Chapter 8

Cell Reproductin

Mitosis and Meiosis

What’s a Chromosome?

Parts of Chromosome

• A chromatid is a ½ of the chromosome or one arm.

• A centromere is the place where the two halves of the chromosome are held together.

• The centromere is at different locations depending on the chromosome type.

Types of Chromosomes

• Sex chromosomes determine the gender of the organism and also carry genes for other characteristics. They are the last or 23rd pair.

• Males are XY and females are XX.• Autosomes are body chromosomes not involved

with sex.• Autosomes carry the rest of the traits for an

organism.• Every organism that reproduces sexually carries

two of every chromosome. The two copies are called homologous chromosomes.

Types of Cells

• A diploid cell have both sets of homologous chromosomes.

• All normal cells except reproductive cells are diploid.

• A haploid cell have ½ the normal number of chromosomes.

• All sex cells such as egg and sperm are haploid because they need to join during fertilization to produce a new organism.

Cell Division

• Mitosis is when one cell divides into two diploid, identical daughter cells.

• Mitosis occurs to reproduce all types of cells except sex cells.

• Meiosis is when one cell will divide and create 4 haploid daughters cell that are different from each other.

• Meiosis occurs in sex cells only.

The Cell Cycle

• The cell cycle is a repeated set of events that make up the life of a cell.

• The time between cell divisions is called interphase.

• Interphase is the longest part of cell life.

• After interphase comes M phase and then cytokinesis.

Interphase

• The first stage of interphase is called G1 phase. Cells grow to mature size.

• The second stage is called S phase. The cell’s DNA is copied.

• The last stage is called G2 phase. The cell prepares to divide and grows a little more.

• Sometimes a cell will exit the cycle. This is called G0. Some cells like nerve cells stop dividing permanently once they reach maturity.

Mitosis

• Mitosis consists of 4 phases.

• Prophase, metaphase, anaphase and telophase/cytokinesis.

• Mitosis happens in diploid cells only.

• 2 identical daughter cells are formed.

• Cancer results when mitosis runs out of control.

Prophase• 1. DNA turns into

chromsomes.• 2. Centrosomes appear

next to the disappearing nucleus.

• 3. Each centrosome contains a pair of centrioles.

• 4. Spindle fibers appear that radiate from the centrosomes.

Metaphase

• 1. The spindle fibers attach to the centromere of each chromosome.

• 2. The chromosomes line up along the center of the cell.

Anaphase

• 1. The chromatids of each chromosome separate at the centromere.

• 2. The chromosomes move to opposite ends of the cell being pulled by the spindle fibers.

Telophase/Cytokinesis

• 1. The spindle fibers begin to disapper.

• 2. The chromosomes begin to coil again become invisible.

• 3. The nucleus reforms in each cell.

• 4. A cleavage furrow forms separating the two new cells.

• *A cell plate forms in plant cells.

Meiosis

• Only occurs in sex cells, sperm and egg.

• Creates 4 haploid cells that are different from each other.

• That is the reason why siblings don’t look exactly alike but may be similar.

• Meiosis is divided into Meiosis I and Meiosis II.

Important Events

• During Prophase I, crossing over occurs.

• Crossing over creates genetic recombinations that result in different cells.

• This assures that the genes from mom and dad are mixed.

• Homologous chromosomes cross over and form a tetrad when they do this.

Crossover

Reproduction• Asexual reproduction occurs with only 1 parent.• No gametes are produced in asexual

reproduction and the offspring is a genetic clone of the parent.

• Sexual reproduction uses meiosis to create gametes (sperm and egg).

• The offspring are genetically different from each other and the parent.

• Some organisms can reproduce both ways.• In hard times, some plants reproduce asexually

while in times of plenty, they can reproduce sexually. Some animals can also do this.