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Transcript
Standard 2: Genetics Mutation and sexual reproduction lead to genetic variation in a population. a. meiosis is an early step in sexual reproduction in which the pairs of chromosomes separate and segregate randomly during cell division to produce games containing one chromosome of each type b. only certain cells in a multicellular organism undergo meiosis c. how random chromosomes segregation explains that a particular allele will be in a gamete d. that new combinations of alleles may be generated in a zygote through the fusion of male and female gametes (fertilization) e. why approximately half of an individual’s DNA sequence comes from each parent f. the role of chromosomes in determining an individual’s sex g. how to predict possible combinations of alleles in a zygote from the make-up of the parents Genetics Have you ever wondered why you have the same eye color as your mother? Or the same hair color as your father? • Genetics is the study of heredity or inheritance. • Genetics helps to explain how traits are passed from parents to their young. • Parents pass traits to their young through gene transmission Inheritance of Traits Gregor Mendel discovered this idea over 100 years ago. The principle has three parts: 1. Hereditary traits are determined by specific genes. 2. Individuals carry two genes for each trait, one from the mother’s egg and one from the father’s sperm. 3. When an individual reproduces, the two genes split up (segregate) and end up in separate gametes. • Genes are located on chromosomes and consist of DNA. • They contain specific instructions for protein synthesis DNA- Deoxyribonucleic Acid Genes on Chromosomes Reproduction • Asexual – Many single-celled organisms reproduce asexually by splitting or budding. – Some multicellular organisms can reproduce asexually, produce clones (offspring genetically identical to parent). Sexual Reproduction • Fusion of two gametes (male and female) to produce a single zygote. • Introduces greater genetic variation, allows genetic recombination. • With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents. Question • Fertilization is the fusion of two gametes (male and female) to produce a ????????????? Answer • Fertilization is the fusion of two gametes (male and female) to produce a ZYGOTE Homologues • Chromosomes exist in homologous pairs in diploid cells. Exception: Sex chromosomes (X, Y). Other chromosomes are known as autosomes (body cells). In humans … • 23 chromosomes donated by each parent called haploid (means half) cells. – haploid number “n” = 23 in humans. • The total we have is 46 called diploid • 23 from egg and 23 from sperm= the complete number of chromosomes for the child In humans … (continued) • Gametes (sperm or ova): – Contain 22 autosomes and 1 sex chromosome. – Are haploid (having a single set of unpaired chromosomes) Mitosis vs. Meiosis Fertilization results in zygote with 2 haploid sets of chromosomes - now diploid. Diploid cell; 2n = 46. (n=23 in humans) Most cells in the body produced by mitosis. Ex: skin cells, tissue cells Only sex cells are produced by meiosis. Question • If an organism has 60 chromosomes, how many chromosomes would be in their gametes produced? Answer • If an organism has 60 chromosomes, how many chromosomes would be in their gametes produced? 30 half the total number Mitosis Meiosis Meiosis I and II • Meiosis I and meiosis II are two separate substages of meiosis. • Meiosis 1 occurs by producing genetic recombination in the daughter cells while in meiosis 2 each of the four daughter cells will contain half the amount of chromosomes of the parent cell. Phases of Meiosis I and II • Meiosis has five phases including: prophase, metaphase, anaphase, telophase and interphase. • It goes through each phase twice – Prophase 1 through telophase 1 then prophase 2 through telophase 2 Meiosis–key differences from mitosis • • • • Meiosis reduces the number of chromosomes by half. Daughter cells differ from parent, and each other. Meiosis involves two divisions, Mitosis only one. Meiosis I involves: – Synapsis – homologous chromosomes pair up. Chiasmata form (crossing over of non-sister chromatids). – In Metaphase I, homologous pairs line up at metaphase plate. – In Anaphase I, sister chromatids do NOT separate. – In Telophase 2, Haploid cells are formed. –. Ticket Out the Door Mnemonic for PMAT • P=Paco Prophase • M=Made Metaphase • A= a Anaphase • T=Taco Telophase Make Your Own!