Cell divisionMitosis and Meiosis

How cells divide

Purposes of Cell Division

1.Growth2.Repair3.Maintenance

Cell Division-the process by which a cell divides into two new daughter cells

- Has 4 phases:

G1InterphaseS

G2

M - Mitosis

The Cell Cycle- the activities of a cell from one cell division to the next

G1 – Gap phase 1• Growth - cell begins to grow in size; gets

nutrients, builds cell parts• If cell not big enough – when it divides the 2

daughter cells won't survive – not enough energy, not enough cell parts

• Mitochondria divide in plant and animal cells to make more (so there is enough for 2 cells)

• In plant cells chloroplasts divide to make more• Cell checks if there is any damage – won't

divide until it is repaired

S - Synthesis phase

- Cell will duplicate, make an exact second copy of all it's chromosomes (1 chromosome = 1 molecule DNA)- This is when DNA Replication takes place

G2 – Gap phase 2

- Cell gets ready to divide- Checks if all DNA is completely replicated- Check if there are no errors during Replication

Eukaryotic Chromosomes Organization

MITOSIS

•P- Prophase•M- Metaphase•A-Anaphase•T-Telophase

Interphase

chromosomes look like tangled rope, no individual chromosomes are visible – not condensed

Prophase

•chromosomes condense•we clearly see them•nuclear membrane breaks down•centrioles move to opposite ends of cell

MetaphaseThe two identical pairs of chromosomes (sister chromatids)are sticking together

Spindle fibers (come out of centrioles) – like ropes with hooks will attach tothe chromosomes and pullthem to the middle of the cell – the metaphase plate

Anaphase•The spindle fibers contract(similar to muscles)

•The glue holding the two identical chromosomes (sister chromatids) breaks down

•The two sister chromatids are pulled by the spindle fibers to opposite ends of the cell

Telophase

•Chromosomes have moved to opposite ends•Nuclear membrane re-forms around "new" chromosomes•Cell begins Cytokinesis

Cytokinesis

- The Cytoplasm and all itscontent divides in two

- Cell division is complete:2 new, daughter cells areformed

- The daughter cells haveidentical chromosomes/DNA

Cytokinesis

Animal Cells- microfilaments form a belt around the equator and contract to form two daughter cells.

Plant Cells- vesicles filled with cellulose fuse together at the equator to form a cell plate

Sexual Reproduction

Meiosis

Cell Division and ReproductionAsexual reproduction-the production of genetically identical offspring from a single parentTypes: Binary fission, budding, regeneration

Sexual reproduction-cells from two parents unite to form the first cell of a new organism

Budding Yeast

Fission Yeast

Meiosis

• The process by which germ cells (sex cells) such as egg and sperm are created

• Has 2 divisions: Meiotic Division I and II• Generates Genetic variety – the products of

meiosis – the daughter cells – are NOT genetically identical

• Sexual Reproduction is responsible for the large variety of life on Earth

DNA replication

DNA replication

2 X 2n = 4n

2n

2n

2 X 2n = 4n

Meiosis I• Similar to Mitosis• Phases are called Prophase I, Methapahse I,

Anaphase I, Telophase I• Major difference: Not only are sister

chromatids together (the 2 exact copies of 1 chromosome), but two related, homologous chromosomes ( 1 from dad and the 1 form Mom) pair and recombine (mix and match)

• They exchange genetic information – mix and match – daughter cells are NOT identical to mother cell

Crossing OverRecombination of DNA occurs during prophase I of meiosis I

Crossover, one piece of DNA from Mom's chromosome is replaced with another form Dad's

This increases the number of different gene combinations that occur in offspring

Meiosis II• Phases are called Prophase II, Metaphase II,

etc.• The two daughter cells form Meiosis I divide

again, but WITHOUT replicating their DNA• Reduces DNA content in half – 2n to n• The product of this division are 4 cells called

gametes which have ½ the DNA – they are not identical to the original cell

Why reduce DNA content in half?• Each gamete has now 1 of each chromosomes,

so when it fuses with a gamete of the opposites sex it will have 2 of each (1 from Dad, 1 from Mom) – n + n = 2n

• Answer: Fertilization• Everyone begins life as this single cell called a

zygote or fertilized egg.• The zygote or fertilized egg then grows by

mitosis, producing Billions and Billions of cells with 46 chromosomes each

Fertilization

• How many chromosomes does each cell in human body have? Answer: 46

• How many chromosomes are in the sex cells? Answer 23

• Why is this important in fertilization? Answer: When sperm and egg fuse they each have 23 chromosomes.

• 23 (mom) + 23(dad) = 46 chromosomes

Genetic Variation

• Why does everyone look different?– The answer is genetic variation: different

combinations of our parent’s DNA create different variations of people.

• Two main sources of genetic variation are recombination and mutations during sexual reproduction

Mutations

• We already know what mutations are - change in the sequence of nucleotides in the DNA, can be beneficial or harmful

• Only mutations in sex cells (sperm and egg) can be passed on to an offspring

• Helps to make different combinations of DNA and as a result different people

Recombination• Crossover of the two related chromosomes (1

Dad and 1 Mom) produces mix and match – new combinations in DNA sequence in a chromosome

• These chromosomes are separated randomly during meiosis II – different gametes (egg or sperm cells) will have different combinations of chromosomes: remember there are 23 chromosomes – how many different combinations?

Recombination continued

• 2 X 23 chromosomes – randomly separated + the mix and match during crossover = Millions of different combinations

• A couple can produce a variety of offspring (how different are you from your siblings?)

• This is why we say that sexual reproduction generates all the variety of life on Earth.