cell division - bioapes.com · –with no big mistakes from cell to cell from generation to...
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CELL DIVISION
The Cell Cycle, Mitosis, & Meiosis
How cells make copies of themselves
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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
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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
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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
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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
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THE CELL CYCLEThe life cycle of cells including growth, DNA replication, and division
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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
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- 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
Fill out your Cell Cycle Worksheet now
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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
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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
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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”
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Licentious division - prostate cancer cells during division
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Osteosarcoma – cancer of the bone
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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
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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
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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
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CHROMOSOMESThe way cells keep their DNA safe during cellular division
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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
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CHROMOSOMESAfter replication, the chromatin will condense
- The special proteins act like a spool and carefully wrap the DNA upThis forms chromosomes
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duplicated
chromosome
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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
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Each chromosome has 2 “arms”
These are sister chromatids
Connected by centromeres
They have the same gene and the same traits
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MITOSISDNA separates in to separate chromosomes in their own nuclei and
then the cell splits
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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
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THE PHASES OF MITOSIS
Mitosis is split up in to 4 phases- These occur in a very strict sequential order
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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
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MEIOSISCreating unique individuals through sexual reproduction
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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
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CHROMOSOME COUNT
Diploid – 2n Haploid – 1n
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MEIOSIS
The reason you look similar but not exactly like your parents
A special kind of cell division for gametes- ‘Reproductive’ or ‘sex’ cells
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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
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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
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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
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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
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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
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Meiosis I
Interphase- G1
• Grows
- S• Replicates DNA
- G2
• Grows & produces proteins
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Meiosis IProphase I- Chromatin condenses- Homologous
chromosomes pair up and align
- Crossing over occurs- Nuclear envelope
breaks down
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Meiosis I
Metaphase I
- Homologous pairs line up at the metaphase plate
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Meiosis I
Anaphase I
- Homologous chromosomes move to opposite poles (ends) of the cell
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Meiosis ITelophase I- Chromosomes gather
into nucleus
- Original cell divides in 2• There are two 2n
cells
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Meiosis II
Prophase II
- Chromosomes condense again after a short interphase
•DNA does NOT replicate
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Meiosis II
Metaphase II
- Centromeres of sister chromatids line up at the metaphase plate in each cell
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Meiosis II
Anaphase II
- Chromatids separate
•Become single chromosomes
- Chromosomes are pulled to opposite poles
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Meiosis II
Telophase II
- Chromosomes gather into nuclei
- Cells divide
- Each cell is genetically different
• Crossing over• Independent Assortment
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Meiosis
The result
- Four cells with their own nucleus
•Haploid number (1n) of chromosomes
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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.)
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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
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FERTILIZATIONHaploid (1n) sperm
combines with a haploid (1n) ovum (egg)
- Producing a diploid (2n) zygote
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FERTILIZATION
The zygote has the ability to divide and produce other cells of its kind or any other kind of cell
- Stem cells