Lecture 1 PDF

“the key to every biological problem must finally be sought in the cell; for every living organism is, or at some time has been, a cell.” E. B. Wilson - Pioneering Cell Biologist Cell Biology is the study of the structure, function, and behavior of cells Molecular biology is the field of science c...

“the key to every biological problem must finally be sought in the cell; for every living organism is, or at some time has been, a cell.” E. B. Wilson - Pioneering Cell Biologist Cell Biology is the study of the structure, function, and behavior of cells Molecular biology is the field of science concerned with studying the chemical structures and processes of biological phenomena that involve the basic units of life, of molecules. - This field is focused especially on nucleic acids (e.g., DNA and RNA) and proteins—macromolecules that are essential to life processes— and how these molecules interact and behave within cells. - Molecular biology emerged in the 1930s, having developed out of the related fields of biochemistry, genetics, and biophysics. - today it still remains closely associated with those fields. THE UNIVERSAL FEATURE S OF CELLS ON EARTH Cells are the fundamental units of life. - small, membrane-enclosed units filled with a concentrated aqueous solution of chemicals and endowed with the extraordinary ability to create copies of themselves by growing and then dividing in two. - This cell includes the machinery to gather raw materials from the environment and to construct from them a new cell in its own image, complete with a new copy of its hereditary information. Living things are curious, intricately organized chemical factories that take in matter from their surroundings and use these raw materials to generate copies of themselves. - These living organisms appear extraordinarily diverse but are fundamentally similar inside. a. All cells Store their Hereditary Information in the same Linear chemical code: DNA - All living cells on Earth store their hereditary information in the form of double-stranded molecules of DNA—long, unbranched, paired polymer chains, formed always of the same four types of monomers. - These monomers, chemical compounds known as nucleotides, have nicknames drawn from a four-letter alphabet—A, T, C, G—and they are strung together in a long linear sequence that encodes the genetic Information. b. All cells replicate their hereditary information by templated Polymerization - The mechanisms that make life possible depend on the structure of the double stranded DNA molecule. - Each monomer in a single DNA strand—that is, each nucleotide— consists of two parts: a sugar (deoxyribose) with a phosphate group attached to it, and a base, which may be either adenine (A), guanine (G), cytosine (C), or thymine (T) - Each sugar is linked to the next via the phosphate group, creating a polymer chain composed of a repetitive sugar-phosphate backbone with a series of bases protruding from it. - The DNA polymer is extended by adding monomers at one end. - For a single isolated strand, these monomers can, in principle, be added in any order, because each one links to the next in the same way, through the part of the molecule that is the same for all of them. - In the living cell, however, DNA is not synthesized as a free strand in isolation, but on a template formed by a preexisting DNA strand. c. All Cells Transcribe Portions of Their Hereditary Information into the same Intermediary Form: RNA - To carry out its information-bearing function, DNA must do more than copy itself. - It must also express its information, by letting the information guide the synthesis of other molecules in the cell - This expression occurs by a mechanism that is the same in all living organisms, leading first and foremost to the production of two other key classes of polymers: RNAs and proteins. - The process begins with a templated polymerization called transcription, in which segments of the DNA sequence are used as templates for the synthesis of shorter molecules of the closely related polymer ribonucleic acid, or RNA. - In RNA, the backbone is formed of a slightly different sugar from that of DNA—ribose instead of deoxyribose—and one of the four bases is slightly different—uracil (U) in place of thymine (T). - But the other three bases—A, C, and G—are the same, and all four bases pair with their complementary counterparts in DNA—the A, U, C, and G of RNA with the T, A, G, and C of DNA. - During transcription, the RNA monomers are lined up and selected for polymerization on a template strand of DNA, just as DNA monomers are selected during replication. - The outcome is a polymer molecule whose sequence of nucleotides faithfully represents a portion of the cell’s genetic information, even though it is written in a slightly different alphabet—consisting of RNA monomers instead of DNA monomers. - The same segment of DNA (template) can be used repeatedly to guide the synthesis of many identical RNA molecules. - Thus, whereas the cell’s archive of genetic information in the form of DNA is fixed and sacrosanct, these RNA transcripts are mass- produced and disposable. - These transcripts function as intermediates in the transfer of genetic information. - Most notably, they serve as messenger RNA (mRNA) molecules that guide the synthesis of proteins according to the genetic instructions stored in the DNA. d. All Cells Use Proteins as Catalysts - Protein molecules, like DNA and RNA molecules, are long unbranched polymer chains, formed by stringing together monomeric building blocks drawn from a standard repertoire that is the same for all living cells. - Like DNA and RNA, proteins carry information in the form of a linear sequence of symbols, in the same way as a human message written in an alphabetic script. - There are many different protein molecules in each cell, and—leaving out the water—they form most of the cell’s mass. a. Amino Acid Sequence of b. Amino Acid Sequence of Insulin Hemoglobin  Nucleotide pairing between different regions of the same RNA polymer chain causes the molecule to adopt a distinctive shape. The conformation of an RNA molecule. e. All Cells Translate RNA into Protein in the Same Way - How the information in DNA specifies the production of proteins was a complete mystery in the 1950s when the double-stranded structure of DNA was first revealed as the basis of heredity.  Applying Chargaff’s Rule of Base Pairing, and using the X-ray diffraction image of DNA (using X-ray Crystallography produced by Rosalind Franklin, Maurice Wilkins and Raymond Gosling) handed to them by Maurice Wilkins (without Franklin’s Knowledge and was Franklin’ colleague at King’s College), James Watson and Francis Crick worked on the double-helix structure of DNA.  X-ray photographs of the A (“crystalline”) and the B (“wet”) form of DNA (Whereas the A form contains sharp spots, the B form has a cross of smudges) produced by Rosalind Franklin and Raymond Gosling using X-ray Crystallography. - The translation of genetic information from the 4-letter alphabet of polynucleotides into the 20-letter alphabet of proteins is a complex process. - The information in the sequence of a messenger RNA molecule is read out in groups of three nucleotides at a time: each triplet of nucleotides, or codon, specifies (codes for) a single amino acid in a corresponding protein. - The number of distinct triplets that can be formed from four nucleotides is 43,Therefore, there are 64 possible codons, all of which occur in nature. - However, there are only 20 naturally occurring amino acids. - That means there are necessarily many cases in which several codons correspond to the same amino acid. - This genetic code is read out by a special class of small RNA molecules, the transfer RNAs (tRNAs). - Each type of tRNA becomes attached at one end to a specific amino acid, and displays at its other end a specific sequence of three nucleotides—an anticodon— that enables it to recognize, through base-pairing, a particular codon or subset of codons in mRNA.