Molecular Genetics Chromosomes found in the nucleus carry the hereditary material -- DNA DNA (deoxyribonucleic acid) DNA controls cellular activity by influencing the production of enzymes. Structure of DNA Molecules: DNA is a very long chain polymer made up of thousands of repeating units called nucleotides. Nucleotide Unit is composed of a phosphate group, a pentose sugar, and a nitrogenous base. The Nitrogenous Bases are; adenine (A) thymine (T) guanine (G) cytosine (C) Watson and Crick (early 1950's) -- determined the structure of the DNA molecule ** Consists of two chains of nucleotide units in a twisted ladder-like structure. (resembles a spiral staircase) This spiral staircase is called an alpha helix. (37 degree turn) -- The sides of the ladder are made up of alternating deoxyribose sugar -phosphate group units. -- The rungs of the ladder are made of 2 nitrogenous bases per rung linked together by a weak hydrogen bond. -- Only 2 combinations of base pairs can form the rungs of the DNA molecule. Adenine - Thymine (A-T) Guanine - Cytosine (C-G) ** This specific matching up of the nitrogenous bases is called complementary base pairing. ** DNA is able to replicate itself. [How does DNA form duplicates of itself during mitosis and meiosis?] 1. The double stranded DNA molecule unwinds and unzips between the weak hydrogen bonds between the nitrogenous base pairs. 2. Free nucleotides present in the nucleus attach themselves by forming new hydrogen bonds with the exposed bases in the single chain. The only base which can reattach is the same type of base that was originally joined to it. A view of DNA replication (Usefulness of the Watson-Crick Model) 1. Explained how mitosis produces exact DNA copies for each daughter cell -- thus the genetic information passes on unchanged. 2. Explained how DNA acts as a code directing the making of enzymes and other proteins by a cell, thus directing cellular activities. Directs the synthesis of RNA which makes proteins. RNA (ribonucleic acid) ** RNA is also a polymer formed by a sequence of nucleotides. How does RNA differ in structure from DNA? 1. The RNA molecule is a SINGLE nucleotide strand, not a double strand as in DNA. 2. The sugar molecule in RNA is RIBOSE -not deoxyribose as in DNA. 3. The base URACIL (U) takes the place of thymine. (T) Synthesis of RNA -- The various types of RNA are made from the template (pattern) of the DNA molecule. Types of RNA 1. mRNA (messenger RNA) made in nucleus 2. tRNA (transfer RNA) on the cytoplasm 3. rRNA (ribosomal RNA) in the ribosomes How are proteins synthesized from a DNA template? (model) 1. DNA serves as a template for the synthesis of mRNA from RNA nucleotides in the nucleus. transcription: transfer of the genetic message from DNA to mRNA 2. mRNA molecules carrying a specific code determined by the base sequence of the DNA template move from the nucleus to the cytoplasm. 3. Strands of mRNA carrying codons transcribed from DNA, move to the ribosomes in the cytoplasm. codon: a triplet in a DNA molecule triplet: grouping of three nitrogenous bases in DNA or RNA molecules (each codon will code for a specific amino acid) Translation: communication between mRNA, rRNA and tRNA to create a protein from mRNA sequence. (steps 4-8) 4. mRNA strands become associated with rRNA on the ribosomes. (rRNA -- arranges date between mRNA and tRNA) 5. Different triplets of nitrogenous bases in tRNA molecules pick up specific amino acids in the cytoplasm and carry them to mRNA at the ribosomes. (tRNA -- I want aa.) 6. Amino acids are put into position on the ribosome with instructions from the triplet codes of tRNA and mRNA. 7. With the aid of enzymes and ATP (energy), the amino acids are bonded to form a polypeptide chain (protein) on the ribosome. An overview of the entire protein synthesis (transcription and translation) process. 8. This protein formation is what directs metabolic activity in any cell. ** One gene codes for one polypeptide chain. gene = the sequence of nucleotides in a DNA molecule necessary to synthesize a polypeptide ** Since the sequence of nucleotides in DNA determines the sequence of nucleotides in messenger RNA, DNA ultimately determines the sequence of amino acids in specific proteins. The specificity of enzymes is dependent on their protein makeup, and, since the individuality of a cell is largely a function of the enzymes it possesses, it is evident that DNA determines the individuality and function of an organism. The work of a cell is carried out by the many different kinds of molecules it assembles, mostly proteins. Proteins are long, folded molecules made up of up to 20 different kinds of amino acids which interact to produce specific protein shapes. The specific shape of the protein (exs. enzymes and hormones) determines the specific function of that protein. Offspring resemble their parents because they inherit similar genes that code for the production of proteins that form similar structures and perform similar functions. How are cell functions regulated? 1.) Gene regulation allows only the selective expression of certain individual genes. 2.) The regulation of certain genes controls the activity and production of certain proteins. ** All this gene regulation allows cells and organisms to respond to their environment and control their growth and division. Body cells of an individual can be very different from each other, even though they have descended from a single cell (zygote) and have essentially the same genetic instructions. This is because different parts of these instructions are used in different types of cells, influenced by the cells environment and developmental history. Genetic Research 1. Cloning: producing a group of genetically identical offspring from the cells of an organism ** This technique shows great promise in agriculture. Plants with desirable qualities can be rapidly produced from the cells of a single plant. 2. Genetic engineering: (recombinant DNA) -- transfer of genetic information from one organism to another -- includes the transfer of entire genes and gene splicing ** A cell can synthesize a new chemical coded for by its new gene(s) -- examples include interferon, insulin, and growth hormone. ** Genetic engineering can correct genetic defects & produce agriculturally more efficient plants and animals. restriction enzymes -- used to cut segments of DNA in one organism so they can be transferred into another organism Characteristics produced by the segments of DNA may be expressed when these segments are inserted into new organisms such as bacteria. Inserting, deleting, or altering DNA segments can alter genes. An altered gene may be passed on to every cell that develops from it. human genome project --- has allowed humans to know the basic framework of their genetic code Knowledge of genetics is making possible new fields of health care. Genetic mapping is making it possible to detect and possibly correct, defective genes that may lead to poor health. A down side to this is that health insurance agencies and other organizations may use this genetic information against individuals. Substances from genetically engineered organisms may reduce the cost and side effects of replacing body chemicals. Human insulin produced in bacteria is already an example of this.