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BIO130H1 (167)

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University of Toronto St. George
Melody Neumann

BIO 130 - Chapter 1: Introduction to the Cell – Cells and Genomes THE UNIVERSAL FEATURES OF CELLS ON EARTH • A single cell functions as the vehicle for hereditary information that defines the species o This single cell includes machinery to gather raw materials from the environment, and to construct out of them a new cell in its own image, complete with a new copy of the hereditary information • DNA is universal – we can take a piece of DNA from a human cell and insert it into a bacterium, and the information will be successfully read, interpreted, and copied All Cells Replicate Their Hereditary Information by Templated Polymerization • DNA strands are made of monomers, or nucleotides, that consist of two parts: (1) a sugar, (deoxyribose), with a phosphate group attached to it, and (2) 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 • A binds with T, and C binds with G – this can facilitate the synthesis of a new strand based of the opposite pairing with an existing strand • Bonds between base pairs are very weak compared with the sugar-phosphate links, and this allows the DNA strands to be pulled apart without the breakage of their backbones o Each pulled apart strand can act as a template for the synthesis of a strand to create a double helix – DNA replication  This process occurs at different rates, with different controls to start and stop it, and different auxiliary molecules to help it along • DNA is the information store, and the template polymerization is the way in which this information is copied throughout the living world All Cells Transcribe Portions of Their Hereditary Information into the Same Intermediary Form, (RNA) • Creation of RNAs and Proteins: o Begins with template 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 o Translation then occurs, in which many RNA molecules direct the synthesis of polymers of a radically different chemical class – the proteins BIO 130 - Chapter 1: Introduction to the Cell – Cells and Genomes • In RNA uracil (U), replaces thymine (T), but the other bases remain the same • RNA monomers will faithfully represent a part of the cell’s genetic information, just written in a slightly different alphabet not reliant on DNA monomers • Many act as intermediates in the transfer of genetic information: they mainly serve as messenger RNA, (mRNA) to guide the synthesis of proteins according to the genetic instructions stored in the DNA • Being single stranded, RNA can bend back on itself to allow one part of the molecule to form weak bonds with another part of the same molecule o This can allow the molecule to fold up into a specific shape dictated by its sequences o Specific shapes may enable it to recognize other molecules by binding to them selectively – which can be used to catalyze chemical changes in the molecules that are bound 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 • The monomers of protein are called amino acids, there are 20 types. o Each amino acid is built around the same core structure through which it can be linked in a standard way to any other amino acid in the set o Attached to this core is a side group that gives each amino acid a distinctive chemical character • Each protein molecule, or polypeptide, folds into a precise three-dimensional form with reactive sites on its surface o These reactive sites can bind with high specificity to other molecules and act as enzymes to catalyze reactions that make or break covalent bonds All Cells Translate RNA into Protein in the Same Way 1. Information in the sequence of a messenger RNA molecule is read out in groups of three nucleotides at a time 2. Each triplet, or codon, specifies a single amino acid in a corresponding protein a. Several codon sequences will code for the same amino acid – since there are only 20 amino acids, there is this redundancy BIO 130 - Chapter 1: Introduction to the Cell – Cells and Genomes 3. The code is read by a special class of small RNA molecules, the transfer RNAs, (tRNAs). a. 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 4. A succession of tRNA molecules charged with their appropriate amino acids have to be brought together with an mRNA molecule and matched up by base-pairing through their anticodons with each of its successive codons 5. Amino acids are then linked together to extend the growing protein chain, and the tRNAs, are released a. This process is carried out by two main chains of RNA, called ribosomal RNAs, (rRNA) b. rRNA latches onto the other end of an mRNA molecule and then trundles along it, capturing loaded tRNA molecules and stitching together the amino acids they carry to form a new protein chain The Fragment of Genetic Information Corresponding to One Protein is One Gene • A gene is a DNA segment that corresponding to a single protein or set of alternative protein variants • Stretches of regulatory DNA are interspersed among the segments that code for protein, and these noncoding regions bind to special protein molecules that control the local rate of transcription • Genome is the total genetic information as embodied in the complete DNA sequence o Dictates not only the nature of the cell’s proteins, but also when and where they are to be made Life Requires Free Energy • For a cell to grow or to make a new cell, it must take in free energy from the environment, as well as raw materials, to drive the necessary synthetic reactions All Cells Are Enclosed in a Plasma Membrane Across Which Nutrients and Waste Materials Must Pass • Each cell is enclosed by a membrane – the plasma membrane BIO 130 - Chapter 1: Introduction to the Cell – Cells and Genomes o Acts as a selective barrier that enables the cell to concentrate nutrients gathered from its environment and retain the products it synthesizes for its own use, while excreting its waste products o Without the plasma membrane, the cell could not maintain its integrity as a coordinated chemical system • The molecules forming the plasma membrane are amphiphilic, meaning they consist of one part that is hydrophobic, (water-insoluble), and another part that is hydrophilic, (water-soluble) o These molecules will aggregate in water to form a bilayer that creates small closed vesicles • Important General Principle: Cells produce molecules whose chemical properties cause them to self-assemble into the structure that a cell needs • All cells have specialized proteins embedded in their membrane, membrane transport proteins, that transport specific molecules from one side to the other o These proteins largely determine which molecules enter the cell, and the catalytic proteins inside the cell determine the reactions that those molecules undergo THE DIVERSITY OF GENOMES AND THE TREE OF LIFE Cells Can Be Powered by a Variety of Free Energy Sources • Organotrophic organisms are those that obtain free energy by feeding on other living things or the organic chemicals they produce • Phototrophic organisms harvest free energy from the sunlight o Primary energy converter • Lithotrophic organisms capture free energy from energy-rich systems of inorganic chemicals in the environment o Primary energy converter o Some lithotrophs get energy from aerobic reactions, using oxygen from the environment, whereas some live anaerobically, using no oxygen • The primary energy converters are the most plentiful form of life The Greatest Biochemical Diversity Exists Among Procaryotic Cells • Organisms can be classified based on their cell structure into two groups: BIO 130 - Chapter 1: Introduction to the Cell – Cells and Genomes o (1) Eukaryotes, which keep their DNA in a distinct membrane-enclosed intracellular compartment called the nucleus.  Plants, fungi, and animals are eukaryotes o (2) Prokaryotes, which have no distinct nuclear compartment to house their DNA.  Bacteria are prokaryotes, as are archaea  Cells are small and simple in outward appearance, usually living as independent individuals or in loosely organized communities, rather than multicellular organisms  Typically spherical or rod shaped, and have a tough protective coat called a cell wall, beneath which a plasma membrane encloses a single cytoplasmic compartment containing DNA, RNA, proteins, and the many small molecules needed for life New Genes Are Generated from Preexisting Genes No gene is ever entirely new. Innovation can, however, occur in several ways: 1. Intragenic mutation: an existing gene can be modified by changes in its DNA sequence, through various types of error that occur mainly in the process of DNA replication 2. Gene duplication: an existing gene can be duplicated so as to create a pair of initially identical genes within a single cell; these two genes may then diverge in the course of evolution 3. Segment shuffling: two or more existing genes can be broken and rejoined to make a hybrid gene consisting of DNA segments that originally belonged to separate genes 4. Horizontal (intercellular) transfer: a piece of DNA can be transferred from the genome of one cell to that of another – even to that of another species. This process is in contrast with the usual vertical transfer of genetic information from parent to progeny Each of these types of change leaves a characteristic trace in the DNA sequence
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