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Chapter 1

Chapter 1: Cells and genomes

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University of Toronto St. George
John Coleman

Chapter 1: Cells and Genomes The universal features of cells on earth - Heredity is central to definition of life; it distinguishes life from other processes o Other processes: burning of a candle, growth of crystal, formation of waves on water In other processes orderly structures are generated without the same type of a link between the peculiarities of the parent and the peculiarities of the offspring Living organisms consume free energy to create and maintain its organization Free energy drives complex chemical processes that are specified by the hereditary information - All living things are generated by cell division from a single cell - The single cell is the vehicle for the hereditary information that defines the species - Specified by this information, the cell includes machinery to gather raw materials from the environment, and to construct out of them new cells in its own image, complete with a copy of hereditary information o Nothing less than a cell has this capability 1. All cells store their hereditary information in the same linear chemical code (DNA) - Living cells like computers, deal in information, and it has been estimated that they have been evolving and diversifying for over 3.5 billion years - All living cells store their hereditary information in a form of double stranded molecules of DNA o Long, unbranched polymer chains, formed always of the same four types of monomers o These monomers have nicknames drawn from a four letter alphabet A, T, C, G, they are stung together in a liner sequence that encodes the genetic information o We can take human DNA and insert it into a bacterium, or vise-versa and the information will be successfully read, interpreted and copied - Using chemical methods, scientists can read out entire sequence of monomers in any DNA molecule (DNA molecules extend for millions of nucleotides), thereby deciphering the hereditary information that each organism contains 2. All cells replicate their heredity information by templated polymerization - Mechanisms that make life possible depend on the double stranded DNA molecule - Each monomer or nucleotide is a single DNA strand consists of two parts; one part: a sugar and phosphate group attached to it and second part: a base, which may be either adenine, guanine, cytosine or thymine o Each sugar is linked up to the nest one via the phosphate group o This creates a polymer chain composed of repeating sugar-phosphate back bone with a series of bases protruding from it www.notesolution.com o DNA polymer is extended by adding monomers at one end o In living cells, DNA is not synthesized as a free strand in isolation, but on a template formed by preexisting DNA strand o Bases protruding from the existing DNA strand bind to the bases of the strand being synthesized, according to a strict rule defined by the complementary structures of the bases: A binds to T, and C binds to G o These base pairings hold fresh monomers in place, thereby controlling the selection of which monomer will be added to the growing strand next - The bonds between the bases are weaker than those between the sugar-phosphate links o This allows the DNA to be pulled apart without breaking its backbone o Each strand of DNA, can serve as a template for the synthesis of fresh DNA complementary to itself, that is, a fresh copy of the hereditary information - DNA replication occurs in different cells at different rates, with different controls to start and stop it and with different auxiliary molecules to help it along o The coping of genetic information by DNA replication in this process, the two strands of DNA are pulled apart, and each serves as a template for synthesis of a new complementary strand - Basics are universal: DNA is the information store, and templated polymerization is the way in which this information is copied throughout the living world 3. All cells transcribe 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 it guide the synthesis of other molecules in the cell o The expression occurs by a mechanism which is the same in all living organisms o The process leads to first and foremost to the production of two other key classes of polymers: RNA and proteins This 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 Later, in the process of translation, many of these RNA molecules direct the synthesis of polymers of a radically different chemical class the proteins From DNA protein, the genetic information is read out and put to use through a two step process, first in transcription, segments of the DNA sequence are used to guide the synthesis of molecules of RNA, then in translation, the RNA molecules are used to guide the synthesis of molecules of protein - RNA backbone is formed out of a slightly different sugar, ribose instead of deoxyribose, and one of the four bases is slightly different uracil in place of thymine, but the other three are the same o All four bases of RNA pair with their complementary counterparts in DNA www.notesolution.com
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