1. Chromosomes Formation of New Cells by Cell Division Cell Division, also called cell reproduction, occurs in humans and other organisms at different times in their life. Bacterial cells undergoing reproduction divide by one type of cell division. Eukaryotic organisms undergoing growth, development, repair, or asexual reproduction, divide by a different type of cell division. And, the formation of gametes (an organism’s reproductive cells, such as sperm and eggs) involves a third type of cell division.

2. Prokaryotes reproduce by a type of cell division called binary fission (a form of asexual reproduction that produces identical offspring). Eukaryotic Cell Reproduction The vast majority of information encoded in DNA is organized into units called genes. A gene is a segment of DNA that codes for a protein or RNA molecule. Genes play an important role in determining how a person’s body develops and functions.

3. As a eukaryotic cell prepare to divide, the DNA and the proteins associated with the DNA coil into a structure called chromosomes. Before the DNA coils up; however, the DNA is copied. The two exact copies of DNA that make up each chromosome are called chromatids. The two chromatids of a chromosome are attached at a point called the centromere.

4. Each human somatic cell (any cell other than a sperm or egg cell) normally has two copies of 23 different chromosomes, for a total of 46 chromosomes. The 23 chromosomes differ in size, shape, and set of genes. Each of the 23 pairs of chromosomes consists of two homologous chromosomes (similar in size, shape, and genetic content). When a cell, such as somatic cell, contain 2 sets of chromosomes, it is said to be diploid. When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid. A haploid number is a human gamete can be written as n = 23. The diploid number in a somatic cell can be written as n = 46. A zygote is a fertilized egg cell, the first of a new individual.

5. Of the 23 pairs of chromosomes in human somatic cells, 22 pairs are called autosomes. Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual. The sex chromosomes, one of the 23 pairs of chromosomes in humans, contain genes that will determine the sex of the individual. In humans and many other organisms, the two sex chromosomes are referred to as X and Y chromosomes. Any individual with a Y chromosome is male, and an individual without a Y chromosome is female. In human males, the sex chromosomes are made up of one X and one Y chromosome (XY). In human females consist of two X chromosomes (XX).

6. The presence of all 46 chromosomes is essential for normal development and function. Humans who are missing even one of the 46 chromosomes do not survive. Humans with more than two copies of a chromosome, a condition called trisomy will occur. Below are two pictures of an infant with trisomy 13. Abnormalities in the chromosome number can be detected by analyzing a Karyotype, a photo of the chromosomes in a dividing cell that shows the chromosome arrangement by size.

7. A portion of a karyotype from an individiual with an extra copy of chromosome 21 is called Down Syndrome, or trisomy 21. When a sperm and egg cell form, each chromosome and its homologue separate in an event called disjunction. If one or more chromosomes fail to separate properly, an event called nondisjunction – one new gamete ends up receiving both chromosomes and the other gamete receives none.

8. The cell cycle is a repeating sequence of cellular growth and division during the life of an organism. A cell spends 90% of its time in the first 3 phases of the cycle, which are collectively called interphase. 1. First Growth (G 1) phase. A cell grows rapidly and carries out its routine functions. Cell’s that are not dividing remain in the G1 phase. 2. Synthesis (S) phase. A cell’s DNA is copied during this phase. At the end of this phase, each chromosome consists of two chromatids attached at the centromere.

9. 3. Second growth (G2) phase. Hollow protein fibers called microtubules are assembled. The microtubules are used to move the chromosomes during mitosis. 4. Mitosis. The process during cell division in which the nucleus of a cell is divided into two nuclei. Each nucleus ends up with same number and kinds of chromosomes as the original (cloning). 5. Cytokinesis. The process during cell division in which the cytoplasm divides.

10. Step 1: Prophase. Chromosomes coil up and become visible during prophase. The nuclear envelope dissolves and a spindle forms. Step 2: Metaphase. During metaphase, the chromosomes move to the center of the cell and line up along the equator. Spindle fibers link the chromatids to opposite poles.

11. Step 3: Anaphase. Centromeres divide during anaphase. The two chromatids (now called chromosomes) move toward opposite poles as the spindle fibers attached to them shorten. Step 4: Telophase. A nuclear envelope forms around the chromosomes at each pole. Chromosome, now at opposite poles, uncoil and the spindle dissolves. The spindle fibers break down and disappear.