Chromosomes, Mitosis &
Meiosis
Chapter 9
Chromosomes
• Located in the nucleus of eukaryotic cells
• Visible only when the cell is dividing
• Made of chromatin – DNA + proteins
– Chromatin is just the chromosomes in an extended,
partially unraveled state
• Chromosomes contain genes
– Humans have less than 30,000
– Genes provide information on cellular function
• Chromosomes package a lot of information into a
small space
Packaging of Chromosomes
• Histones – help to coil the DNA molecule
– Histones have a slight + charge
– DNA has a slight – charge
– This attraction forms a clump of 146 base pairs of
DNA wrapped around 8 histones – a nucleosome
– Nucleosomes help prevent tangling of the DNA
• Scaffolding proteins – nonhistone proteins that
bundle the nucleosomes and help maintain the
chromosome structure
Nucleosomes
During cell division
• Chromosomes consist of identical halves called
sister chromatids
• Sister chromatids are held together at an area
called the centromere
• The centromere has a protein disk called a
kinetochore
The Cell Cycle – the life cycle of a cell
• Interphase
–
–
–
–
Between one cell division and the next
The majority of a cell’s life
When the cell is doing its work
Consists of three phases:
• G1 phase  S phase  G2 phase
• M phase
– Division of the cell
– Consists of two phases:
• Mitosis (prophase, metaphase, anaphase,
telophase)
• Cytokinesis
The Cell Cycle
Interphase – G1 Phase
• G = gap (no DNA synthesis)
• Growth and normal metabolism occurs
• Cells that are no longer actively dividing (nerve,
skeletal muscle, red blood cells) are arrested in
this stage
– These cells are said to be in the G0 phase
• At the end of this phase necessary enzymes are
synthesized
Interphase – S phase
• Synthesis
• DNA replicates and histone is synthesized
• This is when the sister chromatids are produced
Interphase – G2 phase
• Cell is entering final preparations for cell division
M phase – Mitosis: Prophase
• Nuclear envelope breaks down
• Nucleolus disappears
• Chromosomes (made of two identical sister
chromatids) appear as chromatin condenses
• Spindle fibers form between centrioles which are
now at the poles of the cell
Prophase
whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited
M phase – Mitosis: Metaphase
• Spindle fibers attach to the kinetochores of the
chromosomes (located at the centromere of the
two sister chromatids)
• Chromosomes line up along the equatorial plane
of the cell, midway between the two poles
Metaphase
whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited
M phase – Mitosis: Anaphase
• Sister chromatids separate at the centromeres
• Each chromosome is pulled toward the opposite
poles
Anaphase
whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited
M phase – Mitosis: Telophase
• Chromosomes arrive at the poles (now each is a
single sister chromatid)
• Nuclear envelope begins to re-form around each
set of chromosomes
• Cytokinesis begins
Telophase
whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited
M phase – Cytokinesis
• The division of the cytoplasm
• Two new cells are called daughter cells
• In animal cells:
– A cleavage furrow pinches in around the midline of
the cell and divides the two cells
• In plant cells:
– A cell plate forms in the midline and grows across to
divide the two cells
Daughter Cells
whitefish cell mitosis. LM X360 Credit: © Carolina Biological/Visuals Unlimited
Regulation of the cell cycle
• Different types of cells have different time frames
for the cell cycle
• Cell cycle checkpoints help regulate the cell cycle
• Some chemicals can affect the cell cycle:
– Drugs that stop the cell cycle are used in cancer
treatment
• These can cause nausea and hair loss sideeffects
– Hormones may stimulate mitosis
• Cytokinins in plants
• Certain steroids in animals
Name that stage…
Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited
Name that stage…
Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited
Name that stage…
Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited
Name that stage…
Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited
Name that stage…
Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited
Name that stage…
Onion root tip mitosis Credit: © Carolina Biological/Visuals Unlimited
Confocal image of dividing cell. The markers are mitotic spindle
(red), chromosomes (blue) , and peroxisomes (green).
Credit: © Carolina Biological/Visuals Unlimited