Cell Reproduction

Chapter 8.2

Part 2

The discovery of chromosomes

Chromosomes carry the genetic material that is copied and passed down through generations.

They appear in the nucleus just before cell division, and disappear soon after.

They look like long stringy structures, and they become dark when stained.

The structure of eukaryotic chromosomes Chromatin are long

strands of DNA wrapped around proteins called histones.

Chromosomes exist in the form of chromatin for most of the cell’s lifetime.

Chromatin looks like beads on a string.

Before cell division can occur, chromatin must coil up and become tightly packed (see figure 8.10 pg.205)

Chromosome Structure

The Cell Cycle

The Cell Cycle is the sequence of growth and division of a cell.

Divided into two periods;1. Period of growth

2. Period of division

Cell Cycle - Interphase Interphase is a cell growth

phase where the cell increases in size, carries on metabolism, and duplicates chromosomes prior to division.

Interphase is divided into 3 parts:G1 – Cell grows & protein production is high. S – DNA Synthesis – the cell copies it’s chromosomes during this phase.G2 –A second, shorter growth period – mitochondria and other organelles are manufactured.

“Sister Chromatids”

Each duplicated chromosome is made up of two halves called Sister Chromatids.

They are exact copies of each other formed when DNA is copied during interphase.

Sister chromatids are held together by a centromere.

Mitosis begins with Prophase Prophase – the first

and longest phase of mitosis.

Chromatin coils up into visible chromosomes.

The nucleus begins to disappear – nuclear envelope and nucleolus dissolve.

Prophase cont.

Centrioles, small dark cylindrical structures made of microtubules, begin to move to opposite ends of the cell (in Animal Cells only!)

Spindle, a football-shaped, cage like structure consisting of thin fibers made of microtubules, begins to form.

Plant cells do not have centrioles, only spindles!

Stage 2: Metaphase

Chromosomes become attached to the spindle fibers by the centromeres.

Chromosomes are pulled into a line across the equator of the cell by the spindle fibers.

Each sister chromatid is attached to a single spindle fiber, and the fibers extend to opposite poles of the cell.

This is to ensure that each new cell gets an identical and complete set of genetic information!

Stage 3: Anaphase Anaphase begins with the

separation of sister chromatids.

The centromeres split apart, and the sister chromatids separate from each other.

Microtubules in the spindle fibers shorten and pull the chromatids towards opposite ends of the cell.

Final Phase: Telophase Begins when chromatids

reach opposite poles of the cell.

Like Prophase in reverse - Chromosomes unwind. Spindle breaks down. Nucleolus reappears. New nuclear membrane

forms around each group of chromosomes.

A new double membrane begins to form between the two new nuclei.

Cytokinesis After telophase, the cell’s cytoplasm divides in a process

called cytokinesis. Animals – plasma membrane pinches along the equator

until the cell is separated into two new cells. (fig 8.15) Plants – a cell plate forms across the equator, and new

cell membranes and cell walls form on either side of the plate until two new cells are created.

Results of Mitosis Guarantees genetic continuity – two new

cells (Daughter Cells) with identical chromosome sets as the parent cell.

Daughter cells will also divide to create four more identical copies, etc…

In multicellular organisms, this process results in growth and reproduction of cells which work together as tissues, which make up organs, which are a part of organ systems, and so on…