Biology 3.2 Chromosomes (Bioninja 2)

• Prokaryotes have a single circular molecule made of naked DNA and may have additional plasmids (autonomous DNA units).
• Eukaryotes have multiple linear DNA molecules packaged with histones and do not have plasmids (unless genetically modified).
• John Cairn pioneered a technique for measuring the length of DNA molecules while uncoiled (via autoradiography).
• Radioactive thymidine is incorporated into a cell’s DNA.
• Chromosomes were fixed to a photographic surface and treated with silver bromide (AgBr).
• Radiation converts silver ions into insoluble grains that is visible via electron microscopy when a film is developed.
• Diploid versus Haploid: Sexually reproducing organisms receive genetic material from both parents.
• Diploid = 2 sets of chromosomes (i.e. body cells).
• To reproduce, these organisms only pass on half their genetic material.
• Haploid = 1 set of chromosomes (i.e. sex cells / gametes).
• When haploid sex cells fuse, the diploid cell can grow into a new organism.
• Homologous chromosomes are the paired chromosomes inherited from both parents (maternal and paternal) in sexually reproducing animals.
• Homologous chromosomes have the same genes at identical loci positions.
• The specific alleles for each gene may be different.
• Humans have 23 pairs of chromosomes.
• Diploid number (2n) = 46 chromosomes.
• 22 pairs are homologous autosomes.
• Each pair has identical genes and loci.
• The 23rd pair are the sex chromosomes.
• Females have two X chromosomes (XX).
• Males have X and Y chromosomes (XY).
• The Y chromosome is responsible for the development of male sex characteristics.
• Hence, the father always determines sex.
• Chromosome number is a characteristic feature of members of a species.
• Karyotypes identify the number and types of chromosomes in a cell.
• Karyotyping is performed pre-natally to identify the sex of offspring or diagnose potential chromosome abnormalities (e.g. aneuploidies).
• Cells are collected from the amniotic fluid of the pregnant mother for amniocentesis.
• Cells are collected directly from the placental tissue for Chorionic Villi Sampling.
• A karyogram shows the chromosomes of a cell in homologous pairs of decreasing length.
• Down Syndrome (Trisomy 21) is a chromosomal disorder caused by an extra copy of chromosome 21.