Gene a DNA sequence that defines a certain heritable characteristic. Chromosome a DNA molecule which is made of DNA and proteins and carries genes. Alleles Are the different forms of a specific gene. Are the results of mutations of a gene sequence. If an organism has two of the same alleles, it is called homozygous for the trait. If an organism has two different alleles, it is said to be hetrezygous for the trait. Alleles can be dominant and recessive, where dominant alleles are always expressed if present, while recessive alleles are only expressed if present homozygously. Gene locus the specific position of a gene in a chromosome. A gene mutation Is a permanent mutation in the base of DNA. Genome: the whole genetic information of an organism. In humans: 46 chromosomes in nucleus plus mitochondrial DNA* In plants: Chromosomes in the nucleus plus DNA in the mitochondria and chloroplasts. In prokaryotes: Much smaller genome consisting of DNA in a circular chromosome and any plasmids. Human Genome Project refers to the sequencing of the entire human genome. Prokaryotic cell Chromosome is circular (compared to linear eukaryotic chromosomes), One copy of each gene, except during cell division (Binary Fission!), Chromosome is “naked” (i.e. not associated with histone proteins like eukaryotic chromosomes). Eukaryotic cell  DNA molecules are wrapped around globular histone proteins to form units called nucleosomesCoiled into chromatin during Interphase, Supercoiled to form characteristic chromosome shape during Mitosis. John Cairn, British biochemist (1922- ), Used autoradiography to visualize and measure DNA molecules in E. coli in 1963, E. coli grown with Thymine nucleotides that contained radioactive Hydrogen isotope, Tritium, Thymine used so only DNA labele, Cells place, on dialysis membrane and lysed with lysozyme enzyme, Coated in photographic emulsion and left in dark room for 2 month, High energy electrons from tritium decay reacted with film emulsion, leaving dark lines showing DNA outline. Humans have 23 different types of chromosomes22 pairs of Autosomes (1-22), 1 pair ofSex Chromosomes(23), Each chromosome carries a different set of genes in a specific order, The location of each gene on the chromosome is the gene locus. Homologous Chromosomes: Chromosomes of the same size and structure, with the same sequence of genes (i.e. same genes at the same gene loci), Despite having the same genes, may have different alleles for the gene!, In humans, one of each chromosome type comes from the mother and one of each type comes from the father to make 22-23 homologous pairs*, In order to interbreed, two organisms must have homologous chromosome sets. Haploid Nuclei: One of each chromosome type, In humans haploid cells contain 23 chromosomes, Haploid cells are sex cells (gametes) that fuse together during sexual reproduction – egg and sperm, Haploid = “Half” of a full set of chromosomes Diploid Nuclei: Two of each chromosome type (and so 2 copies of each gene*), In humans haploid cells contain 46 chromosomes (2 x 23) Haploid sperm and egg fuse in fertilization to produce diploid Zygote which then divides by Mitosis to form embryo, Diploid cells are body cells (somatic cells). Chromosome number is a fundamental feature that defines a species/All members of a species have the same chromosome number/Chromosome number tends to be conserved across millions of years of evolution/In rare cases chromosomes can fuse together or split to change chromosome number of a species/ During human evolution, two ancestral ape chromosomes fused to form modern day Chromosome 2 (As a result, humans have 23 types of chromosomes, while ape species have 24). Karyotype is the characteristic pattern of chromosomes of an organism, referring to their size, shape and the banding pattern. Karyogram is an image of all the chromosomes of an organism’s cell, shown in decreasing size of the homologous pairs. Meiosisis a process that divides one diploideukaryotic nucleus to form four haploid nuclei/The original diploid cell is divided twice in this process – Meiosis I and Meiosis II/Since the chromosome number is halved, Meiosis is called a reduction division. Homologous pairs are a set of one maternal and one parental chromosome that have the same genes in the same loci along the chromosome. Remember that diploid organisms have two variations of each chromosome number (for example, you have two chromosome 21s) Sister chromatids are identical copies formed by the replication of a chromosome. These are joined at the centromere in a replicated chromosome. Once sister chromatids separate, they are referred to as individual chromosomes. Prophase 1 Prior to this phase, the DNA has already duplicated (S phase) and the cell contains double the number of chromosomes (depicted as two chromatids of each chromosome)/ DNA supercoils and chromosomes shorten/ Nuclear envelope breaks down/ Centrioles move to the poles. Metaphase 1 Homologous chromosomes pair up at the cell equator (this means that the pairs of each chromosomes line up on top of each other)/ Each pole’s spindle microtubule attaches to one chromosome from each homologous pair (recall that in mitosis, one chromosome would have one of each pole’s spindle microtubules). Anaphase 1 Each spindle microtubule pulls one whole chromosome of the homologous pair towards it’s pole, causing a division of the chromosome pairs across the cell (recall that in mitosis, at this step, only the chromatids would separate so that a full set of chromosomes would be present at each pole)/ The movement of the chromosomes is achieved through shortening of the spindle microtubules. Telophase I Nuclear envelope forms around each set of chromosomes/ The cell divides into two cells with haploid number of nuclei (only one chromosome from each homologous pair)/ Chromosomes partly uncoil/ The cell will proceed with meiosis II. Prophase 2 Chromosomes supercoil again and become shorter/ Centrioles again move to the poles of the cell/ Nuclear envelopes break down. Metaphase 2  Chromosomes line up at the metaphase plate, one next to each other across the equator/ Spindle microtubules (one from each pole) attach to the centromeres of the chromosomes (centromeres). Anaphase II  Spindle microtubules pull the sister chromatids apart, so that one chromatid of each chromosomes travels to the opposite pole/ Therefore, each pole of the cell will receive one DNA copy of each chromosome. Telophase 2 At this last stage, each pole of the cell contains half the number of chromosomes compared to beginning of meiosis I, but the same number of chromosomes (just half the chromatids) compared to meiosis II./ The nuclear envelope forms and the cell divides into two cells/ Recall that in the first meiotic division, two cells were formed, meaning that now, each of those two cells divided into two, yielding a total of 4 cells, each with half the number of chromosomes. Mendel’s principles Because of his work with pea plants Mendel is considered the father of modern genetics. He planted 1000s of seeds per trial and carried out many trials to be sure of his results. His published work (1865) is now considered important, but at the time was ignored for 30 years. Gel Electrophoresis:Samples of either DNA or protein are inserted into wells in a gel/ Gel is placed in a conducting fluid and a current is passed through it/ Molecules move through gel based on their charge (ex. DNA molecules move to positive electrode since they’re negatively-charged)/ Smaller fragments / molecules move further through the pores in the gel, while larger pieces don’t travel as far. The Polymerase Chain Reaction (PCR) Synthetic method of amplifying specific sequences of DNA. Useful when only a small amount of DNA is available for testing e.g. crime scene samples of blood, semen, hair, etc. Process:  PCR occurs in a thermal cycler and involves 3 steps:1)Denaturation:DNA sample is heated to 95⁰C to break hydrogen bonds and separate it into two strand.2)Annealing:  DNA sample is cooled to 54 ⁰C, allowing primers attach to opposite ends of the target sequence 3)Elongation:  A heat-tolerant DNA polymerase (Taq) copies the strands.