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Chromosomes and Cell Reproduction Chromosome structure and role in development and reproduction What allows a few cells to grow into a human being? During cell division, what allows each cell to receive the proper genes? Objectives • Identify examples of cell division in prokaryotes and eukaryotes • Distinguish between a gene, a DNA molecule, a chromosome, and a chromatid • Differentiate between homologous chromosomes, autosomes, and sex chromosomes • Compare haploid and diploid cells • Predict how changes in chromosomes can affect development Why cell division? • • • • Growth Tissue repair Replace worn out cells Asexual reproduction Why are we interested in chromosomes? • Vitally important that each new cell receives the proper set of chromosomes to function properly • Each organism has a characteristic number of chromosomes • We need to understand the structure of chromosomes and the role of chromosomes in the cell Different types of cell division – all involve passing on DNA • Bacterial cell division – Binary Fission • Division of body cells in eukaryotes - Mitosis • Formation of gametes (reproductive cells) in eukaryotes - Meiosis Prokaryotic Cell Division • Called Binary Fission • Form of asexual reproduction • Single parent passes on exact copies of all DNA to offspring • First stage = single circular DNA molecule is copied • Second stage = Cell divides Eukaryotic Cell Division • Much more complicated process due to larger amount of genetic information (genes) • Genes = segment of DNA that codes for a protein or RNA molecule • When genes being used, DNA (chromatin – DNA and its associated proteins) is stretched out • Before cell division, DNA must divide and then coil up into chromosomes Chromatids and Centromere • Chromatids will become separated during cell division • 1 Chromatid will be placed into each new cell • This ensures that each new cell will have the same genetic material as the original cell Chromosome Number and Structure • Each human body cell (excluding sperm and egg cells) has 2 copies of 23 different chromosomes • Total chromosomes = 46 • Each chromosome contains thousands of genes • Genes play important role in how person’s body develops and function Homologous Chromosomes • Each pair of chromosomes = homologous chromosomes • Chromosomes similar in size, shape and genetic content • Each homologue comes from one of the two parents • One set = Ma • One set = Pa Somatic Vs Sex Cells = Diploid Vs Haploid • Body cells (non-sex cells) = somatic cells • A cell that contains 2 sets of chromosomes (i.e. a somatic cell) = diploid or 2n • A cell that contains only one set of chromosomes (i.e. a sex cell or gamete) = haploid or n • For humans n = 23. That is how many chromosomes our gametes have. 2n = 46. That is how many chromosomes our somatic cells have Zygote • Fusion of two haploid gametes = fertilization • Diploid zygote formed by fertilization = first cell of a new individual Different Types of Chromosomes Autosomes Sex Chromosomes • Chromosomes that are not directly involved in determining the sex (gender) of an individual • One of the 23 pairs of chromosomes in humans • Determine the sex of the individual • Humans and many other animals - X and Y chromosomes • Anyone with a Y chromosome is male. • Female = XX; Male = XY • Female can only give an X, so sex of offspring determined by father Change in chromosome number • Genes help determine how body develops and functions • All 46 chromosomes must be present for normal development/function • Too many or too few chromosomes leads to problems • Abnormalities in chromosome number detected by a karyotype - a photo of chromosomes in dividing cell - shows chromosomes arranged by size Down Syndrome – Trisomy 21 • Extra copy of chromosome 21 • Short stature, round face, mental retardation • Occurs more frequently in older mothers • Mothers younger than 30 – 1 in 1500 births • Mothers over 45 – 1 in 46 births How does someone end up with an extra chromosome? • When sperm and egg cells form (during meiosis) each homologous pair separates (disjunction) • If chromosomes fail to separate (non-disjunction) one gamete ends up with both chromosomes and the other gamete gets none • Trisomy – gamete with both chromosomes fuses with normal gamete Changes in chromosome structure • Mutations = changes in chromosome structure • Chromosome breakage 4 types of mutations • Duplication = chromosome fragment attaches to homologous chromosome (which then carries 2 copies of set of genes) • Inversion = chromosome fragment reattaches to original chromosome but in reverse order • Translocation = chromosome fragment reattaches to non-homologous chromosome • Deletion = a piece of a chromosome breaks off Types of mutations Inversion Translocation Types of Mutations Deletion Duplication