Modern Genetics

Chemical Basis For Genetics In the 1940’s and 1950’s experiments showed that genes are made up of the chemical compound DNA, or deoxyribonucleic acid. DNA is a large complex molecule found in the nucleus of the cell. DNA is responsible for passing genetic information from generation to generation. DNA also controls the manufacture of enzymes.  Enzymes control cellular activity.

Nucleotides The DNA molecule is made up of a long chain of repeating nucleotide units. Nucleotides contain nitrogenous bases, a sugar, and a phosphate group. The four different bases in DNA nucleotides are  Adenine  Guanine  Cytosine  Thymine

Nitrogenous Bases of DNA

DNA Structure The DNA double helix has two strands or sides. The strands are connected at the rungs. The sides of the ladder consist of alternating sugar and phosphate molecules. The rungs are pairs of nitrogenous bases.  The nitrogen bases are attached to each other by weak hydrogen bonds.

DNA Structure The four nitrogenous bases in DNA bond (join) together in a certain way known as base pairing. Adenine (A) and Thymine (T) bond together. Guanine (G) and Cytosine (C) bond together. No other combinations are possible.

DNA Replication During reproduction, DNA makes exact copies of itself (replicates). The process begins with an untwisting of the DNA helix. The two strands “unzip” leaving two strands of nucleotides.

DNA Replication Each strand is a pattern for the new nucleotide strand. Free nucleotides present in the cytoplasm pair with the free nitrogenous bases of both strands. Two identical DNA molecules that are identical to the original are produced.

DNA Replication

RNA - Ribonucleic Acid RNA also controls cellular activities. RNA contains the sugar “ribose.” RNA is single stranded. RNA also has 4 nitrogen bases:  Adenine  Guanine  Cytosine  Uracil instead of Thymine.

RNA - Ribonucleic Acid RNA has different functions than DNA:  Messenger RNA - mRNA – is a single, uncoiled strand that carries the genetic code information of DNA from the nucleus to the ribosomes in the cytoplasm.  Transfer RNA - tRNA – is a single folded strand that reads the message for protein formation carried by mRNA. tRNA then transfers amino acids to form proteins.  Ribosomal RNA - rRNA – is a form of RNA that makes up most of the ribosomes in a cell.

RNA Codon Set of 3 nucleotides

DNA Structure The structure of DNA was discovered in the 1950’s by James Watson and Francis Crick. According to Watson and Crick, DNA molecules are shaped like a twisted ladder. The twisted ladder structure is called a double helix.

Mutations A mutation is any change or mistake in the genes or chromosomes of an organism that can be inherited. These changes usually produce new characteristics. To be inherited, these changes must occur in the gamete or sex cell. As a result of fertilization, the changed gene or chromosome in the sex cell is passed on to the new organism. If they occur in other cells, they can be passed on only to other body cells.

Mutations Mutations occur as random chance events and may occur naturally within the cell or be caused by forces outside the cell called mutagenic agents.  X rays, ultraviolet light, radioactive substances, cosmic rays, and chemicals such as formaldehyde and benzene, are mutagenic agents.  Asbestos fibers and drugs, such as LSD, marijuana, cigarettes and alcohol, are also known to cause changes in genes and chromosomes. Unborn children can be injured when their pregnant mothers are exposed to mutagenic agents.

Human Genetic Diseases Most genetic diseases in humans are inherited as recessive genes and involve mistakes in enzyme formation. When there is a mistake in enzyme formation, the person’s metabolism is incorrect. The presence of many genetic disorders can be detected either before or after birth.  Screening (chemical analysis of body fluids)  karyotyping (detecting chromosome abnormalities by matching homologous chromosomes)  amniocentesis (removing a small amount of amniotic fluid from around the fetus) are tests used to detect (identify) genetic diseases

Genetic Engineering Today, scientist can deliberately remove genes from one organism and add them to the genetic material of another organism. Genetic Engineering will alter (change) the DNA of a cell. The changed DNA is called recombinant DNA. The cell that received the recombinant DNA receives new traits, such as, ability to prevent a certain disease.

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