I
NTRODUCTION
Genetics is the scientific study of inherited variation. Human genetics is the scientific study of inherited
human variation. This field has been energized in recent years by the Human Genome Project.
Scientists expect that the project will lead to the development of new drugs targeted to specific genetic
disorders. Increasingly, modern genetics involves genetic engineering; a technique used to manipulate
genes and has produced many advances in medicine.
in medicine.
PRINCIPLES OF GENETICS
The site where genes work is the cell.
Each cell’s function within an organism.
In eukaryotes (organisms whose cells contain a nucleus), DNA resides within membrane-
bound structures in the cell (nucleus, mitochondria, and chloroplasts in plants).
In prokaryotes (one-celled organisms that lack internal membranebound structures), DNA
floats freely within the cell body.
DNA is packaged into structures called chromosomes within a cell.
Every chromosome in a cell contains many genes, and each gene is located at a particular
site, or locus, on the chromosome.
Chromosomes usually occur in matched pairs called homologues.
The number of homologous chromosomes in the human body contain 23 pairs of
chromosomes.
Cell
•An animal cell typically contains several types of membranebound organs, or organelles.
•The nucleus directs activities of the cell and carries genetic information from generation to generation.
•The mitochondria generate energy for the cell.
•Proteins are manufactured by ribosomes, which are bound to the rough endoplasmic reticulum or float free
in the cytoplasm.
•The Golgi apparatus modifies, packages, and distributes proteins.
•Lysosomes store enzymes for digesting food.
The entire cell is wrapped in a lipid membrane that selectively permits materials to pass in and out of the
cytoplasm.
Cell Division and Reproduction
•Organisms use two types of cell division to ensure that DNA is passed down from cell to cell during
reproduction.
•Simple one-celled organisms reproduce by a process called mitosis. During mitosis a cell doubles its DNA
before dividing into two cells and distributing the DNA evenly to each resulting cell.
•Organisms that reproduce sexually produce special cells called gametes, or egg and sperm.
•During sexual reproduction, an egg and sperm unite to form a zygote, in which the full number of
chromosomes is restored.
Mitosis occurs in five stages:
1.Interphase: the start of mitosis, the DNA of each chromosome replicates. Each chromosome then reorganizes into
paired structures called sister chromatids, with each member of the pair containing a full copy of the DNA
sequence.
2.Prophase: the sister chromatids condense, thickening until they appear joined at a single site, known as the
centromere.
3.Metaphase: the sister chromatids line up in the middle of the cell.
4.Anaphase: the chromatid pairs split apart at the centromere, and each half of the pair then moves toward opposite
poles of the cells.
5.Telophase: the final stage of mitosis, a nuclear membrane forms around the chromosomes at each pole of the cell.
•Mitosis ends with the formation of two new cells, each with a matching full set of chromosomes.
The cytoplasm divides; the cell membrane pinches inward ultimately producing two daughter cells (Cytokinesis).
•Meiosis comprises two successive nuclear divisions.
•First division of meiosis:
1.Prophase I: Each chromosome duplicates and remains closely associated. These are called sister
chromatids.
2.Metaphase I: Homologous chromosomes align at the equatorial plate.
3.Anaphase I: Homologous pairs separate with sister chromatids remaining together.
4.Telophase I: Two daughter cells are formed with each daughter containing only one chromosome of the
homologous pair.
•Second division of meiosis:
1.Prophase II: DNA does not replicate.
2.Metaphase II: Chromosomes align at the equatorial plate.
3.Anaphase II: Centromeres divide and sister chromatids migrate separately to each pole.
4.Telophase II: Cell division is complete. Four haploid (n) daughter cells are obtained.
DNA Structure
•The structure of DNA encodes all the information every cell needs.
•DNA molecules form chains of building blocks called nucleotides.
•Each nucleotide consists of a sugar molecule called deoxyribose that bonds to a phosphate molecule and to
a base.
•DNA uses four bases in its structure: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).
•The pairing of bases in the DNA double helix is highly specific (A always joins with T, and G always links
to C). These base combinations, known as complementary base pairing.
•Genes line up in a row along the length of a DNA molecule.
•In humans a single gene can vary in length from 100 to over 1,000,000 bases.
•Genes make up less than 2% of the length of a DNA molecule. The rest of the DNA molecule is made up of
long, highly repetitive nucleotide sequences "junk” DNA".
• The process begins when the two strands of a DNA molecule separate, a task directed by the enzyme
RNA polymerase.
• After the double helix splits apart, one of the strands serves as a template, for the formation of a
complementary mRNA molecule.
• Free-floating individual bases within the cell bind to the bases on the DNA template to form a strand
of mRNA.
• In eukaryotes, the mRNA strand consists of coding regions called exons (link together to form an
mRNA strand) separated by regions