The bases on complementary strands of DNA bond with each other in a specific way A-T and G-C

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1 1 Bio 1101 Lecture 6 (Guided Notes) Ch. 8: Cellular Basis of Reproduction Cellular Basis of Reproduction & Inheritance In order for an organism to replace dead cells or to grow and produce new cells, existing cells must reproduce Cells must replicate their DNA (chromosomes) before they divide, so that each new cell has all of the necessary genetic information = the molecule in our cells that codes for proteins (see pages for a review of DNA and protein structures) Deoxyribonucleic acid DNA consists of two strands of Nucleotides are composed of a, a group (phosphorous and oxygen), and a -containing base There are 4 different kinds of these bases: adenine (A), thymine (T), guanine (G), and cytosine (C) The bases on complementary strands of DNA bond with each other in a specific way A-T and G-C Some Terminology = the complete set of an organism s genes = a discrete nucleotide sequence in DNA coding for a protein Recent studies have revised the estimated number of genes in humans down to about 19,000 That s about the same as a microscopic nematode C. elegans (also 19,000 genes)! = a threadlike, gene-carrying structure, composed of DNA and proteins (chromatin) 7 Each eukaryotic organism contains one or more chromosomes Each chromosome consists of a long piece of DNA, along with proteins ( = DNA plus proteins) Each chromosome may carry several, or many, genes cells have 2 copies of every chromosome one they inherited from their mother, and one from their father cells have only 1 copy of every chromosome 1

2 When a cell divides to make a copy of itself (for example, when your hair is growing or skin cells are replacing themselves), how does it: Make another copy of its DNA? Get one entire set of chromosomes into each daughter cell? Process = The general process: Duplication of the chromosomes Sorting of the chromosomes into two identical sets ( sorting the spaghetti ) Cytokinesis splitting of the cell into two This entire cycle is called the Cell cycle has several steps 1. Interphase 2. Prophase Anaphase 5. Telophase 6. (Ends with ) Chromosomes condense Sister join Microtubules form in cytoplasm Nuclear membrane breaks down Chromosomes line up on metaphase plate Mitotic spindle tubules attach to each chromosome Sister chromatids are pulled apart 2

3 Cell elongates & When the two groups of chromatids reach opposite ends of the cell, the nuclear membrane re-forms around the chromosomes Mitotic spindle diappears The cell splits in two (either a forms in animal cells, pinching the cell in two; or a new cell wall forms in the middle, splitting the cell in two, in plant cells. The division of the cytoplasm into two new cells = Mitosis again, with diagrams from textbook Identify the Phases of Mitosis: The result of Mitosis: 2 Genetically Daughter Cells Identical to the Parent Cell Mitosis is the form of cell division used in reproduction Regulation of Cell Growth Cells have various genes that produce special proteins that they use to signal each other and coordinate their growth Some of these proteins are called and stimulate division If a mutation occurs and the cell produces too much growth factor, it may become cancerous divide very rapidly Chemotherapy and Radiation Therapy work by stopping Radiation Therapy tumors exposed to radiation, which disrupts cell division Chemotherapy different drugs interfere with different stages of mitosis (from periwinkle flower) prevents mitotic spindle from forming (from bark of Pacific Yew tree) freezes the spindle once it is formed 5-minute break 3

4 Sexual Reproduction When two individuals mate, how do they pass on their genes to their offspring? Special reproductive cells called are formed Eggs and sperm cells are Each egg and sperm cell has only copy of every gene How are gametes produced? How do we divide the spaghetti this time? Cell goes through divisions (whereas Mitosis only involves one division) In the process, pieces of chromatids can break apart and recombine with sister chromatids (this is called and produces new combinations of genes generates diversity) Results of Meiosis: Haploid Cells The daughter cells are genetically from each other and from the parent cell The resulting cells are = when gametes fuse to produce a new individual This restores the diploid number of chromosomes (2 copies of each, one from the mother, 1 from the father) If there were no meiosis, then with each successive generation, the amount of DNA in a cell would double! Many simple organisms, such as bacteria, do not reproduce sexually Binary fission in prokaryotes Budding in many protists 4

5 Much easier, so What good is sex? Why not just reproduce by mitosis? Genetic Diversity! Each individual is a unique mix of the genes from its mother and father Diversity is also generated during prophase 1 of meiosis (recall crossing over ) unique combinations of genes created on chromosomes Remember, Mitosis creates exact copies of cells If we reproduced by mitosis, our offspring would inherit the exact same genes we have By using Meiosis to create sex cells (gametes), which combine via fertilization to form a new individual, each offspring produced may inherit one of hundreds of millions of different possible combinations of genes from the mother and father Is Cell Division Perfect? No, errors can occur Example: When chromosomes are supposed to be pulled apart in Anaphase 1 or 2 what if they don t get pulled apart to separate cells? Then one of the gametes produced will have either too many or too few copies of that chromosome (= ) Example: Trisomy 21 in humans Human Karyotype with Trisomy 21: Female with Down s Syndrome Sex Chromosomes and Sex-linked Traits Humans have different pairs of chromosomes, including the sex chromosomes XX = normal XY = normal Therefore, females only have X chromosomes to pass on to their offspring may either pass on an X or a Y it is therefore the who determines the sex of the baby (in mammals) Since males will produce half X and half Y sperm, the child has a 50/50 chance of being male/female 45 5

6 The Y chromosome (the one associated with maleness) is much smaller and contains fewer genes than the X chromosome Over 1,000 on X About 78 on Y X s and Y s carry different types of genes Sex-linked recessive traits: Normal females carry two copies of all the genes located on the X chromosome Less likely to have genetic problems associated with the sex chromosome, because one of those genes is likely to be functioning properly In Males, only 1 copy of the X chromosome and its associated genes Therefore, if one of those genes has problems, will display that trait Examples: color-blindness hemophilia Nondisjunction of Sex Chromosomes Disorders associated with incorrect number of sex chromosomes XXY a male with Syndrome (male has some feminine characteristics, such as enlarged breasts, smaller testes, and sterility) XYY normal male; no defined disorder, but generally taller XXX ; limited fertility XO female with Syndrome; tend to be shorter, web of skin between neck and shoulders, and are sterile Other errors during meiosis a fragment of a chromosome is lost a piece of a homologous chromosome breaks off and combines with the other If it inserts in the wrong direction, it is called an inversion a piece of a nonhomologous chromosome breaks off and combines with the other Random Animal of the Day! The! A species of fully aquatic, giant salamander in eastern US They have among the largest genomes of any tetrapods billion base pairs! 6

7 (compared to 3 billion in humans) 51 All for today Next time, we ll discuss inheritance and the structure and function of DNA 7