Cell division
Mitosis and Meiosis
How cells divide

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

The Cell Cycle- the activities of a cell from one cell division to the next
- Has 4 phases:
G1
S
G2
M - Mitosis
Interphase

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

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

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

The two identical pairs of chromosomes (sister chromatids) are sticking together
Spindle fibers (come out of centrioles) – like ropes with hooks will attach to the chromosomes and pull them to the middle of the cell – the metaphase plate

• 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

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

Cytokinesis
- The Cytoplasm and all its content divides in two
- Cell division is complete:
2 new, daughter cells are formed
- The daughter cells have identical chromosomes/DNA

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 Reproduction
Asexual reproduction-the production of genetically identical offspring from a single parent
Types: Binary fission, budding, regeneration

Budding Yeast
Fission Yeast

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

• Pairs of chromosomes; one from male and one from female
– Diploid- (2n) a cell that contains both sets of chromosomes; somatic cells-body cells
– Haploid-(n) a cell that contains only one set of chromosomes; gamete-sex cells

Homologous chromosomes sister chromatids sister chromatids

• The process that separates homologous chromosomes forming a haploid gametes -sex cells; egg or sperm
• Occurs in ovaries or testes

Meiosis
• Has 2 stages: Meiosis 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
2 X 2n = 4n
2n
DNA replication
2 X 2n = 4n
2n

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 Over
Recombination 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 1:
Homologous chromosomes separate

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.

Comparison of Mitosis & Meiosis