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Lecture # 14 Chapter 7 Mitosis and Meiosis 7.1 Prokaryotes Have a Simple Cell Cycle • Prokaryotic cells divide asexually – These cells possess a single circular chromosome, containing about 1000 genes – The chromosome is replicated – The cell then divides into two cells, a process called binary fission Fig. 7.1 7.2 Eukaryotes Have a Complex Cell Cycle • Cell division in eukaryotes is more complex than in prokaryotes because – 1. Eukaryotic contain far more DNA – 2. Eukaryotic DNA is packaged differently • It is in linear chromosomes compacted with proteins • Some eukaryotes also make exact copies of themselves via asexual reproduction amoeba 7.2 Eukaryotes Have a Complex Cell Cycle Interphase Mitosis Cytokinesis – G1 phase • Primary growth phase – S phase • DNA replication – G2 phase • Microtubule synthesis – M phase • Chromosomes pull apart – C phase • Cytoplasm divides Fig. 7.2 How the cell cycle works 7.3 Chromosomes • Diploid cells have two similar copies of each chromosomes • Chromosomes exist in somatic cells as pairs – Homologous chromosomes or homologs • Replicated chromosomes consist of two identical sister chromatids – These are held together at the centromere Fig. 7.3 • Before a cell starts dividing, the chromosomes duplicate • This process produces identical sister chromatids Chromosome duplication Sister chromatids • When the cell divides, the sister chromatids separate • Each of the two daughter cells has a complete and identical set of chromosomes Centromere Chromosome distribution to daughter cells • Humans have 46 chromosomes or 23 pairs of homologs • The chromosomes can be organized by size into a karyotype Fig. 7.4 • • A typical human chromosome contains about 1000 genes and about 140 million nucleotides in its DNA • About 5 cm in stretched length • 2,000 printed books of 1,000 pages each! In the cell, however, the DNA is coiled into chromatin – Complex of DNA (~ 40%) and proteins (~ 60%) Levels of eukaryotic chromosome organization •The DNA helix is wrapped around positively-charged proteins, called histones •200 nucleotides of DNA coil around a core of histones, forming a nucleosome •The nucleosomes coil into solenoids •Solenoids are then organized into looped domains •The looped domains appear to form rosettes on scaffolds Fig. 7.5 7.2 Eukaryotes Have a Complex Cell Cycle Eukaryotic cells divide in one of two ways – Meiosis • Occurs in germ (reproductive) cells • Results in the production of gametes – Mitosis • Occurs in somatic (nonreproductive) cells Haploid gametes (n = 23) Egg cell Sperm cell MEIOSIS FERTILIZATION Diploid zygote (2n = 46) Multicellular diploid adults (2n = 46) Mitosis and development 7.4 Mitisos Mitotic cell division allows a fertilized egg to develop into an embryo, an adolescent and eventually an adult sea urchin embryo • Organisms can regenerate some body parts These sea stars are regenerating using mitosis. What can humans regenerate? skin Dead cells - Asexual reproduction, development, growth and cell replacement are mitotic divisions Epidermis, the outer layer of the skin Dividing cells Dermis Prophase • Chromosomes coil • Nuclear membrane breaks down • Spindle fibers form Metaphase • Chromosomes line up on the midline • Spindle fibers attach to centromeres Anaphase • Centromeres divide • Spindle fibers shorten • Sister chromatids separate and move to opposite poles Telophase • Cell elongates • Nuclear membrane reforms • • Chromosomes uncoil • Spindle disappears Cytokinesis • Division of the cytoplasm • Cleave furrow forms at equator of the cell • Constriction tightens by contraction of filaments • Cell is divided into two identical cells • Cytokinesis • Animal cells – Cleavage furrow forms, pinching the cell in two Fig. 7.8 • Plant cells – Cell plate forms, dividing the cell in two
