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Chapter 2

Textbook Chapter 2 - Human Genetics

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Esteban Parra

Notes From Reading CHAPTER 2:H UMAN G ENETICS(PGS.33-59) Introduction - Our biology is a reflection of both genetics and environment, often interacting in complex ways - Genetics is also part of our evolution, as we inherit genetic material from the previous generation (our parents) and pass some of it on to the next generations (our children) - It is important to view genetics as a long-term dynamic process that unites all life – past, present and future - Molecular Genetics – The branch of genetics concerned with the structure and function of genes and DNA sequences at the molecular level - Mendelian Genetics – The branch of genetics concerned with patterns and processes of inheritance, named after Gregor Mendel, the first scientist to work out these principles - Genetics can also be studied in groups, such as populations within a species o Interested in describing the patterns of genetic variation within and among different populations, and their relationship with biological evolution - Population Genetics – The branch of genetics concerned with the changes in the frequency of genes and DNA sequences in populations over time o Primary concern is on genetic changes within populations from one generation to next, often termed as microevolution o Microevolution – Short-term evolutionary change o Macroevolution – Long-term evolutionary change Molecular Genetics DNA: The Genetic Code - The study of genetics at the molecule level concerns the amazing properties of a molecule known as deoxyribonucleic acid, or DNA - DNA – The molecule that provides the genetic code for biological structures and the means to translate this code o Basically a set of instructions for determining the makeup of biological organisms o Provides information for building, operating and repairing organisms The Structure of DNA - In many organisms, most of the DNA is contained in the nucleus of cells o A small amount of DNA also exists in the mitochondria, the parts of the cell that produce energy - Bases – Chemical units (adenine, thymine, guanine, cytosine) that make up part of the DNA molecule and specify genetic instructions - All biological structures, from nerve cells to blood cells to bone cells, are made up predominantly of proteins o Proteins are made up of amino acids, whose chemical properties allow them to bond together to form proteins o Each amino acid is coded for by three of the four chemical basis - There are 64 (4 ) possible codes that can be specified, using some combination of three bases o There are only 20 amino acids that need to be specified by the genetic code - The ability of the DNA molecule to use the different amino acid codes derives from a simple property of the chemical bases o The base A bonds with the base T, and base G bonds with base C o This chemical properly enables the DNA molecule to carry out a number of functions, including making copies of itself and directing the synthesis of proteins Notes From Reading CHAPTER 2:H UMAN GENETICS (PGS.33-59) Function of DNA - The DNA molecule can make copies of itself - DNA molecule is made up of two strands that form the long arms of the ladder - The DNA molecule can separate into two distinct strands o Once separate, each strand attracts free-floating bases o When the new bases have attached themselves to the original strands, the results is two identical DNA molecules - The ability of the DNA molecule to control protein synthesis also involves the attraction of complementary bases, but with the help of RNA - RNA – The molecule that functions to carry out the instructions for proteins synthesis specified by the DNA molecule o Serves as the messenger and decoder for the information in the DNA molecule - Difference between DNA and RNA is that in RNA the base A attracts a base called U (uracil) instead of T - In protein synthesis, the DNA molecule separates into two strands, and one strand becomes inactive - The active strand, attracts free-floating bases to form a strand of mRNA o Messenger RNA – The form of RNA that transports the genetic instructions from the DNA molecule to the site of protein synthesis - The strand then travels to the site of protein synthesis where the strand of mRNA transfers its information via tRNA o Transfer RNA – A free-floating molecule that is attracted to a strand of messenger RNA, resulting in the synthesis of a protein chain - The sequence of mRNA attracts a tRNA molecule with complementary sequence which results in the amino acid proline is included in the chain of amino acids making up a particular protein - Summary: o One strand of the DNA molecule produces the strand of tRNA, which carries the specified amino acid Chromosomes and Genes - Nuclear DNA – The DNA that is contained in the nucleus of the cell - Chromosomes – A long strand of DNA sequences - With certain exceptions, each cell in the human bod contains a complete set of chromosomes and DNA Genes - Gene – A DNA sequence that codes for a functional polypeptide or RNA product o A section of DNA that has an identifiable function o Example: a gene that determines a particular blood group o Hemoglobin – The molecule in blood cells that transports oxygen - No all DNA contains genes; much of our DNA is made up of noncoding sequences of DNA whose purpose (if any) is unknown - Exon – A section of DNA that codes for the amino acids that make up proteins. Its is contrasted with an intron - Intron – A section of DNA that does not code for the amino acids tat make up proteins. It is contrasted with exon - The formation of mature RNA involves the removal of the noncoding sections and the splicing together of the coding sections - Regulatory Gene – A gene that acts as a genetic switch to turn protein-coding genes on or off (affecting how these genes are expressed) - Differences in regulatory genes have great evolutionary significance and help explain how large physical differences can exist between species that have similar genes Notes From Reading CHAPTER 2:H UMAN G ENETICS PGS .33-59) - Homeobox Genes – A group of regulatory genes that encode a sequence of 60 amino acids regulating embryonic development - The same genes are found in different species, but their expression is modified by regulatory genes to produce physical differences in body shape o Expression and regulation of genes is affected by mechanisms apart from the actual DNA sequence Mitosis and Meiosis - The DNA molecule provides for the transmission of genetic information - Mitosis – The process of replication of chromosomes in body cells o When a cell divides, each chromosome duplicates and then splits o Each chromosome has replicated itself so when cell finishes dividing, the result is two cells with the full set of chromosomes - The genetic code is passed on from parents to offspring through the sex cells – sperm in males and eggs in females - Meiosis – The creation of sex cells by replication of chromosomes followed by cell division o Chromosomes replicate themselves, and then the cell divides and then divides again without replicating o Sperm cells: 4 sex cells are produced from the initial set of 23 pairs of chromosomes (similar for eggs except only 1 of the four eggs is functional) - Genome – The total DNA sequence of an organism o In humans, the genome is approx. 3 billion base pairs in length Mendelian Genetics - In the 19 century, Gregor Mendel carried out an extensive series of experiments in plant breeding - Before Mendel, it was thought that inheritances involves blending together of genetic information in the egg and sperm (thought as the same way and paint is mixed) - Mendel’s experiments showed that genetic information is inherited in discrete units (genes) - In one experiment, Mendel crossed pea plants who seeds were yellow with pea plants who seeds were o Mendel found that all the offspring plant had yellow seeds suggesting that somehow one trait (yellow color) dominated its effects - When he crossed this new generation, he found that some of their offspring had yellow seeds and some had green seeds o Ratio of plants with yellow seeds to green seeds was very close to 3:1 - Mendel formulated several principles of inheritance Genotypes and Phenotypes - Locus – The specific location of a gene or DNA sequence on a chromosome - Alleles – The alternative form of a gene or DNA sequence that occurs at a given locus. Some loci have only one allele, some have two, and some have many alternative forms. Alleles occur in pairs, one on each chromosome - Example: some human adults are lactose intolerant (cannot digest milk or milk products) o The gene that controls lactase activity is located on chromosome 2 o One allele codes for lactase persistence (enzyme continues to be produced) o One allele codes for lactase restriction (enzyme stops being produced) - Example: the type of molecules that are present on the surface of red blood cells o MN system determines whether you have M molecules, N molecules or both - Example: ABO system o Three alleles – A, B, and O Notes From Reading CHAPTER 2:H UMAN G ENETICS PGS .33-59) Mendel’s Law of Segregation - Mendel’s Law of Segregation – States that sex cells contain one of each pair of alleles o Within body cells, alleles occur in pairs, and when sex cells are formed, only one of each pair is passed on - Genotype – The genetic endowment of an individual from the two alleles present at a given locus - Homozygous – When both alleles at a given locus are identical - Heterozygous – When the two alleles at a given locus are different - Phenotype – the observable appearance of a given genotype in the organism - The relationship between genotype and phenotype is affected by the relationship between the two alleles present at any locus Dominant and Recessive Alleles - Dominant Allele – An allele that makes the effect of the other allele (which is recessive) in a heterozygous genotype - Recessive Allele – An allele whose effect is masked by the other allele (which is dominant) in a heterozygous genotype - Example: The ability to taste certain substances, including a chemical known as PRC o
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