
• animal cells - actin microfilaments contract
Squeeze in middle
plant cells – grow
a new wall
Cancer: no checkpoints
• cells divide as fast as possible
• treatment
– surgery
– administration of chemicals: chemotherapy
Fighting cancer
• Surgery to remove the
cancerous tumor
– not effective if cancer
• Radiation therapy
– high-energy radiation
– kills dividing cells
– damages DNA
– triggers apoptosis
Fighting cancer
• Chemotherapy
– Drugs interfere with cell division
• Taxol
Cancer and cell division
• Taxus brevifolia, the
pacific yew, is one
species of a family
of related evergreen
• extract from bark
kills cancer cells
• Taxol
Cancer and cell division
• interferes with the
normal organization
of microtubule
• prevents
microtubules from
• chromatids not
Michèle Shuster • Janet Vigna • Gunjan Sinha • Matthew Tontonoz
Biology for a
Changing World
Lecture PowerPoint
Single-Gene Inheritance and
Meiosis - Why have sex?
• Asexual reproduction - offspring usually
identical to the parent
• Same vulnerabilities
• Sexual reproduction – some DNA from each
– Similar to, but not the same as either parent
– May have better combination of genes
• Better chance to survive and breed
Humans are diploid have two sets of DNA (2n)
In humans n = 23
“Maternal” = from mom
1n from mom, 1n from dad
“Paternal” = from dad
Homologous = “the same as”
Chromosomes in homologous pairs
Chromosome 1 from dad is the same kind of
chromosome as chromosome 1 from mom
Meiosis: Making cells for sex
Gametes – cells for reproduction
Sperm & egg
haploid (1n)
1 of each type of chromosome
“one of each homologous pair of chromosomes”
Meiosis = how we get Gametes
“double the DNA, Divide, Divide Again”
Nerd Words for making babies
Haploid (1n) sperm + haploid (1n) egg = diploid (2n) zygote
Made via meiosis
In humans n = 23
sex chromosomes
Zygote  embryo  adult
Offspring = different from parents
Gene combination neither parent has
• Gamete – sex cell, ½ the DNA of normal cell
• Each cell in you
½ DNA from dad’s sperm (a gamete)
½ DNA from mom’s egg (a gamete)
Cell to use in sex = made by meiosis
“haploid” = one set of DNA (Half the normal amount)
1n = only one set of DNA
• “Somatic Cell”
Normal cell = made by mitosis
“diploid” = two sets of DNA (dad and mom)
2n = has two sets
Meiosis: Making Gametes
Special division to
go from diploid to
6 chromosomes
Double @ S
of interphase
6 doubled
3 doubled
divide again
Meiosis: Two Cell Divisions
• “Double the DNA, Divide, Divide again”
• Meiosis 1 – first division
Separates homologous pairs
• Separates homologous pairs
• Meiosis II – second division
tears chromosomes apart
(like mitosis)
• Meiosis II
– separates sister
– four haploid
daughter cells
– develop into egg
or sperm
– Polar body
Meiosis: Two Cell Divisions
• Named like Mitosis
• Meiosis 1 - first division
Prophase I, metaphase I, anaphase I, telophase I
Separates homologous pairs
• Meiosis II – second division II
Prophase II, metaphase II, anaphase II, telophase II
tears chromosomes apart (like mitosis)
Meiosis: Terms to know
• Tetrad = “group of four”
Four “chromatids” = Two doubled chromosomes
(these are four future chromosomes)
• Metaphase Plate
imaginary line in middle of cell
Where chromosomes line up during metaphase
Meiosis I - separate the pairs
• Each chromosome was doubled during interphase
• Metaphase 1 = “Homologous pairs” line up
doubled chromosomes of same kind line up
• Tetrad = another name for pairs of doubled chromosomes
Four “chromatids” = four future chromosomes
Meiosis I - separate the pairs
• Anaphase 1 = separate the Homologous pairs
• Spindle fibers NOT in a tug of war.
– Each pulls a chromosome to the pole
Meiosis II: separate doubled chromosomes
• Just like mitosis
• Metaphase II
Line up down the Middle
• Anaphase II
Tear them Apart
Meiosis II: Double, Divide, Divide
• Double DNA in Interphase
• Meiosis I = 1 parent  2 daughter cells
• Meiosis 2 = each daughter cell divides again
• Total of four cells at the end
Polar body
Compare: Mitosis & Meiosis
Before Interphase
Double in Interphase
Divide again
1 Parent (2n)
Temporarily 4n
End with 2 somatic cells (2n)
1 Parent (2n)
Temporarily 4n
Have 2 cells (2n)
End with 4 gametes, each is 1n
Meiosis & sexual reproduction
creates genetic variation
better chance to survive
Sex Determination
• xx = female, xy = male
• All eggs have ‘x’
• 50% of sperm have ‘x’
• 50% of sperm have ‘y’
sex determination video
• Junction: where things
come together
• Disjunction: where
things separate
Things that should separate do not
Often caused by broken spindle fibers
Nondisjunction: abnormal gametes
Inherited Chromosomal Abnormalities
• “eu”(normal) + “ploidy”(number) = euploidy
• “An” = NOT
• “aneuploidy” = abnormal # of chromosomes
serious abnormality for animals
Often lethal for embryo
Caused by problem during meiosis
spindle fiber(s) can break
Aneuploidy in autosomes
Big chromosomes: probably lethal to animals
Small chromosomes: side effects
Aneuploidy: trisomy 21
Age of mother
Aneuploidy: trisomy 21
Father’s age matters too
Advanced paternal age combined with maternal age
significantly influences the incidence of Down
Aneuploidy in sex chromosomes
Effects often more minor than in autosomes
• XO – Turner’s Syndrome
• XXY – Kleinfelter’s syndrome
Sexual Reproduction = Genetic Variation
Three Sources Of Genetic Variation
1. Independent Assortment
2. Crossing Over
3. Random Fertilization
Gene variation: 1 independent assortment
pairs line up randomly
Many possible gametes
(Exception: Platypus sex
chromosomes are weird. Ask
outside class)
Genetic Variation: 2 – crossing over
ONLY MEIOSIS 1’s prophase or anaphase
Paternal and maternal chromosomes can swap parts.
Crossing Over: Meiosis & Variation
Increases variation
Four possible gametes
Many chromosomes
Many possible gametes
Genetic recombination
Meiosis and genetic diversity
• No two gametes are identical
• Due to recombination and independent
Genetic Variation: 3 – random fertilization
Lots of sperm (more than needed for reproduction)
Different gene combinations in each
Most don’t get to fertilize
• Cell division is, necessary for normal growth, development, and repair of
the body.
• The cell cycle is the sequence of steps that a cell undergoes in order to
divide. Stages of the cell cycle include interphase, mitosis, and cytokinesis.
• In mitosis, which takes place in several stages, replicated chromosomes
segregate to opposite poles of the dividing cell; during cytokinesis, the cell
physically divides into two daughter cells.
• Cell cycle checkpoints ensure accurate progression through the cell cycle;
repair mechanisms at each checkpoint can fix mistakes that occur, such as
DNA damage.
• Mistakes in the course of cell division can lead to cancer, which is
unregulated cell division.
• Chemotherapy drugs work by interfering with some part of the cell cycle.
Stem Cells and Germ Cells
• Germ cells = cell that will be used to make
• Stem Cells
Unspecialized (can make lots of cell types)
growth, regenerate, and repair tissues
Review Questions
Cell Division
Concept Quiz
Which of the following is not true about
The cell grows larger during interphase.
Chromosomes are duplicated during interphase.
Interphase is divided into three phases.
Chromosomes segregate to daughter cells during
Concept Quiz
Which of the following is true about
homologous chromosomes?
A. Both were received from the same parent.
B. One of each segregates to each daughter cell
during mitosis.
C. Both stay together in meiosis I.
D. All 23 pairs are always homologous.
Concept Quiz
Meiosis insures that
A. Each gamete receives the same genes
B. Chromosome number is doubled in the gametes
C. Zygotes produced by fertilization have the normal
number of chromosomes
D. All paternal chromosomes end up in the same
Have you taken the latest quiz?
When is your next paper due?
If you are not sure, you need to
Log into PAL