Nucleus

The nucleus is the largest organelle and usually the only one visible with a light microscope. They are the control centre of the cell and act as a genetic library. Some cells have more than 1, such as skeletal muscle. The nuclei are usually shaped like the cell.

Nuclei have three (3) recognisable regions or structures: the nuclear envelope (membrane), nucleoli, and chromatin.

The nuclear envelope is a double membrane with the 2 layers meeting at certain places to form pores, which are like channels, that allow substances in and out. The nuclear envelope holds nucleoplasm (acts like cytosol) inside the nucleus.

 A nucleolus ("little nucleus") is also contained in the nucleus. They are 1 or more dense bodies within the nucleus. They are the sites of ribosome assembly. The ribosomes move out of the nucleus through the nuclear pores to the cytoplasm where they aid in protein synthesis. Chromatin is composed of DNA, histone proteins and RNA chains. The histone proteins package the very long DNA molecules in a compact, orderly way, but they also play an important role in gene regulation (this is a complex process and you will not need to learn this in this subject).

__**Protein Synthesis**__ In short protein synthesis can be described as "DNA makes RNA makes Protein" this is known as the cental dogma of molecular biology.

This process is a little more complicated than that statement and the actual process can be divided into two parts:

1. **Transcription:** Before the synthesis of a protein begins, the corresponding RNA (Ribonucleic acid) molecule is produced by RNA transcription. One strand of the DNA double helix is used as a template by the RNA polymerase to synthesize a messenger RNA (mRNA). This mRNA migrates from the nucleus, through the nuclear pores, to the cytoplasm. During this step, mRNA goes through different types of maturation including one called [|splicing] where the non-coding sequences are eliminated. The coding mRNA sequence can be described as a unit of three nucleotides called a codon.

2. **Translation:** In the cytoplasm the ribosome binds to the mRNA at the start codon (AUG) that is recognised only by the initiator tRNA (transfer RNA). The ribosome proceeds to the elongation phase of protein synthesis. During this stage, complexes, composed of an amino acid linked to tRNA, sequentially bind to the appropriate codon in mRNA by forming complementary base pairs with the tRNA anticodon. The ribosome moves from codon to codon along the mRNA. Amino acids are added one by one, translated into polypeptidic sequences dictated by DNA and represented by mRNA. At the end, a release factor binds to the stop codon, terminating translation and releasing the complete polypeptide from the ribosome.



[|Protein synthesis animation]

__**Replication**__

Before a dividing cell enters mitosis, it undergoes a period of growth called interphase. Some 90 percent of a cell's time in the normal cellular cycle may be spent in interphase. During interphase DNA makes DNA (it replicates)


 * Stages of Interphase**
 * G1 phase: The period prior to the synthesis of DNA. In this phase, the cell increases in mass in preparation for cell division. Note that the G in G1 represents gap and the 1 represents first, so the G1 phase is the first gap phase.


 * S phase: The period during which DNA is synthesised. Note that the S represents synthesis.([|Replication animation])


 * G2 phase: The period after DNA synthesis has occurred but prior to the start of prophase. The cell synthesises proteins and continues to increase in size. Note that the G in G2 represents gap and the 2 represents second, so the G2 phase is the second gap phase.


 * In the latter part of interphase, the cell still has nucleoli present.


 * The nucleus is bounded by a nuclear envelope and the cell's chromosomes have duplicated but are in the form of chromatin

Once the genetic material has replicated the cell must undergo a process of splitting the genetic material to make two identical daughter cells. Mitosis consists of 4 phases: prophase, metaphase, anaphase and telophase.


 * Prophase:**
 * Chromatin fibres become coiled into chromosomes with each chromosome having two chromatids joined at a centromere.


 * The mitotic spindle, composed of microtubules and proteins, forms in the cytoplasm.


 * The two pair of centrioles move away from one another toward opposite ends of the cell due to the lengthening of the microtubules that form between them.


 * Late prophase:**
 * The nuclear envelope breaks up.


 * Polar fibers, which are microtubules that make up the spindle fibers, reach from each cell pole to the cell's equator.


 * Kinetochores, which are specialised regions in the centromeres of chromosomes, attach to a type of microtubule called kinetochore fibers.


 * The kinetochore fibers "interact" with the spindle polar fibers connecting the kinetochores to the polar fibers.


 * The chromosomes begin to migrate toward the cell center.


 * Metaphase:**
 * The nuclear membrane disappears completely.


 * The two pair of centrioles align at opposite poles of the cell.


 * Polar fibers (microtubules that make up the spindle fibers) continue to extend from the poles to the center of the cell.


 * Chromosomes move randomly until they attach (at their kinetochores) to polar fibers from both sides of their centromeres.


 * Chromosomes align at the metaphase plate at right angles to the spindle poles. (they line up across the equator)


 * Chromosomes are held at the metaphase plate by the equal forces of the polar fibers pushing on the centromeres of the chromosomes.


 * Anaphase:**
 * The paired centromeres in each distinct chromosome begin to move apart.


 * Once the paired sister chromatids separate from one another, each is considered a "full" chromosome. They are referred to as daughter chromosomes.


 * Through the spindle apparatus, the daughter chromosomes move to the poles at opposite ends of the cell.


 * The daughter chromosomes migrate centromere first and the kinetochore fibers become shorter as the chromosomes near a pole.


 * In preparation for telophase, the two cell poles also move further apart during the course of anaphase. At the end of anaphase, each pole contains a complete compilation of chromosomes.


 * Telophase:**
 * The polar fibers continue to lengthen.


 * Nuclei (plural form of nucleus) begin to form at opposite poles.


 * The nuclear envelopes of these nuclei are formed from remnant pieces of the parent cell's nuclear envelope and from pieces of the endomembrane system.


 * Nucleoli (plural form of nucleolus) also reappear.


 * Chromatin fibers of chromosomes uncoil.


 * After these changes, telophase/mitosis is largely complete and the genetic "contents" of one cell have been divided equally into two.


 * Cytokinesis**, from the greek //cyto-// (cell) and //kinesis// (motion, movement), is the process in which the cytoplasm of a single cell is divided to form two daughter cells. It usually initiates during the late stages of mitosis, splitting a mitotic cell in two, to ensure that chromosome number is maintained from one generation to the next.

[|Mitosis animation]

The Cell: Contents