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Chromosome “Wrap-up” How does so much DNA fit in Nucleus? Even the shortest human chromosome contains more DNA than would fit into the nucleus (46 b. base pairs) If it weren’t condensed, it would take up 1.5 cm of length Chromosomes must condense to fit in nuclei “Packing” a punch! The smallest chromosome DOESN’T take up 1.5 cm, it only takes up 2 micrometers during its condensed state of mitosis/meiosis How? DNA is organized into a hierarchy of “packing” Levels of DNA Packing in Chromosomes Level 1 – 4 nm DNA is wound around a protein core to look like a bead called a “nucleosome” 10 nm thick Levels of DNA Packing in Chromosomes Level 2 – Further coiling results in 30 nm nucleoprotein fibers – Looped addtionally to form metaphase chromosomes 700nm DNA coiling The Coils of Chromosomes Basic chromosome structure P arm kinetochore Q arm of chromosome chiasmata nucleosome Telomere Recombinant DNA Nucleo protein fiber Chromosomes Carry Genes CHROMOSOMES Human chromosomes are stored in the nucleus Chromosomes have DNA wrapped around special histone proteins. “Beads on string”: nucleosomes organize DNA in human genome DNA Base-Pairs: DNA Helix A-T G-C How Many Chromosomes Are There? Bacteria usually have one circular chromosome and no nucleus Organisms with nuclei have variable numbers of chromosomes depending on the species: • Mosquito 6 • Chimpanzees 48 • Goldfish 94 http://gnn.tigr.org/whats_a_genome/Chp1_3_1.shtml You Inherited Two Copies of Each Chromosome, One from Mom and One from Dad You inherited two copies of each gene, one from Mom and one copy from Dad. One copy of chromosome #3 from Mom All 46 Human chromosomes together is called a human karyotype. The X and Y chromosomes are the sex chromosomes. The 22 chromosomes (not the sex chromosomes) are called autosomes. One copy of chromosome #3 from Dad Human Cells Have 22 Sets of Paired Chromosomes 22 SETS OF PAIRED AUTOSOMES EACH 2 sex chromosomes 2 sex chromosomes Almost every cell in your body (except RBC, eggs, sperm) has two sets of genes carried on 22 sets of paired chromosomes, plus 2 sex chromosomes. What is the Relationship Between DNA and Chromosomes? ? Each Chromosome Contains Hundreds of Genes! DNA CELL CHROMOSOMES DNA G E N E NUCLEUS DNA Helix DNA GENE 1 GENE 6... A GENE 2 GENE 5 GENE 3 GENE 4 DNA is Packaged Into Chromosomes DNA double helix GENE (blue) Several GENES along a length of DNA DNA is coiled around proteins (more on this later) Small region of “unwrapped” chromosome Loosely wrapped DNA in chromosome Tightly wrapped chromosome Adapted from Alberts et al. Molecular Cell Biology Chromosomes are Dynamic Structures Human Chromosomes CONDENSED EXTENDED Metaphase Chromosome Fruit Fly One Human Chromosome Contains One ds DNA Helix Short arm (‘p’) Centromere Long arm (‘q’) ONE double-stranded (ds) DNA molecule extends from one end of the chromosome to the other end of the chromosome. The chromosome on the left has duplicated (replicated). How many ds DNA helices does it contain? Human Chromosomes Have Two Ends and a Middle! The ends of a linear chromosome are called telomeres. Telomeres play a special role when the chromosome DNA duplicates during cell division. Centromeres are usually in the middle of the chromosome, but can be near one end. What essential role do centromeres play in cell division? Telomere Centromeres are Stained Centromere Telomeres are Stained Telomere Human Chromosome Karyotypes 1 2 3 4 6 7 8 9 10 11 Which gender results from inheriting these chromosomes? Banding patterns on chromosomes represents different staining of A-T and G-C rich regions in the DNA What Happens to Chromosomes and Genes When Cells Divide? CENTROMERE ONE DOUBLESTRANDED DNA HELIX DNA Replication (Duplication) DNA Replication (Duplication) TWO DOUBLESTRANDED DNA HELICES CENTROMERE Telomeres are needed to duplicate the very ends of the chromosomes Before A Cell Can Divide To Make Two Cells, It Must Duplicate It’s Genes, Chromosomes and DNA 46 chr Chromosomes duplicate and line-up on metaphase plate [46 x 2 = 92] Start here Chromosomes move to opposite poles chromosomes attach to spindle fibers 46 chromosomes In each daughter cell Centromeres are Critical for Chromosome (Gene) Inheritance Spindle fibers attach to centromeres on the Metaphase chromosomes. Spindle Pole Spindle Microtubule Centromeres Molecular motors at centromere move the chromosome along the spindle fiber to the pole. Anaphase Down Syndrome: A Consequence of Centromere Failure and Incorrect Chromosome Inheritance A special type of cell division (meiosis) generates germ cells: eggs and sperm 46 46 A mistake in meiosis during egg production creates an egg with one additional chromosome (#21) Chromosome number 46 46 46 46 meiosis meiosis 23 23 germ cells 24 23 germ cells 23 23 fertilization 24 23 fertilization 46 fertilized zygote 47 fertilized zygote How Does Down Syndrome Affect People Who Inherit an Extra Copy of Chromosome 21? Down Syndrome identified by John Langdon Down (1866) who noticed that some people looked similar although they were not biologically related. Down Syndrome features: a broad flat face, small nose and variable levels of mental impairment. Called “Mongolism” in 1866 but now properly called Down Syndrome. Down Syndrome is caused by Trisomy 21 (3 copies of chr 21) Extra copy of chromosome 21 causes the “Down” characteristics. Most Down people have 47 chromosomes in every cell. Others have 46 chromosomes in some cells and 47 chromosomes in other cells (mosaic) and have less severe Down symptoms. http://www.genetics.com.au Genetics Fact Sheet #24A Down Syndrome Risk Increases with Age of Mother Down Syndrome occurs more frequently in women over age 35 years. Down Syndrome can also be caused by inheriting an extra chromosome 21 from the father’s sperm. Risk does not increase with age. Age of Mother: 20-24 yrs risk is 1 in 1474 35 yrs risk is 1 in 384 40 yrs risk is 1 in 112 45 yrs risk is 1 in 28 There is no cure for a child with Down Syndrome. Note: Risk of having child with a chromosome abnormality also increases with age of mother: There are pre-natal screening tests available for Down Syndrome. Discuss this in unit on genetic testing. 20-24 yrs 1 in 506 35 yrs 1 in 179 40 yrs 1 in 64 45 yrs 1 in 19 Scientists are looking for the regions of chromosome 21 that are most likely to be involved in causing Down Syndrome. Other Genetic Disorders Caused by Chromosome Abnormalities Klinefelter Syndrome: 46,XXY (“Normal” is 46,XY) Klinefelter affects only boys with risk of 1 in 500 to 1 in 1000 males born. (fairly common) Symptoms include sparse facial hair, inability to form sperm (infertility but sexuality is “normal”), learning difficulties Klinefelter caused by a mistake in segregation of the X chromosome at or soon after conception. When the mistake occurs after conception, Klinefelter boys can be mosaic (have some 46,XY cells and some 46,XXY cells) Klinefelter is not related to the age of the mother or father at conception. There is no cure although treatment with testosterone can reduce symptoms. There are pre-natal screening tests available. Discuss this in unit on genetic testing. Klinefelter occurs sporadically for unknown reasons. http://www.genetics.com.au Genetics Fact Sheet #24B