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CELL DIVISION
The Cell Cycle, Mitosis, & Meiosis
How cells make copies of themselves
DNA REPLICATION
In the last unit, we talked about DNA Replication- DNA helicase breaks the hydrogen bonds between base
pairs• This unwinds and unzips the DNAAllows each strand to act as a template for making a new
strand- DNA polymerase adds complementary nucleotides one
by one to both templates• This results in 2 daughter identical strands of DNAEach strand has one new strand and one old strand
CRITERIA 2
Watson & Crick knew that their structure for DNA must allow it to be copied with precision
- This is because DNA stores an organisms’ genetic information• That information must be passed down largely unaltered
– with no big mistakesFrom cell to cellFrom generation to generation
CELL DIVISION
DNA replication is only a part of a complicated process that allows cells to pass information on to the next generation of cells
CELL DIVISION
Cell division is a complicated coordination of several processes
- DNA replication – making a copy of DNA to pass on
- Mitosis – dividing that DNA between daughter cells
- Cytokinesis – dividing cytoplasm and organelles
THE CELL CYCLEThe life cycle of cells including growth, DNA replication, and division
THE CELL CYCLEThe Cell Cycle is the life cycle that all cells follow
- Very carefully regulated- 2 major stages
• Interphase – growing and developing; “daily living of a cell”
G1 phase S phase
G2 phase
• M stage – carefully separating DNA and cytoplasm so the cell can split in 2Mitotic phase
Cytokinesis
- G1 phase• Cell grows
physically larger
•Makes organelles
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- S phase• DNA replication –
all of an organisms chromosomes
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- G2 phase• Cell grows more• Makes proteins and
more organelles
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- Mitotic phase• Chromosomes
condense and divide
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- Cytokinesis• Splitting the
cytoplasm in 2
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- G1 phase• Cell grows physically larger• Makes organelles
- S phase• DNA replication – all of an
organisms chromosomes
- G2 phase• Cell grows more• Makes proteins and more
organelles
- Mitotic phase• Chromosomes condense and
divide
- Cytokinesis• Splitting the cytoplasm in 2
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WHEN THE CELL CYCLE GOES WRONG
The cell cycle is very tightly regulated and controlled
- When something goes wrong•Cells can replicate much faster than normalLeading to the development of what we
would call cancer
WHAT GOES WRONG?
Cellular replication is controlled by many things- Cell cycle regulators• Inhibitors•Promoters
If a mutation causes a change in one of those regulators - Cell cycle activity runs rampant with nothing to stop it
CELLS GROW UNCONTROLLABLY
Cancer cells do not behave like normal cells- They ignore signals to stop growing and dividing- They ignore signals to die•Apoptosis – scheduled cell death
- They grow faster than normal- They can continue to divide for much longer than
other cells• “Immortal”
Licentious division - prostate cancer cells during division
Osteosarcoma – cancer of the bone
Henrietta Lacks
An African American woman who was being treated for cervical cancer
- Her doctor biopsied a tumor without consent during treatment and cultured the cellsHer cancerous cells reproduced
at a very high rate and could be kept alive a long time•They survived long enough to
perform many different tests on the same sample
The line of HeLa cells has been maintained since they were first collected in 1970
- Her cells were the first to be mass produced commerciallyThey are still used today
for research and education
The use and commercialization of her cells occurred without her and her family’s knowledge for a long time
- Although this was deeply immoral, scientists have used them in many major scientific breakthroughs
• Development of polio vaccine
• Determining whether cancer was contagious
• Determining whether one could become immune to cancer
CHROMOSOMESThe way cells keep their DNA safe during cellular division
PREPARING DNA FOR CELL DIVISIONIn the nucleus, DNA actually
exists as a complex of DNA and supporting proteins
- This complex is called chromatinChromatin spends the
majority of time in an uncondensed state (95% of time)
nucleus
chromatin
nucleolus
CHROMOSOMESAfter replication, the chromatin will condense
- The special proteins act like a spool and carefully wrap the DNA upThis forms chromosomes
duplicated
chromosome
Karyotypes are arranged pictures of chromosomes in their most condensed state
Most chromosomes exist as homologous pairs –
basically, we have 2 copies of every chromosome
One from the mother and one from the father
These have the same gene, but different traits
Each chromosome has 2 “arms”
These are sister chromatids
Connected by centromeres
They have the same gene and the same traits
MITOSISDNA separates in to separate chromosomes in their own nuclei and
then the cell splits
MITOSIS
A type of cell division- One cell (the ‘mother’) divides to- Produce two new cells (the ‘daughters’) that- Are genetically identical to itself
→ This is the “last” part of the Cell Cycle where the DNA of the cell’s nucleus is split into 2 equal sets of chromosomes
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THE GOAL
To ensure that every daughter cell gets a perfect, full set of chromosomes
- Remember Watson & Cricks’ 2nd criteria for their DNA molecule model
THE PHASES OF MITOSIS
Mitosis is split up in to 4 phases- These occur in a very strict sequential order
Late Interphase
- The cell has grown and prepared for division• It has 2 copies of
uncondensed DNA• It has 2 copies of its
centrosome
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Early Prophase
- Chromosomes start to condense
- Mitotic spindle (aka spindle apparatus) forms
- Nucleolus disappears
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Late Prophase
- Nuclear envelope breaks down, freeing the chromosomes from the nucleus
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Metaphase
- Spindle captures all of the chromosomes
- Chromosomes align in the center of the cell, called the metaphase plate
Meta →Middle
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Anaphase
- Sister chromatids are pulled apart from each other
- Microtubules elongate and make the cell longer
Ana → Against
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Telophase
- Spindle apparatus starts to disassemble
- 2 nuclei begin to form, one for each set of chromosomes
- Chromosomes begin to decondense
Telo→ Distance
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Cytokinesis
- Starts towards the end of anaphase/beginning of telophase
- Ends after telophase with 2 identical daughter cells
- Animal Cells• A protein called actin pinches
the cell in at the cleavage furrow
- Plant Cells• A cell plate forms down the
middle of the cell
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Animal Cellcleavage furrow
Plant Cellcell plate
MEIOSISCreating unique individuals through sexual reproduction
CHROMOSOME COUNT
Diploid – 2n
• 2 sets of genetic information
• 2 homologous chromosomes
• Somatic “body” cells
• di = 2
Haploid – 1n
• 1 set of genetic information
• Gamete “reproductive” cells
• hap = half
CHROMOSOME COUNT
Diploid – 2n Haploid – 1n
MEIOSIS
The reason you look similar but not exactly like your parents
A special kind of cell division for gametes- ‘Reproductive’ or ‘sex’ cells
2 OUTCOMES OF MEIOSIS
1. During Meiosis, 1 diploid (2n) cell splits and makes 4 haploid (1n) cells
- This is a good idea for reproductive cells because you can take half of the maternal DNA and combine it with half of the paternal DNAThis results in offspring that is diploid
• You do not want offspring that have more than the diploid number of chromosomes because these can lead to problems
2 OUTCOMES OF MEIOSIS
2. The specific genes the maternal and paternal gametes have get scrambled in the offspring
- This provides for genetic diversityWhich is necessary for adaptation and evolution
HOW GENES GET MIXED UP
Crossing over- When chromosomes line up,
homologous pairs can trade DNAA gene from Chromosome A
can trade places with the same gene from Chromosome BoThe genes are for the same
protein, but can have slightly different traits
HOW GENES GET MIXED UP
Law of Independent Assortment
- The alleles, or different traits, of a gene are inherited independently of each otherThe trait one gamete gets does
not affect the trait another gamete getsoThis allows for many
combinations of traits
STAGES OF MEIOSIS
Meiosis is a 2 part process- Meiosis IFirst round of cell divisionoHomologous chromosomes
separate
- Meiosis IISecond round of cell divisionoSister chromatids separate
Meiosis I
Interphase- G1
• Grows
- S• Replicates DNA
- G2
• Grows & produces proteins
Meiosis IProphase I- Chromatin condenses- Homologous
chromosomes pair up and align
- Crossing over occurs- Nuclear envelope
breaks down
Meiosis I
Metaphase I
- Homologous pairs line up at the metaphase plate
Meiosis I
Anaphase I
- Homologous chromosomes move to opposite poles (ends) of the cell
Meiosis ITelophase I- Chromosomes gather
into nucleus
- Original cell divides in 2• There are two 2n
cells
Meiosis II
Prophase II
- Chromosomes condense again after a short interphase
•DNA does NOT replicate
Meiosis II
Metaphase II
- Centromeres of sister chromatids line up at the metaphase plate in each cell
Meiosis II
Anaphase II
- Chromatids separate
•Become single chromosomes
- Chromosomes are pulled to opposite poles
Meiosis II
Telophase II
- Chromosomes gather into nuclei
- Cells divide
- Each cell is genetically different
• Crossing over• Independent Assortment
Meiosis
The result
- Four cells with their own nucleus
•Haploid number (1n) of chromosomes
MEIOSIS IN HUMANS
Women- Eggs are produced while the
female is still in uteroLimited number of mature eggs
- Eggs have everything they need to grow (organelles, proteins, etc.)
MEIOSIS IN HUMANS
Men- Sperm production begins
around puberty and continues throughout the lifetimeRoughly 2.5 million produced
every day
- Sperm contains only the nucleus and an energy source that allow it to move
FERTILIZATIONHaploid (1n) sperm
combines with a haploid (1n) ovum (egg)
- Producing a diploid (2n) zygote
FERTILIZATION
The zygote has the ability to divide and produce other cells of its kind or any other kind of cell
- Stem cells