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Chromosomes PART 1 Honors Genetics Ms. Day Why is Cell Division Important? ¡ Unicellular organisms l Reproduce by cell division à increasing the population. 100 µm (a) Reproduction. An amoeba, a single-celled eukaryote, is dividing into two cells. Each new cell will be an individual organism (LM). Why Do Multicellular Organisms Depend on Cell Division? 1. 2. 3. Development & Growth Repair (ex: tissue renewal) Maintenance 200 µm (b) Growth and development. This micrograph shows a sand dollar embryo shortly after the fertilized egg divided, forming two cells (LM). 20 µm (c) Tissue renewal. These dividing bone marrow cells (arrow) will give rise to new blood cells (LM). Cell Division (Mitosis) 2 genetically identical daughter cells from 1 parent cell ¡ Before cells divide l They duplicate their genetic material à ensures that each daughter cell receives an exact copy of the genetic material, DNA ¡ Makes Organization of the Genetic Material ¡ All of a cell’s DNA (genetic information) is called its genome l In prokaryotes ¡ Genome=single, long DNA molecule in a circle l In eukaryotes ¡ Genome = several DNA molecules grouped in clumps (called chromosomes) Forms of DNA ¡ DNA (in nucleus of eukaryotes) can be in 2 forms l Chromatin : DNA is not tightly packed together (loosely coiled) ¡ Occurs during interphase (most of cell’s life) l Chromosomes : tightly packed together (TIGHTLY coiled) ¡ Occurs during mitosis (cell division) Tightly coiled? Not tightly coiled? How Does DNA Fit into a Small Cell? ¡ Prokaryotic organisms l No nucleus; instead have a nucleiod region ¡ DNA = 1 chromosome How Does DNA Fit into a Small Cell? ¡ Eukaryotic l chromosomes stored within the cell's nucleus ¡ DNA is coiled around proteins known as histones l Group of histones is called a nucleosome ¡ helps form a “supercoil” DNA + histones è form nucleosomes (help to “super coil DNA) Nucleosomes • Like beads on a string Histone Tails +! -! Tails have positive charge; DNA has negative charge Histone Modification ¡ Adding acetyl group to the tail (acetylation) 1. 2. 3. Neutralizes charges Making DNA less tightly coiled Increasing transcription DNA Modification ¡ Adding a methyl group to the DNA (methylation) Maintains the positive charge l Makes DNA more coiled l Reduces transcription l Gene is “active”! • Switched on • Open chromatin • Transcription can happen • Unmethylated cytosines (white circles) • Acetyl groups ARE on histones Gene is “inactive” ! • No transcription • Methylated cytosines (red circles) • No acetyl groups on histones Making Chromosomes **Occurs right before cell division (mitosis) Steps: 1. DNA copies itself through DNA Replication 2. The DNA coils (wraps) around proteins called histone proteins. 3. Chromosomes are made. The DNA molecules in a cell Are packaged into chromosomes 50 µm Chromosome Structure Two shapes of chromosomes 1. Single chromosome (1 copy of DNA); “V” shaped 2. Double Chromosome (2 copies of DNA); “X” shaped; a.k.a-duplicated chromosome Occurs right before cell division (mitosis) Chromosomes and Cell Division preparation for cell division (mitosis) l DNA is replicated (single à double chromosome) ¡ In l V shape à X shape has two sister chromatids, which separate during cell division 0.5 µm A eukaryotic cell has multiple chromosomes, one of which is represented here. Before duplication, each chromosome has a single DNA molecule. Once duplicated, a chromosome consists of two sister chromatids connected at the centromere. Each chromatid contains a copy of the DNA molecule. Mechanical processes separate the sister chromatids into two chromosomes and distribute them to two daughter cells. Figure 12.4 Chromosome duplication (including DNA synthesis) Centromere Separation of sister chromatids Centromeres Sister chromatids Sister chromatids Double Chromosome Structure Kinetochore attaches to spindle fibers Sister Another view… Draw and Label the Pieces of a Duplicated Chromosome Chromosomes ¡ Every eukaryotic species has a characteristic, unique # of chromosomes in EACH cell nucleus l Ex: Humans have 46 chromosomes ¡ # of chromosomes does NOT necessarily equal complexity Different Types of Cells ¡ There are 2 types of cells in the human body 1. Somatic cells: all body cells 2. Gametes: reproductive cells (sperm and egg) Different cell types à different cell divisions ¡ Eukaryotic cell division consists of 1. Mitosis l Occurs in somatic cells 2. meiosis l Occurs in GERM LINE CELL (a special somatic cell in gonads) l CREATES gamete cells Chromosome Number ¡ Gametes (sperm/egg) have 1 copy of each chromosome l called haploid l ¡ Somatic (body) have 2 copies of each chromosomes l called diploid l Chromosomes come in PAIRS l All pairs of chromosomes differ in size, shapes, and set of genes. Haploid vs. Diploid ¡ Haploid= when a cell has 1 copy of each chromosome. ¡ Expressed ¡ as n = 23 Diploid= when a cell has 2 copy of each chromosome. (a.k.a TOTAL # of C’s in organisms) ¡ Expressed Haploid egg cell as 2n = 46 n n Haploid sperm cell Fertilization 2n 1st diploid Zygote cell of an organism Set of Chromosomes ¡ ¡ ¡ Each chromosome set contain 2 “homologues” or “twins” Chromosomes have a “twin” or matching pair Homologous Chromosome Pair = 2 chromosomes that are similar in shape, size, and genes. ¡ Each homologue in a pair comes from each parent!