Cells divide because new body cells are needed for growth, repair, and replacement throughout the body (in skin, muscles, etc.), as well as to make gametes (sperm and egg cells) for sexual reproduction.
There are 3 types of cell division: • Binary fission- (show first part of animation) • occurs in bacteria only • DNA (single, circular chromosome) is copied and attached to cell wall • New cell membrane and wall form between 2 chromosomes • Cell splits into 2 identical cells • asexual reproduction – two genetically identical organisms are made from only one parent
Mitosis - process that makes new diploid body cells • diploid (2n) – full set of chromosomes (one from each parent) • somatic cells - diploid body cells • eukaryotic cells go through mitosis • Meiosis - process that makes new haploid gametes • haploid (n) – one set of chromosomes • gametic cells – haploid sex cells (egg and sperm) • eukaryotic cells
DNA and Chromosomes: • DNA is contained inside the nucleus of a cell. • When the cell is not dividing, DNA is all stretched out and wound around histone proteins—this is called chromatin. It looks like spaghetti noodles! • After DNA replication, DNA loops and folds to form X- shaped chromosomes • One side of the X is called a chromatid. • One side of the X is identical to the other side = sister chromatids • Two chromosomes that are the same size with the same bands are homologous pairs
After DNA is replicated, you have 2 sets of each chromosome • Sister chromatids are joined at a region called the centromere • The centromere contains special proteins called kinetochores to attach spindle fibers later
Each human has the same 46 chromosomes, or 23 pairs of chromosomes, in every cell of their body • Chromosome pairs 1-22 look alike and code for body traits—they are called autosomes. • The 23rd pair determines the sex of the individual (XX=female, XY=male)—they are called sex chromosomes. • Matching pairs of chromosomes are called homologous pairs. Females (XX) have 23 homologous pairs while males (XY) have 22 homologous pairs and 1 non-homologous pair • Half of the chromosomes come from the mom, and half come from the dad. • Example: 1 homologous pair is made of 1 chromosome from mom and 1 chromosome from dad
How many: Homologous pairs? Autosomes? Sex chromosomes?