Chromatin, Chromosomes

Chromatin is an uncoiled mass of DNA and associated proteins called histones.

When cell division begins, DNA coils around the proteins forming visible structures called chromosomes.

Below: Human chromosomes (female)

Haploid, Diploid

Diploid cells (2N) have two complete sets of chromosomes.   The body cells of animals are diploid.

Haploid cells have one complete set of chromosomes.  In animals, gametes (sperm and eggs) are haploid.

Homologous Chromosomes

Homologous chromosomes are two chromosomes that are the same. This happens because humans and other diploid organisms have two of each chromosome. Each of the pairs is a homologous pair. One of the homologous chromosomes was inherited from the individual's mother and the other one was inherited from the individual's father. For example, the two chromosomes #1 are homologous. However, a chromosome #1 and a chromosome #2 are not homologous because they are different chromosomes.

Genes

A small segment of DNA that contains the information necessary to construct a protein or part of a protein (polypeptide) is called a gene.  Genes are the unit of inheritance.

A cell divides by pinching into two. Each of two daughter cells produced contains genetic material inherited from the original (parent) cell.

Single-celled organisms divide to reproduce.

Cell division in multicellular organisms enables the organism to grow larger while the cells remain small.  A large surface:volume ratio is due to small cell size.

Organisms with many cells can have cells which are specialized for different functions and tasks.  For example, red blood cells are specialized for carrying oxygen but neurons (nervous tissue) are specialized for conducting signals from one cell to another.

Some cells of multicellular organisms must divide to produce sex cells (gametes).

Mitosis

Mitosis produces two daughter cells that are identical to the parent cell.  If the parent cell is haploid (N), then the daughter cells will be haploid.   If the parent cell is diploid, the daughter cells will also be diploid. 

N --> N

2N --> 2N

This type of cell division allows multicellular organisms to grow and repair damaged tissue.

Meiosis

Meiosis produces daughter cells that have one half the number of chromosomes as the parent cell. 

2N --> N

Meiosis enables organisms to reproduce sexually. Gametes (sperm and eggs) are haploid.

Meiosis involves two divisions producing a total of four daughter cells.

Click here to go to the chapter on meiosis.

A chromatid is a single DNA molecule.

Double-stranded chromosomes have two chromatids; normally, each one is identical to the other. The point where the two chromatids are attached is called the centromere.

Chromosome Doubling vs DNA Synthesis

Splitting chromosomes into two will double their number because each chromatid is identical. 

DNA replication occurs when a single-stranded chromosome produces a second chromatid.

Click here to review DNA synthesis (replication).

Overview of the Cell Cycle

	Interphase (G1 and G2) Chromosomes are not easily visible because they are uncoiled
	Prophase The chromosomes coil. The nuclear membrane disintegrates. The spindle apparatus forms.
	Metaphase The chromosomes become aligned.
	Anaphase The chromatids separate (The number of chromosomes doubles).
	Telophase The nuclear membrane reappears. The chromosomes uncoil. The spindle apparatus breaks down. The cell divides into two.
	G1 Interphase The chromosomes have one chromatid.
	G2 Interphase The chromosomes have two chromatids.

Mitosis Animation

The link below is an animation that shows chromosome movement during mitosis in a hypothetical species with 2N = 4.

Click here to begin the animation. After the screen opens, press Ctrl-F to view the animation in full screen mode.

The cell cycle alternates between interphase and mitosis as diagrammed below.

 

Mitosis has these four phases: prophase, metaphase, anaphase, and telophase.

Prophase

During prophase, chromosomes begin condensing (forming) as DNA becomes coiled. The genes cannot function (produce mRNA and therefore protein) when the DNA is coiled. Coiling facilitates movement.

The nucleolus disappears.

The nuclear membrane becomes fragmented and disappears by the end of prophase.

A system of microtubules needed to move the chromosomes begins to form during prophase. The microtubules, also called spindle fibers, form from an area of the cell called the centrosome. During interphase, the cell has one centrosome but just before prophase, the centrosome duplicates, producing a second centrosome.  During prophase, microtubules radiate from each centrosome. Some of the microtubules extend from one centrosome toward the other.

The entire complex of centrosomes and spindle fibers is called the spindle apparatus.

Each centrosome  of an animal cell contains two centrioles. Plant cells do not have centrioles but they do form spindle fibers. 

Click on the image above to enlarge it.

During metaphase, the chromosomes have moved to the center of the cell (diagram below, photograph above). This line of chromosomes is referred to as the metaphase plate.

The structures in the diagram below are referred to as the spindle apparatus. Kinetochore microtubules are attached to the chromosomes. Polar microtubules are not attached to chromosomes but overlap each other. Asters are short microtubules that radiate from the centrosomes. The spindle apparatus can be seen on the drawing of a cell in metaphase below.

Metaphase ends when chromosomes split, thus doubling the number of chromosomes.

When the chromosomes split at the end of metaphase, the chromosome number is doubled. For example, the number of chromosomes and chromatids during each phase in a human cell is:

Phase	# Chromosomes	# Chromatids
Prophase	46	92
Metaphase	46	92
Anaphase	92	92
Telophase	92	92

Chromosome movement

Microtubules lengthen and shorten by the addition or removal of tubulin dimers. Click here for details in the chapter on cells.

During anaphase, the chromosomes move toward poles of cell.

Cytokinesis (division of the cytoplasm) begins in anaphase. A cleavage furrow forms as actin filaments underneath the plasma membrane constrict in a band called the contractile ring. Two cells will be produced as this process continues.

Telophase begins when chromosomes reach the poles of the daughter cells.

Many of the events in telophase are the reverse of prophase.  The chromosomes uncoil, the nuclear membranes around daughter nuclei appear, the spindle apparatus breaks down, and the nucleolus reappears.

Cytokinesis is completed as telophase ends.

This is the non-dividing phase.

During interphase, the nucleus is visible and the chromosomes are uncoiled and invisible.

Interphase includes G₁, S and G₂.

G₁

Each chromosome has one chromatid.

The cell grows in size.

Synthesis of organelles occurs.

S

This is when DNA synthesis occurs.

G₂

Each chromosome has two chromatids.

The synthesis of enzymes and other proteins in preparation for mitosis occurs during this period.

 

Cells that permanently leave the cycle

Some cells remain permanently in G₁. Examples: skeletal muscle, nerve cells

Some cells remain permanently in G₂. Example: cardiac muscle

Below: Whitefish blastula X 400

Practice

How many chromosomes are there in each of the three diagrams below? How many chromatids?

If a parent cell had 6 chromosomes, how many during each phase listed below?

Phase	# Chromosomes	# Chromatids
prophase
metaphase
anaphase
telophase
G1 interphase
G2 interphase

If a cell had 4 chromosomes that were single-stranded, how many chromosomes and chromatids during each phase listed below?

Phase	# Chromosomes	# Chromatids
prophase
metaphase
anaphase
telophase
G1 interphase
G2 interphase

Draw each phase of mitosis (prophase, metaphase, anaphase, telophase) in a cell that has 2N = 4 chromosomes. Show the chromosomes, microtubules, and nucleus where appropriate.