Cell Reproduction

Chapter 8

Cell Reproduction

• As a cell prepares to divide, the DNA inside the nucleus becomes organized into chromosomes

• This is to ensure that both of the new cells get all of the genetic information from the original cell

Chromosome Structure

• Chromosomes are rod shaped structures made of DNA and proteins found in the nucleus of cells

Chromosome Structure

• Chromosomes consist of 2 identical halves called chromatids

– When a cell divides, each of the two new cells will receive one chromatid

• Two chromatids are attached at the centromere

Chromosome Structure

• Between cell divisions, DNA is not so tightly coiled

• This form is called chromatin

Chromosome Numbers

• Each species has a characteristic number of chromosomes in each cell

Chromosome Numbers

• Animal chromosomes are categorized as either sex chromosomes or autosomes

– Sex chromosomes determine the sex of an organism

• In humans, sex chromosomes are X or Y (females = XX, males = XY)

– All of the other chromosomes are autosomes

Chromosome Numbers

• Every cell of an organism produced by sexual reproduction has two copies of each autosome (one from each parent)

• The two copies are called homologous chromosomes

– Same size and shape and carry genes for the same traits

Chromosome Numbers

• Karyotype – picture of the chromosomes in a normal dividing cell found in a human

Diploid and Haploid Cells

• Diploid – cells that have 2 sets of chromosomes (46 total in humans)

• Haploid – cells that only have 1 set of chromosomes (23 total in humans)

• Only sperm and egg cells are haploid

CELL DIVISIONSection 2

Cell Division in Prokaryotes

• Binary fission is the division of a prokaryotic cell into two offspring cells

Cell Division in Eukaryotes

• Two types of cell division

– Mitosis results in new cells that are identical to the original cell

– Meiosis occurs during the formation of gametes(haploid reproductive cells)

The Cell Cycle

• A repeating set of events in the life of a cell

• Division is one phase of the cycle

• Time between cell divisions is called interphase

Interphase

1. G1 phase – offspring cells grow to mature size

2. S phase – the cell’s DNA is copied

3. G2 phase – cell prepares for cell division

• Cells can exit the cycle and enter into the G0

phase to stop dividing (ex: fully developed brain cells)

Stages of Mitosis

Prophase

• DNA condenses into chromosomes

• Nuclear membrane breaks down

• Centrosomes form and microtubules grow from them (called the mitotic spindle)

Metaphase

• Chromosomes line up in the middle of the cell

• Spindle fibers attach to the centromere of each chromosome

Anaphase

• Chromatids are pulled apart and move to opposite ends of the cell

Telophase

• Chromosomes reach opposite ends of the cell

• Spindle fibers disassemble

• Chromosomes unravel

• Nuclear membranes reform

Cytokinesis

• Animal cells –cytoplasm divides in two

• Plant cells – cell plate forms between new cells

Control of Cell Division

• A cell spends most of its time in interphase

• What tells the cells to exit interphase and begin dividing?

• There are three main checkpoints that act as “traffic signals” for the cell to divide or not to divide

Control of Cell Division

1. Cell growth (G1) checkpoint – controls whether the cell will divide

2. DNA synthesis (G2) checkpoint – make sure DNA was copied properly

3. Mitosis checkpoint –signals tell the cell to exit mitosis

When Control Is Lost: Cancer

• If a mutation occurs in one of the genes that regulates the cell cycle, cell growth and division could be disrupted

• This disruption could lead to cancer – the uncontrolled growth of cells

MEIOSISSection 3

Meiosis

• A process of nuclear division that reduces the number of chromosomes in new cells to half the number in the original cell

Formation of Haploid Cells

• Meiosis produces gametes, which are haploid reproductive cells

• Cells preparing to divide by meiosis undergo the G1, S, and G2 phases of interphase

– Cells begin meiosis with a duplicate set of chromosomes, just like mitosis

• Cells undergoing meiosis divide twice, resulting in 4 haploid (1n) cells

Two Stages of Meiosis

• First cell division = Meiosis I

– Prophase I, Metaphase I, Anaphase I, Telophase I and Cytokinesis I

• Second cell division = Meiosis II

– Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis II

Meiosis I

Meiosis II

Prophase I

• DNA coils tightly into chromosomes

• Spindle fibers appear

• Nuclear membrane disassembles

• Synapsis occurs - homologous chromosomes line up next to each other

– Each pair is called a tetrad

Prophase I

• Crossing-over occurs – portions of chromatids may break off and attach to adjacent chromatids

• Genetic recombination results – genetic material between maternal and paternal chromosomes is exchanged

Prophase I

Metaphase I

• Tetrads line up along the middle of the cell

• Spindle fibers attach to the centromere of each homologous chromosome

Anaphase I

• Each homologous chromosome moves to an opposite end of the cell

• Random separation of homologous chromosomes is called independent assortment and results in genetic variation

Telophase I and Cytokinesis I

• Chromosomes reach opposite ends of the cell and cytokinesis begins

• Two new cells are produced, each containing one chromosome from each homologous pair (haploid – 1n)

Prophase II

• Spindle fibers form and begin to move the chromosomes toward the midline of the dividing cell

Metaphase II

• Chromosomes move to the midline of the dividing cell

Anaphase II

• Chromatids separate and move toward opposite ends of the cell

Telophase II and Cytokinesis II

• Telophase II – nuclear membrane forms around the chromosomes in each of the four new cells

• Cytokinesis II – 4 new cells are formed, each with half of the original cell’s number of chromosomes

Development of Gametes

• In animals, the only cells that divide by meiosis are those that produce gametes within the reproductive organs

– In humans – testes (males) and ovaries (females)

Development of Gametes

SPERMATOGENESIS

• In the testes – male gametes known as sperm cells or spermatozoa are produced

• One diploid cell divides meiotically to form four haploid cells called spermatids – each develops into a mature sperm cells

Development of Gametes

OOGENESIS

• Production of mature egg cells, or ova

• A diploid reproductive cell divides meiotically to produce one mature egg cell (ovum) and three polar bodies which will degenerate – cytoplasm is not evenly distributed in cytokinesis

Sexual Reproduction

• Production of offspring through meiosis and the union of a sperm and an egg

• Offspring are genetically different because genes are combined in new ways

• Advantage: enables species to adapt rapidly to new conditions