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Chapter 3

Textbook Chapter 3 Notes

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University of Toronto Mississauga
Heather Miller

Notes From Reading CHAPTER 3:H EREDITY AND EVOLUTION PGS .37-68) Learning Objectives  Explain why cells are basic to life and describe the two different types of cells found in animals  Compare and contrast the two types of cell division  Describe the basic structure of DNA and explain how it relates to DNA replication  Describe the basic concepts of heredity that are found in all sexually reproducing organisms, including humans Introduction  People have tried to explain how offspring inherit characteristics from their parents  One common belief was that traits of offspring resulted from the blending of parental characteristics (this is not true)  We need to examine the fundamental principles of genetics  Genetics – is the study of how traits are transmitted from one generation to the next o The study of gene structure and action and the pattern of transmission of traits from parent to offspring o Genetic mechanisms are the foundation for evolutionary change  It is genetics that ultimately links the various subdisciplines of biological anthropology The Cell  Cells are the basic units of life in all living things  In some forms (ie. Bacteria), a single cell constitutes the entire organism  More complex multicellular forms, (ie. Plants, insects, birds and mammals) are composed of billions of cells  Life on earth can be traced back to at least 3.7 billion years to prokaryotic cells o They are single-celled organisms represented today by bacteria and blue-green algae.  Structurally more complex cells appeared approx. 1.2 billion years ago and these are called eukaryotic cells o Found in multicellular organisms  A eukaryotic cell is a three-dimensional structure that contains a variety of structures, called organelles, enclosed within a cell membrane  One of these organelles is the nucleus, a discrete unit surrounded by a thin nuclear membrane o Nucleus – a structure (organelle) found in all eukaryotic cells. The nucleus contains chromosomes (nuclear DNA)  Within the nucleus are two acids that contain the genetic information that controls the cell’s functions: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) o Deoxyribonucleic Acid (DNA) – The double-stranded molecule that contains the genetic code. DNA is a main component of chromosomes. o Ribonucleic Acid (RNA) – a molecule similar in structure to DNA. Three different single-stranded forms of RNA are essential to protein synthesis.  The nucleus is surrounded by a gel-like fluid called the cytoplasm, which contains several other types of organelles o Cytoplasm – the portion of the cell contained within the cell membrane, excluding the nucleus. The cytoplasm consists of a semifluid material and contains numerous structures involved in cell function  These organelles are involved in various activities, such as breaking down nutrients and converting them into other substances, storing and releasing energy, eliminating waste and manufacturing proteins  Proteins – three-dimensional molecules that serve a wide variety of functions through their ability to bind to other molecules  Protein Synthesis – the assembly of chains of amino acids into functional protein molecules. The process is directed by DNA Notes From Reading CHAPTER 3:H EREDITY AND EVOLUTION PGS .37-68)  There are two types of cells: somatic cells and gametes  Somatic Cells – all the cells in the body except those involved with reproduction. Are the cellular components of body tissues, such as muscle, bone, skin, nerves, heart and brains  Gametes – reproductive cells (eggs and sperm in animals) developed from precursor cells in ovaries and testes. (aka sex cells) are specifically involved in reproduction and are not structural components of the body o There are two types of gametes: egg cells (produced in the ovaries in females) and sperms (developed in male testes)  Zygote – a cell formed by the union of an egg and a sperm cell. o It contains the full complement of chromosomes (in humans, 46) and has the potential to develop into an entire organism DNA Structure and Function  Cellular function are directed by DNA  Molecule – a structure made up of two or more atoms o Molecules can combine with other molecules to form more complex structures  The DNA molecule is composed of two chains of even smaller molecules called nucleotides o Nucleotides – basic units of the DNA molecule, compose of a sugar, a phosphate unit, and one of four DNA bases  Nucleotides are stacked on top of one another to form a chain that is bonded along its bases to another nucleotide  Together the two twist to form a spiral, resulting DNA molecule, then, is two-stranded and is described as forming a double helix (that resembles a twisted ladder)  The specificity of nucleotide base pairing is essential to the DNA molecule’s ability to replicate, or make an exact copy of itself DNA Replication  Cells multiply by dividing to make exact copes of themselves  There are two kinds of cell division  Cells divide in a way that ensures that each new cell receives a full set of genetic material o A cell cant function properly without the appropriate amount of DNA  Before a cell can divide, its DNA must first replicate  Prior to cell division, enzymes break the bonds between bases in the DNA molecule, leaving the two previously joined strand of nucleotides with their bases exposed o Enzymes – specialized proteins that initiate and direct chemical reactions in the body  The exposed bases then attract unattached nucleotides that are constantly being manufactured elsewhere in the cell nucleus  Complementary – referring to the fact that DNA bases form base pairs in a precise manner o For example, Adenine can only bond to Thymine. These two bases are said to be complementary because one requires the other to form a complete DNA base pair  Each of the two previously joined parental nucleotide chains serve as models, or templates, for the formation of a new strand of nucleotides  As new strands are formed, its bases are joined to the bases of an original strand  When the process is complete, there are two double-stranded DNA molecules exactly like the original, and each new molecule consists of one original nucleotide chain joined to a newly formed chain Protein Synthesis  One of the most important functions of DNA is to direct the manufacture of proteins (protein synthesis) within the cell  Proteins are complex, three-dimensional molecules that function through their ability to bind to other molecules  Hemoglobin – a protein molecule that occurs in red blood ells and binds to oxygen molecules Notes From Reading CHAPTER 3:HEREDITY AND EVOLUTION (PGS.37-68)  Collagen, is the most common protein in the body, and it is a major component of all connective tissues  Enzymes are also proteins, and they regulate chemical reactions  ie. a digestive enzyme called lactase breaks down lactose or mill sugar into two simpler sugars  Another class of proteins includes many kinds of hormones  Hormones – substances (usually proteins) that are produced by specialized cells and travel to other parts of the body, where they influence chemical reactions and regulate various cellular functions  Ie. Insulin is a hormone produced by cells in the pancreas, and it causes cells in the liver to absorb energy-producing glucose (sugar) from the blood  Regulatory proteins can enter a cell’s nucleus and attach directly to the DNA and can switch genes on and off, thereby influencing how the genes function  Protein are made up of chains of smaller molecules called amino acids o Amino Acids – small molecules that are the components of protein  A protein can not function correctly unless its amino acids are arranged in the proper order  Protein synthesis involves a molecule similar to DNA called RNA  RNA reads the instructions and assembles amino acids to form proteins  Entire sequence of DNA bases responsible for the synthesis of a protein is referred to a gene  Gene – a sequence of DNA bases that specifies the order of amino acids in an entire protein, a portion of a protein, or any functional product. o A gene may be made up of hundreds or thousands of DNA bases  If the sequence of DNA bases is changed by a mutation, some proteins may not be manufactured, and the cell (or indeed the organism) may not function properly at all o Mutation – a change in DNA. The term can refer to changes in DNA bases as well as changes in chromosome number of structure  The DNA segments that ultimately are translated into amino acids are called exons  Most of the DNA in a genes isn’t expressed during protein synthesis, and these unexpressed segments are called introns  Regulatory Genes – genes that code for the production of proteins that can influence the action of other genes (Many are active only during certain stages of development)  Many of the anatomical differences between humans and chimpanzees are the results of evolutionary changes in regulatory genes in both lineages  Homeobox (Hox) Genes – an evolutionarily ancient family of regulatory genes. Hox genes direct the segmentation and patterning of the overall body plan during embryonic development o Are present in all invertebrates (ie. worms and insects) and vertebrates, and they don’t vary greatly from species to species o Therefore, not only are these genes vitally important, but also that they evolved from genes that were present in some of the earliest form of life  The genetic code is universal where DNA is the genetic material in all forms of life  DNA is composed of the same molecules using the same kinds of instructions  Similarities imply biological relationship among, and un ultimate common ancestry for, all forms of life  What makes trees different from humans isn’t differences in their DNA material, but differences in how that material is arranged and regulated Cell Division: Mitosis and Meiosis  Throughout much of a cell’s life, its DNA exists as an uncoiled, threadlike substance  Cell division results in the production of new cells, and during this process, the DNA becomes tightly coiled  Visible under a light microscope are structures called chromosomes o Chromosomes – discrete structures, composed of DNA and protein, found only in the nuclei of cells. Chromosomes are visible only under magnification during certain stages of cell division Notes From Reading CHAPTER 3:H EREDITY AND EVOLUTION (PGS.37-68)  If chromosomes were visible during normal cell function, they would appear as single- stranded structures  During the early stages of cell division, they are made up of two stands, or two DNA molecules, joined together at a constricted area called the centromere o Centromere – the constricted portion of a chromosome. After replication, the two strands of a double-stranded chromosome are joined at the centromere  Two strands because: the DNA molecules have replicated and one strand is an exact copy of the other  There are two basic types of chromosomes: o Autosomes – all chromosomes that carry genetic information that governs all physical characteristics except primary sex determination (sex chromosomes) o Sex Chromosomes – the X and Y chromosomes. The Y chromosome determines maleness; in its absence, an embryo develops as a female  All autosomes occur in pairs  Normal human somatic cells have 22 pairs of autosomes ad one pair of sex chromosomes  To function normally, a human cell must possess both members of each chromosomal pair, or a total of 46 chromosomes Mitosis  Cell division in somatic cells is called mitosis  Mitosis – simple cell division; the process by which somatic cells divide to produce two identical daughter cells o The cell is involved in metabolic activities. DNA replication occurs, but chromosomes are not visible o The nuclear membrane disappears, and double-stranded chromosomes are visible o The chromosomes align themselves at the center of the cell o The chromosomes split at the centromere, and the strands separate and move to opposite ends of the diving cells o The cell membrane pinches in as the cell continues to divide. The chromosomes begin to uncoil o After mitosis is complete, there are two identical daughter cells. The nuclear membrane is present and chromosomes are no longer visible  DNA replication ensures that the amount of genetic material remains constant from one generation of cells to the next Meiosis  While mitosis produces new cells, meiosis can lead to the development of an entire new organism because it produces reproductive cells  Meiosis – cell division in specialized cells in ovaries and testes. Meiosis involves two divisions and results in four daughter cells, each containing only half the original number of chromosomes. These cells can develop into gametes.  During meiosis, specialized cells in male testes and female ovaries divide and eventually develop into sperm and egg cells  Initially, these cells contain the full complement of chromosomes, but after the first division (called reduction division), the number of chromosomes in the two daughter cells is half of the original number  Product of this union is a zygote inherits the exact amount of DNA it needs to develop and function normally  During the first division of meiosis, partner chromosomes come together to form pairs of double-stranded chromosomes  The pairs of chromosomes line up along the cell’s equator  Pairing of partner chromosomes is extremely important because while they’re together, the members of each pair exchange genetic information in a critical process called recombination o Recombination – the exchange of DNA between paired chromosomes during meiosis; also called crossing over Notes From Reading CHAPTER 3:H EREDITY ANDE VOLUTION (PGS.37-68)  Pairing is also important because it facilitates the accurate reduction of chromosome number by ensuring that each new daughter cell receives only one member of each pair  As a cell begins to divide, the chromosomes themselves remain intact (double stranded), but member of pairs pull apart and move to opposite end of the cell  After the first division, there are two new daughter cells, but they aren’t identical to each other or to the parent cell  The second meiotic division happens pretty much the way it does in mitosis  Once this second division is completed, there are four daughter cells, each with 23 single- stranded chromosomes The Evolutionary Significance of Meiosis  Meiosis occurs in all sexually reproducing organisms and its an extremely important evolutionary innovation because it increases genetic variation in populations  Clone – a clone is an organism that is genetically identical to another organism. The term may also be used to refer to genetically identical DNA segment and molecules  Each individual represent a unique combination of genes that (in all likelihood) has never occurred before and will never occur again The Genetic Principles Discovered by Mendel  Gregor Mendel addressed the question of heredity that the crucial biological process began to be scientifically resolved  His experiments led him to explore the various ways in which physical traits, such as color or height, could be expressed in plant hybrids o Hybrids – offspring of mixed ancestry; heterozygotes  Mendel worked with garden peas, concentrating on seven different traits, each of which could be expressed in two different ways Mendel’s Principle of Segregation  Mendel suggested through his results that different expressions of a traits are controlled by discrete units or particles, which are known as genes  The units occur in pairs, and offspring inherit one unit from each parent  Mendel also realized that the members of a pai
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