CELL GROWTH AND DIVISION CELL DIVISION

Every cell must copy its genetic information before cell division takes place. Each daughter cell then gets a complete copy of that information. In prokaryotes, the process after that is to separate the contents of the cell into two parts. In eukaryotes, the process is more complex and occurs in two main stages.

CELL DIVISION The first stage is the division of the nucleus called mitosis. The second stage is the division of the cytoplasm and is called cytokinesis. Many organisms, especially unicellular ones, reproduce just by mitosis and cytokinesis. Reproduction by mitosis is classified as asexual, since the cells produced by mitosis are genetically identical to the parent cell.

CELL DIVISION Mitosis is also the source of new cells in multicellular organisms. In humans, mitosis begins just after the egg is fertilized, producing the vast numbers of cells needed for the embryo to take form. Chromosomes – In eukaryotes, genetic information is passed from one generation to the next on chromosomes.

CELL DIVISION – DNA makes up chromosomes. – DNA carries the genetic information. – Each organism has its own specific number of chromosomes. Fruit flies have eight Humans have 46 Carrots have 18 – Chromosomes are not visible except during division. The DNA and protein molecules are spread throughout the nucleus. They appear at the beginning of cell division and condense into compact structures that can be seen with a light microscope.

CELL DIVISION – Before division each chromosome is replicated or copied. There are two identical chromatids. When division occurs one chromotid goes to each of the two new cells. Each pair of chromotids are attached at an area called the centromere. The centromeres are located near the middle of the chromotids. Humans have 46 chromosomes, each with two chromotids.

CHROMOSOMES

MITOSIS

Biologists divide the events of mitosis into four phases: prophase, metaphase, anaphase, and telophase. Prophase – This is the first and longest phase and can take % of the total time required to complete mitosis. – Chromosomes become visible

MITOSIS

CENTRIOLES

MITOSIS – The centrioles, two tiny structures located near the nuclear envelope in the cytoplasm, separate and take up positions on opposite sides of the nucleus. – The centrioles are in a region called the centrosome that helps to organize the spindle. Helps to separate the chromosomes. – The chromososmes become attached to fibers in the spindle near the centromere of each chromatid. Plant cells don’t have centrioles but organize the spindles in similar regions.

MITOSIS Near the end of prophase the nucleolus disappears, the nuclear envelope breaks down, and the chromosome coil. Metaphase – This is the second phase, lasting only a few minutes. – The chromosomes line up across the center of the cell. – Microtubules connect the centromere of each chromosome to the two poles of the spindle.

MITOSIS

METAPHASE

MITOSIS

Anaphase – This is the third phase of mitosis. – The centromeres that join the sister chromatids split, allowing them to separate and become individual chromosomes. – The chromosomes continue splitting and move toward the poles of the spindle. – This phase ends when the chromosomes stop moving.

ANAPHASE

MITOSIS

Telophase – This is the fourth and final phase. – The chromosome become less distinct and disperse into dense material. – The nuclear envelope reforms around each cluster of chromosomes. – The spindle breaks apart. – The nucleolus becomes visible. – Mitosis is complete but cell division is not.

TELOPHASE

MITOSIS Cytokinesis – This is the division of the cytoplasm itself. – This usually occurs at the same time as telophase. – The process in animals is: Cell membrane is drawn inward and pinched in nearly two equal parts. Each part then contains its own nucleus and cytoplasmic organelles. – The process in plants is: A cell plate forms midway between the divided nuclei. This gradually forms a membrane and cell wall.

ANIMAL CYTOKINESIS

PLANT CYTOKINESIS

PLANT AND ANIMAL CYTOKINESIS

ANIMAL CYTOKINESIS

PLANT CYTOKINESIS