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Mitosis and Cell Division
Mitosis• Cells divide to make more cells. -this allows organisms to grow,
develop, maintain and repair themselves
• Mitosis is Asexual (1 parent)
• While all of the organelles can be randomly separated into the daughter cells, the chromosomes must be precisely divided so that each daughter cell gets exactly the same DNA.
The Challenge of MitosisNormal human cells have 23 pairs of chromosomes
- each pair consists of one copyof the chromosome from each parent – 46 total
- after replication, the total number of chromosomes = 92
Each cell produced by cell divisionneeds exactly the correct setof 46 chromosomes
Still More Chromosomes
In mitosis, the two chromatids of each chromosome separate, with each chromatid going into a daughter cell.
• Each of the chromatids is a copy of the original chromosome
Machinery of Mitosis
• Cells build a structure called the spindle, which is made of microtubules to facilitate the movement of chromosomes during mitosis
• The spindle fibers are attached to each chromosome at structures called the kinetochore found at the centromere
Machinery of Mitosis: The Spindle
In animal cells, the spindle fibers are anchored at either end to a centrosome
In all cells, the opposite ends of the cell are referred to as the poles
- each centrosome is composed of a pair of centrioles
Plants lack centrosomes and their spindle formation is poorly understood
Mitosis Occurs in Phases• mitosis occurs in a logical
sequence of steps
- the occurrences in one phase prepare the cell for the next stage in the process
-The phases of mitosis:
Prophase
Metaphase
Anaphase
Telophase
Interphase
• Preparatory phase• Chromosomes
replicate here• Where cell spends
most of its time
• G1-cell growth to mature size
• S-cell DNA copied (synthesized)
• G2- preparation for cell division
Prophase – the first step in cell division
• 1. DNA Condenses. -Chromatin coils into
chromosomes • 2.The nucleus & nucleolus
disappear
• 3. Centrioles migrate to poles
• 4. The spindle starts to form, chromosomes attach to spindle fibers
• 5. Longest Phase
Metaphase
• The chromosomes line up along the middle of the cell – known as the equator
• Everything is now aligned for the rest of the division process to occur. Shortest Phase.
Anaphase• the chromosomes separate at
the centromere. - the sister chromatids move
toward opposite poles
• Then the spindle fibers shorten, and the chromosomes are pulled to opposite poles, towards the centrosomes.
Telophase• The chromosomes uncoil
back to chromatin
• The spindle disintegrates
• The nuclear envelope re-forms around the two sets of chromosomes, forming nucleus and nucleolus
• The cytoplasm is divided into 2 separate cells, the process of cytokinesis begins.
Cytokinesis• The organelles get divided up into the 2 daughter cells
passively: they go with whichever cell they find themselves in.
• Plant and animal cells divide the cytoplasm in different ways.
Plant Cell Cytokinesis
• No pinching because of cell wall.
• Cell plate forms from the middle to split the cytoplasm.
• Cell plate is formed by the Golgi Bodies.
Animal Cell Cytokinesis
• In animal cells, a ring of actin fibers (microfilaments are composed of actin) forms around the cell equator and contacts, pinching the cell in half.
• Cleavage furrow-cell membrane pinches inward.
Summary of Mitosis• Prophase:
• Chromosomes condense• Nuclear envelope disappears• Centrioles move to opposite sides of the cell• Spindle forms and attaches to centromeres on
the chromosomes• Metaphase
• Chromosomes lined up on equator of spindle• centrosomes at opposite ends of cell
• Anaphase• Centromeres divide: each 2-chromatid
chromosome becomes two 1-chromatid chromosomes
• Chromosomes pulled to opposite poles by the spindle
• Telophase• Chromosomes uncoil• Spindle disappears• Nuclear envelope reappears• Cytokinesis: the cytoplasm is divided into 2
cells
Vocabulary
• Histones• nucleosomes• chromosomes• chromatin• centromere• kinetochore• spindle• centrosomes• poles
• Prophase• metaphase• anaphase• telophase