mitosis & meiosis

Mitosis & Meiosis

General Biology Mr. Cobb

Cell Theory According to the cell theory, all

cells come from pre-existing cells. But why do cells reproduce?

Cell Reproduction Cells reproduce to replace lost or

damaged cells with new healthy cells.

New cells gradually replace dead cells.

Cell reproduction is responsible for your growth.

Reproduction Asexual reproduction occurs when

one organism makes an exact duplicate of itself.

Sexual reproduction occurs when the genetic information from two organisms combine to make a new, genetically different, organism.

Genetic Material Genetic material (DNA or RNA)

can exist in your cells in three forms: Chromatin Chromosomes Chromatids

Chromatin Genetic material

is called chromatin when it is in a mass of very long fibers.

Chromosomes As the cell prepares to divide, the

genetic material condenses and winds around itself, forming chromosomes.

Chromosomes are visible under powerful microscopes

Chromosomes Chromosomes are pictured in a

karyotype. Humans have 23 pairs of

chromosomes. These pairs are called homologous

pairs of chromosomes.

Diploid Cells that have homologous pairs

of chromosomes are called diploid. (2n)

Most organisms are diploid.

Chromosomes

Chromatids Before the cell divides, the genetic

material is copied. The two identical pieces of DNA join together to form “sister chromatids.”

These chromatids are joined at the centromere.

Chromosomes & Chromatids

Genetic Information

Cell Cycle In order for cells to reproduce,

they must first go through the cell cycle, which lasts about 10-20 hours.

During the cell cycle, the DNA and organelles are copied, the cell increases in size, and then divides.

Cell Cycle The cell cycle is composed of

several steps: Interphase Mitosis

Prophase Metaphase Anaphase Telophase

Cytokinesis

Interphase Interphase is made up of 3 phases: G1 – growth and function. During S-phase, the cell’s DNA is

synthesized (copied). This phase lasts for 3 – 6 hours.

During G2 phase the cell prepares to divide (organelles and cytoplasm increase)

Cell Division When the cell is actually dividing,

it is called mitosis, followed by cytokinesis.

Mitosis During mitosis,

the nucleus and the duplicated DNA divide and are moved into two daughter cells.

Cytokinesis During cytokinesis, the cytoplasm

of the cell divides into two parts. At the end of cytokinesis, the two

daughter cells completely separate, forming two identical cells.

FLIP BOOK – Label flaps like this CELL CYCLE (title page) MITOSIS – Prophase Metaphase Anaphase Telophase/Cytokinesis Interphase

Mitosis

Cell Cycle

Cell Cycle The cell cycle is divided into three

parts: Interphase Mitosis Cytokinesis

Interphase During

interphase, the cell is very active.

Organelles and DNA are duplicated

Interphase Individual

chromosomes are not visible because they are loosely packed chromatin

ChromatIN is found only during INterphase

Mitosis Mitosis is divided into 4 phases:

Prophase Metaphase Anaphase Telophase

Prophase During prophase,

the chromatin fibers condense and become chromosomes

Sister chromatids join at the centromere.

Sister chromatIDs are IDentical

Prophase The nucleolus

disappears. The nuclear

membrane disappears

Spindle fibers form from spindle

Metaphase All the

chromosomes line up in the middle of the cell (called the “metaphase plate”).

Metaphase Spindle fibers

attach to the chromosomes at the centromere.

Anaphase Spindle fibers

condense, causing the sister chromatids to separate.

Each chromatid is now a “daughter chromosome.”

Telophase Telophase

begins when the chromosomes reach the poles of the spindle.

Processes that occurred during prophase are reversed.

Telophase The nuclear

envelope and nucleolus reappear.

The spindle disappears

Chromosomes uncoil

Cytokinesis During

cytokinesis, the cytoplasm separates into the two daughter cells.

The two daughter cells separate from each other.

Cytokinesis In animal cells a

“cleavage furrow” forms when the two cells begin to separate.

In plants, a cell plate forms and separates the two cells.

Cell division The result of the cell cycle is one

cell becoming two identical cells.

Meiosis

Mitosis Mitosis occurs in “somatic” cells. Somatic cells are body cells (ex:

muscle cells, brain cells, liver cells, skin cells, etc.)

But what about sex cells?

Meiosis Sex cells divide by a different

process. This process, called Meiosis,

allows 4 unidentical cells to be formed from one cell.

Meiosis In females, meiosis occurs in egg

cells before birth, and again during puberty.

In males, meiosis occurs in sperm cells throughout their lives.

Definitions Somatic Cells - Body cells Sex cells - Sperm & Egg Homologous chromosomes -

chromosomes that are similar in size, shape, and genetic content. They are NOT identical, just similar.

Homologous Chromosomes Homologous

chromosomes are similar, but not identical.

Definitions Diploid - Have both pairs of

homologous chromosomes (2n). Humans have 46 chromosomes

Haploid - have one pair of homologous chromosomes. Ex: Human sex cells have 23 chromosomes

Diploid Diploid

organisms have pairs of chromosomes.

Human somatic cells are diploid.

Haploid Haploid cells

have single chromosomes.

Meiosis Meiosis is divided into two parts:

Meiosis I Meiosis II

Both Meiosis I and II go through the steps of Mitosis: Prophase, Metaphase, Anaphase,

Telophase

Meiosis I During Meiosis I, DNA is copied

and CROSSING OVER occurs. Crossing over allows for genetic

variability. Meiosis I includes: Prophase I,

Metaphase I, Anaphase I, Telophase I

Prophase I After the

chromosomes condense, the homologous chromosomes come together in pairs (the pairing up of chromosomes is called synapsis)

Prophase IThese pairs of chromosomes are known as Sister Chromatids, they are alike but not identical.

Prophase I The four chromatids come together form a

tetrad. The chromosomes exchange portions of

their DNA. This exchange of DNA is called crossing over.

Metaphase I The centromeres of the

chromosomes line up at the equator of the cell.

Each tetrad (group of chromosomes) is attached to a spindle fiber.

Anaphase I During anaphase I, the

homologous chromosomes separate, with one going to each pole of the cell.

The sister chromatids do not separate.

Anaphase I The homologous

chromosomes separate, NOT the sister chromatids.

Telophase I During telophase I, the cytoplasm

divides, forming two daughter cells.

Each daughter cell has half the number of chromosomes as the parent, but each chromosome is double stranded.

Meiosis I Overview

Meiosis II After telophase I has ended,

Meiosis II begins. NO G1, S, or G2 occurs!

The end product of meiosis II is four haploid daughter cells that are NOT genetically identical.

Meiosis II Meiosis II includes: prophase II,

metaphase II, anaphase II, and telophase II.

The process of meiosis II is very similar to mitosis.

Prophase II Each of the

daughter cells forms a spindle and the double stranded chromosomes move toward the middle of the spindle.

Metaphase II The chromosomes

line up at the equator of the cell.

Each chromosome still consists of two chromatids

Anaphase II The centromeres

divide and the two chromatids separate.

The single stranded chromosomes move toward the opposite ends of the cell.

Telophase II Both daughter

cells divide, forming four haploid cells.

Meiosis II At the end of meiosis II, the

haploid cells return to their interphase state– the DNA uncoils and the nuclear membrane reappears.

Oogenesis Oogenesis is the formation of an egg

cell through meiosis. During oogenesis, the cytoplasm

divides unevenly. One cell gets the majority of the cytoplasm, the other three cells are much smaller.

The larger cell becomes the egg, while the others are called “polar bodies” and are disintegrated by the body.

Oogenesis

Spermatogenesis Spermatogenesis is the formation

of sperm cells through meiosis. The cytoplasm divides evenly and

four haploid sperm cells are generated from one diploid cell.

Spermatogenesis

Spermatogenesis