BIO 10 Lecture 9 REPRODUCTION: MITOSIS AND MEIOSIS Life and DNA • Living organisms are “survival machines” for DNA molecules • DNA molecules carry the information that enables host cells to protect and perpetuate them • • • • Enclose and protect them from “other” Gather and use energy to fight entropy Reproduce Evolve • A Genome = complete collection of an organism’s genetic information as linked genes in one or more long strands of DNA – Genes provide information for building proteins – Information for making proteins is stored digitally in the sequence of chemical bases A, C, G, and T in the double helix. • In order to perpetuate this information, each human cell must be able to replicate its DNA • Then divide the DNA equally into daughter cells – In eukaryotes, this process is called mitosis • Mitosis occurs before the cytoplasm of the cell splits in two (cytokinesis), giving each “daughter cell” one copy of the original parent DNA molecule and half the cytoplasm of the parent cell DNA Replication Chromosomes • DNA is packaged in chromosomes – Each DNA molecule is very long (~0.02 m) compared to its width (0.000000002 m) • Ten million times longer than it is wide – Compare to a piece of sewing thread (1 mm wide) • Would stretch 10,000 m = 6.21 miles! – Nucleus is only ~10 um in diameter but must house ~1 m of DNA (100,000 times longer than the nucleus is wide) • Like having a softball stuffed with 6.21 miles of sewing thread • DNA must be packaged around proteins to keep it organized = chromatin • Like storing sewing thread on spools • The human genome carries about 25,000 genes spread out over 24 different types of chromosomes – 22 autosomes (# 1-22) – 2 sex chromosomes: X and Y – Each chromosome carries its own genes in a linear order along its length • Many genomes are smaller than the human genome, some are larger – E. coli genome: 1,000 x smaller than humans – Amoeba Chaos chaos: 200 times larger than humans! http://www.genomesize.com/statistics.php • Because DNA is packaged into chromosomes, when it divides, the chromosomes also divide – A replicated chromosome is comprised of two daughter DNA molecules held together – The daughter DNA molecules are called “sister chromatids” • Many eukaryotes carry two copies of each chromosome in the genome – Protects against the loss of a single gene (and its protein) due to mutation – Like having a “back-up” system – Pairs of like chromosomes are called homologous chromosomes • Analogy—ask someone how many shoes they have, and they will tell you how many different shoes, like 10 pairs. But if you counted the total, there are actually 20 shoes. Homologues are not identical; they are variants, like a left and right shoe – Humans have 2 sets of 23 chromosomes each • Sex chromosomes: humans and other mammals use chromosomes to distinguish between the sexes. – Males have one X and one Y – Females have two Xs. • Autosomes: All other chromosomes. • Humans have 22 different autosomes (numbered 1-22) and two different sex chromosomes (X and Y) • Karyotypes are ordered, pictorial arrangements of chromosomes The Cell Cycle • The eukaryotic cell cycle is made up of a repeating pattern of growth, DNA replication, mitosis, and cytokinesis. – Typical animal cell cycle lasts about 24 hours. • Two main phases: – Interphase (about 23 hrs, 30 min) • Cell growth, productive activity • DNA replication – Mitosis and Cytokinesis (about 30 min) • Chromosomes segregated to daughter cells • Cell divides in two MITOSIS • Process by which the cell’s newly duplicated chromosomes (pair of sister chromatids still attached) condense, align themselves, and separate evenly prior to cell division • Used for growth, replacement of damaged/ dead cells or (in some organisms) asexual reproduction • Occurs in a series of four stages: – – – – Prophase Metaphase Anaphase Telophase Variations on the Theme • Plant cells—everything is similar except for cytokinesis, because plant cells have to break down and reform the cell wall. – Vesicles fuse near the metaphase plate to form a cell plate that grows outward to form a cell wall. • Prokaryotes (no nucleus)—binary fission. – Single circular chromosome – DNA replicates and daughter “rings” unlink – Simpler than mitosis Meiosis: Making Sex Cells MITOSIS MEIOSIS • Purpose: – Asexual reproduction in single-celled eukaryotes – Growth and replacement of dead cells in multicellular eukaryotes • Process: – Clones the original parent cell – Daughter cells genetically identical to parent cell – One cell division • Location: – All tissues • Outcome: – 2 identical daughter cells • Purpose: – Creation of eggs and sperm for sexual reproduction • Process: – Number of chromosome sets halved – Daughter cells contain half the DNA of the parent cell and the DNA is “mixed up” – 2 cell divisions - Location: - Sexual organs only - Outcome: - 4 genetically different eggs or sperm Why Sex? • Not found in prokaryotes – Asexual reproduction only • Often an option for single-celled eukaryotes (e.g. yeast) – May switch from asexual to sexual reproduction when the environment changes • Usually a requirement for multi-celled eukaryotes – Most are absolutely dependent on sexual reproduction • Advantage of Sex – Introduces new combinations of heritable traits in the offspring – Critically important for large, multi-cellular organisms that have limited numbers of offspring and evolve slowly – Must keep up with the rapid evolution of parasites, bacteria, and viruses • These organisms can evolve very quickly because they have enormous numbers of offspring • Asexual reproduction has the advantage of being very rapid Halving the number of Chromosome Sets • Although superficially meiosis looks similar to mitosis, it is really very different • In addition to creating new combinations of heritable traits in the eggs and sperm, meiosis halves the number of chromosome sets in the eggs and sperm • If combining traits were accomplished just by combining two cells, life would get incredibly complicated. – Analogy to couples hyphenating their names when married. • If n = number of different chromosomes. – Human cells are diploid = 2 of each chromosome (homologues) 2n. – If you took a somatic cell from a woman (say a bit of skin) and a man and combined them, you would have 46 + 46 = 92 (4n) chromosomes. The next generation would repeat this to 184 (8n), and so on. • After only 30 generations, a billion chromosomes per cell!! • Must have a way of reducing the chromosome number by half, to get 23(n) + 23(n) = 46 (2n) total for the new organism each generation – Cells with half the number of chromosomes (one of each) are haploid and are called gametes (sperm and eggs) – These cells are made only in sexually reproducing organisms and only in a special organ called a gonad (testis and ovary) Steps in Meiosis • Meiosis begins like mitosis does, with replication of DNA in S phase of interphase – Involves two cell divisions instead of just one – Meiosis I and Meiosis II • Meiosis I – When homologues separate – Chromosome number halved – Genetic variation created by “scrambling up” chromosomes through two processes: • Crossing over of homologous pairs during prophase of MI • Independent assortment of homologue pairs during metaphase of MI In Meiosis I, homologous pairs line up, exchange parts, and “dance together” ... • Meiosis II – Is a haploid mitotic division – Is when sister chromatids separate Role of Meiosis in Sex Ratio • Humans have 22 pairs of autosomes and one pair of sex chromosomes; females have two X chromosomes, but males have one X and one Y. • Sex chromosomes pair like homologues and separate in meiosis I, so each haploid cell that results has either an X or a Y in males. – The sex of a child is therefore determined by the sperm at fertilization Spermatogenesis in Humans • Spermatogonia: diploid cells that can divide by mitosis to make more of themselves, or make cells destined to become sperm called primary spermatocytes. – Primary spermatocytes complete meiosis I to give rise to two secondary spermatocytes. – Secondary spermatocytes complete meiosis II to make four total haploid spermatids. – Spermatids are matured in about three weeks to form sperm with flagella tails, concentrated mitochondria, and a haploid nucleus. – About 250 million sperm are made each day, about same number in one ejaculate. Oogenesis in Humans • Oogonia: diploid cells that divide by mitosis early in embryogenesis, never divide after birth. Most die before birth, but the remaining oogonia make cells destined to become eggs, called primary oocytes. – Primary oocytes begin meiosis I in the embryo, but do not complete it until ovulation to give rise to one secondary oocyte and a polar body. • Secondary oocytes complete meiosis II after being fertilized by a sperm to make one haploid egg. The other products of meiosis are polar bodies. – Oocyte cytokinesis is unequal to ensure that one large cell, 200,000 times bigger than the sperm, has enough materials to drive early divisions and feed the rapidly dividing embryonic cells. – All mitochondria inherited by the baby are from the mother’s egg – Usually only one secondary oocyte is released per 28-day cycle in an adult woman. Short Review of Lecture 9 • How does DNA carry genetic information? What is a chromosome? What is a gene? • What is a genome? • What is the cell cycle? What happens during interphase? Mitosis? • Compare mitosis and meiosis. How are these processes similar? How are they different? • What types of organisms under mitosis? Meiosis? • What is the advantage of sex? Why are most complex eukaryotes dependent on sex for the perpetuation of their genes? • How is sex determined in humans?