Modern Molecular Genetics

By the early 1920’s, scientists knew that chromosomes were made up of two substances, DNA and protein.

1. In recent years, biochemists have found that the DNA of chromosomes is the genetic material that is passed form generation to generation. (It is known as the molecule of life. 2. To demonstrate that DNA was the substance that determined which traits were inherited, many experiments (including the British researcher Frederick Griffth) were performed

Frederick Griffith In 1928, Griffith found that a substance from dead pneumonia bacteria was transformed into pneumonia causing ones. He called the substance a transforming factor. It was later proven that DNA was the transforming factor. The transmission of genetic material from the pneumonia-causing bacteria into the harmless pneumonia bacteria changed it into pneumonia- causing bacteria.

(I) DNA Structure A very large molecule consisting of thousands of smaller, repeating units known as nucleotides. DNA is found within the nucleus of the cell.

(A) DNA Nucleotide A DNA nucleotide is composed of three parts: 1. A phosphate group 2. A deoxyribose (5-carbon sugar) molecule 3. A nitrogenous base of either adenine, thymine, guanine, or cytosine

(B) Watson-Crick Model In 1935 James Watson and Francis Crick developed a model of the DNA molecule. In this model, the DNA molecule consists of two complimentary chains of nucleotides in a “ladder” type organization. The four nitrogenous bases of the DNA molecule bond together in only one way: adenine (A) with thymine (T) cytosine (C) with guanine (G)

James Watson (L) and Francis Crick (R), and the model they built of the structure of DNA

Double-helix Structure of DNA Each “step” of the ladder consists of nitrogenous bases bonded together by weak hydrogen bonds. The two chains of the DNA molecule are twisted to form a spiral, or double-helix.

(II) DNA Replication 1. DNA, unlike any other chemical compound, can make exact copies of itself by a process known as replication. 2. In replication, the double- stranded DNA helix unwinds; the two strands then separate, or unzip, by the breaking of the hydrogen bonds between pairs of bases. 3. Free nucleotides in the nucleus then bond to the complimentary bases of the DNA strands.

Replication produces two identical DNA molecules that are exact copies of the original molecule. DNA Replication Animation DNA Replication Animation

Genes and Proteins Every cell can be thought as a chemical factory. Genes, which instruct cells to make enzymes, are therefore really packages of information that tell a cell how to make proteins (long chain of amino acids). Genes are specific sections of DNA molecules that are made up of linear sequences of nucleotides.

(III) RNA (Ribonucleic acid) RNA is a nucleic acid, like DNA, composed of nucleotide building blocks. There are three major differences between the structure of DNA and RNA: 1. In RNA, ribose is substituted for deoxyribose. 2. uracil (U) is substituted for thymine (T) 3. RNA consists of only a single strand of nucleotides.

Genetic Code A genetic code contains the information for the sequence of amino acids in a particular protein. This code is present in mRNA molecules and is three bases long. This is known as a codon. Ex: UAG - is a codon

Genetic Codes

DNA Sequencing

From DNA to RNA DNA is copied into RNA by a process called transcription. Transcription is similar to DNA replication: 1. The DNA double-helix opens up. 2. Special enzymes begin to match up RNA nucleotides with the correct nucleotides in DNA. 3. A messenger RNA or mRNA molecule is built.

Messenger RNA (mRNA) 1. When portions of DNA molecules unwind and separate, RNA nucleotides pair with complimentary bases on the DNA strand. This forms a mRNA that is complimentary to the DNA strand. 2. The sequence of nucleotides in the mRNA contain the genetic code. 3. The genetic code for each amino acid is a sequence of three nucleotides forming a codon.

mRNA

tRNA Known as transfer RNA Contains a triplet of nucleotides called the anticodon. At the other end of the molecule, the amino acid is attached.

The anticodon of tRNA matches the codon of the mRNA.

(IV) Translation 1. Also referred to as Protein Synthesis. 2. In the cytoplasm, the mRNA becomes associated with a ribosome. 3. Amino acids in the cytoplasm are “picked-up” by molecules of transfer RNA (tRNA). 4. Each codon on the mRNA bonds with a corresponding anticodon on a tRNA, which carries a specific amino acid. 5. These amino acids are joined together by peptide bonds. 6. The resulting chain of amino acids is a polypeptide.

Protein Synthesis Animation

V. Gene Expression and Cell Differentiation The human body is made up of many different types of cells. All of these cells have the same DNA in them, so why are they so different from each other? The answer is that only certain genes are used in certain cells. The use of the information from a gene is called gene expression (which genes are turned on). Creating the special types of cells through controlled gene expression is called cell differentiation.

Without cell differentiation, our bodies would be made up of only one type of cell.

VI Genetic Engineering Genetic Engineering- is a new technology that humans use to alter the genetic instructions in organisms. a) Biotechnology- The application of technology to biological science. ex: removal of dinosaur DNA from a mosquito’s last meal. b) Selective Breeding- A process that produces domestic animals and new varieties of plants with traits that are particularly desirable.

An Example of Selective Breeding Brahman cattle: Good resistance to heat but poor beef. English shorthorn cattle: Good beef but poor heat resistance. Santa Gertrudis cattle: Formed by crossing Brahman and English shorthorns; has good heat resistance and beef.

DNA Technology Makes it possible to put “new” genes into organisms. 1. Human genes can be inserted into bacteria. 2. These altered bacteria become factories that produce human protein. ex: Gene Splicing Recombinant DNA

Plasmids Are small DNA fragments, are known from almost all bacterial cells. Plasmids carry between 2 and 30 genes. Some seem to have the ability to move in and out of the bacterial chromosome

Gene Splicing Allows a scientist to make cuts of DNA from 2 complimentary different organisms, perhaps a frog cell and a bacterium. Pieces of DNA from one organism can now be glued, or spliced, into the DNA of another organism.

Recombinant DNA Allows scientists to insert the insulin gene into bacterial plasmids. The bacteria that contain this gene produce insulin, which is used by people with diabetes.

Cloning Is a technique that accomplishes the same end result as asexual reproduction. It is a way of making identical genetic copies. Cloning is done by inserting a nucleus from a “parent” organism’s cell (one that has a complete set of genetic information from that individual) into an egg cell from which the nucleus has been removed. The result is an egg that now contains not 50%, but 100% of the genetic information from a single parent. If this new egg cell with all of its genes can be made to develop normally, the resulting offspring is a clone of the individual that donated the original cell (In mammals, the egg would be implanted and develop inside the body of the female).

In Vitro Fertilization IVF (illustrated in the diagram at right) is often used when a woman's fallopian tubes are blocked. First, medication is given to stimulate the ovaries to produce multiple eggs. Once mature, the eggs are suctioned from the ovaries (1) and placed in a laboratory culture dish with the man's sperm for fertilization (2). The dish is then placed in an incubator (3). About two days later, three to five embryos are transferred to the woman's uterus (4). If the woman does not become pregnant, she may try again in the next cycle.