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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

→More on this later

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- Cytokinesis• Splitting the

cytoplasm in 2

→More on this later

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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