The Nucleus The largest most prominent organelle found

The Nucleus The largest most prominent organelle found only in eukaryotic cells. It is surrounded by a membrane called the nuclear envelope (or nuclear membrane) and contains the genetic information necessary for control of cell structure and function.

Structurally, the nucleus is composed of three main parts: � Nuclear Envelope: �Double membrane structure �Contains many holes called, pores, that allow RNA and other chemicals to pass in an out of the nucleus but not DNA. � Nucleolus: �Where ribosomes are constructed �Contains RNA, DNA and proteins �Has some of the ribosomes that synthesis proteins (others are in the ER) � Chromatin: �Contains DNA and proteins formed into packets of code called chromosomes. �When the cell divides, the chromosomes fold up on themselves, getting wider.

Understanding the cell cycle �Without cell division, living organisms cannot grow and develop. �The process that ensures each new cell has a nucleus with a complete set of instructions (DNA) is called mitosis. �When the cell divides, the nuclear material splits in half so that each daughter cell gets genetics that is the same as that of the parent cell.

�The terms mitosis and cell division are often referred to as describing the same process. �HOWEVER!! �Mitosis refers to the dividing of nuclear material �Cell Division is the COMPLETE process of copying and dividing the WHOLE CELL!

Getting ready for mitosis 1. The nucleus must make a copy of its chromatin. All of our body cells (somatic cells) contain 46 chromosomes we call them diploid. After cell division the body cells must still have 46 chromosomes. This is why it is important for the nucleus to make a copy of its genetic information.

2. Replicated chromatin coils up- Once the replicated chromatin coils up it forms double stranded chromosomes which are joined by a centromere Tightly coiled chromatin centromere

Phase 1 - Prophase �Mitosis begins. �During prophase, the nucleus prepares for cell division. The genetic material shortens and thickens. The chromosome copies are held together at their centers (centromere), so they form a sort of ‘X’. �The centrioles (or poles) appear and begin to move to the opposite ends of the cell �The nuclear membrane and nucleolus are no longer visible.

Phases of Mitosis • Densely packed coiled chromosome pairs become visible with a light microscope. • The nucleolus and the nuclear membrane disappear. • “Spindle fibers” begin to form from centrioles at either end of the cell and stretch out to connect to the centromeres on each chromosome pair.

Phase 2 - Metaphase �During metaphase, The spindle fibres (protein structures) attach themselves to the centromeres of the chromosomes and align the chromosomes at the equatorial plate (center of the cell).

Phases of Mitosis • Spindle fibers pull on the centromeres. • Chromosome pairs line up across the center of the cell.

Phase 3 - Anaphase �During anaphase, the copies separate. One complete set of chromosomes is pulled to one side of the cell. The other complete set is pulled to the other side of the cell.

Phases of Mitosis • Spindle fibers contract and shorten • Centromeres are pulled apart • One of each replicated strand move to opposite ends of the cell

Phase 4 - Telophase �In this stage, the chromosomes reach the poles of their respective spindles. Nuclear membrane forms before the chromosomes uncoil. The spindle fibres disintegrate. Mitosis Ends!

Phases of Mitosis • One complete set of chromosomes is at each end of the cell • Spindle fibers begin to disappear • Nuclear membrane forms around each set of chromosomes • A nucleolus appears within each new nucleus. • Single stranded chromosomes start to uncoil into thin strands of chromatin. • Two nuclei in one cell and the cell is ready to divide.

Cytokinesis �Cytokinesis is the final stage of cell division. During this stage, the cytoplasm pinches in at the center of the cell, dividing the original cell in half. When cell division is complete, two new daughter cells are formed. The daughter cells are identical to the parents cell.

Cell Cycle The cell cycle can be broken down into three parts. • mitosis- copying the nucleus • cell division- separating the cytoplasm • interphase Interphase - The cell is in this stage of the cell cycle for the longest time. This is where the cell grows and performs its functions. This stage gets the cell ready for cell division.

Interphase �Interphase is the stage before cell division starts. As a cell prepares to divide, DNA has replicated, but has not formed the condensed structure of chromosome. They remain as loosely coiled chromatin.

A typical cell cycle se op ha pr sis o t i M (2 h) se ha Growth and preparation for division (4 h) erp t In Rapid growth (4 h) ase h p telo ase ph se a an ha p a et m Growth and DNA replication (1 oh)

�Work on questions 1, 2, 3, 5 on page 23. If not completed in class, they are for homework. �Complete worksheets- to be handed out next day.