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 side-effects

– 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