Chromosomes and Cell Cycle All genetic material of a cell is called the genome Genome is composed of DNA Long molecules of DNA organized for cell division are called chromosomes Each chromosome contains thousands of genes Chromosomes contain genes Chromosomes stained orange DNA material in a cell Found to be loosely coiled in the cell and is called chromatin For cellular division the DNA is condensed to form chromosomes DNA condensation generates chromosomes DNA is packaged into chromosomes DNA wraps around a histone core forming nucleosomes. Multiple nucleosomes are organized into solenoids solenoid Condensed further to form chromosomes What happens to a chromosome if you remove the scaffolding proteins? Histones have been removed from this metaphase chromosome Scaffolding proteins retain shape of the chromosome Do all organisms have DNA? Yes! Do all organisms have The same amount of DNA? The same number of chromosomes? The number of chromosomes varies with each organism Humans have 23 pairs of chromosomes 22 pairs of autosomes 1 pair of sex chromosomes Chromosome analysis - Giemsa staining Giemsa stain is used to visualize chromosomes There are 22 pairs of autosomes and 1 pair of sex chromosomes Chromosome analysis - Fluorescence in-situ hybridization (FISH) A technique to visualize specific regions of chromosomes using fluorescent DNA probes. Chromosome analysis - Chromosome Paints DNA is labeled with different fluorescent dyes Dyes paint specific regions Remember that chromosomes represent duplicated DNA These structures are formed to facilitate the proper partitioning of DNA during cell division The structure represents twice as much DNA, since DNA replication has occurred Chromosome Structure centromere q (long arm) p (short arm) Chromosome split the chromatids during cell division All cells need to divide to pass on their genetic information by cell division Cell Cycle reflects the life of a cell Cell Cycle G1 phase First growth phase, cells are making proteins S phase DNA replication occurs G2 phase Growth phase, checking of DNA M phase Cell division Mitosis How does cancer relate to the cell cycle? There are checkpoints in the cell cycle that regulate each stage G1 G2 M Cancer cells have lost the control of cell division Checkpoints in the cell cycle Do all human cells divide by mitosis? Remember that mitosis provides a new cells with a copy of the EXACT same chromosomes, 23 pairs What about the sex cells? How many chromosomes are present in them? Meiosis is a special type of cellular division Sex cells have ONE set of 23 chromosomes Sex cells are considered haploid When two haploid cells fuse they form a diploid cell (one that has 23 PAIRS of chromosomes) How do we end up with half the number of chromosomes in meiosis? Need TWO rounds of division Meiosis I Meiosis II Final result is 4 daughter cells with haploid chromosomes Meiosis Overview Meiosis I Meiosis II Crossing Over in Meiosis Summary Comparison of mitosis and meiosis Key points to notice Only in meiosis I does synapsis occur which allows for recombination or crossing over of chromosomes Does this process occur in mitosis? Nondisjunction occurs when chromosomes fail to separate Can occur either during mitosis or meiosis What happens if it occurs in Meiosis I? What happens if it occurs in Meiosis II? Trisomy 21 Down syndrome Most common chromosome number abnormality Occurs at a rate of 1/700 births Incidence of Down syndrome increases with age of mother Human Sex Determination Textbook Story: XX = Female XY = Male With a few exceptions. XXY = Klinefelter Syndrome- phenotypically male with normal intelligence, sterile XO = Turner syndrome- phenotypically female, sterile, usually normal intelligence XYY = no general traits (tall) XXX = healthy, normal Aneuploidy can also occur with Sex Chromosomes The SRY Gene Down Syndrome may be inherited Robertsonian translocation accounts for 5% of down syndrome cases Occurs between chromosomes 21 and 14