By Aimee Chavez

 Regular body cells continuously make copies of themselves for growth and repair.  This process is called the Cell Cycle.

 The amount of time for the cell cycle to occur depends on the type of cell involved for example bean plants can take 19 hours, animal embryos 20 min., and some human cells 16 hours. The majority of the time occurs in the interphase of the cell cycle.

 Most of the life of any eukaryotic cell is spent in a period of growth and development called interphase. Nerve and Muscle Cells are always in interphase. The DNA which is part of chromosomes is copied and it is now a chromatid. Other organelles are also copied.

 In this process the nucleus divides.  The Nuclear membrane/envelope and the nucleolus break (small dense region within nuclei in which the assembly of proteins begins) down.  Has four parts:  Prophase  Metaphase  Anaphase  Telophase 

 1. Prophase: pairs of chromatids are visible  Centrioles separate and spindle fibers begin to form.  2. Metaphase: Chromatids line up in the center of the cell and the centromeres attach to spindle fibers.

 Anaphase: The sister chromatids separate into individual chromosomes and are moved apart.  Telophase: The chromosomes gather at opposite ends of the cell and lose the distinct shapes. Two new nuclear membranes form.

 Cytokinesis: The cytoplasm pinches in half. Each daughter cell has an identical set of duplicate chromosomes.  After the cell cycle there are two cells  each cell has 46 chromosomes and they are exact copies of each other.

 Sexual reproduction: 2 parents and one or many offspring with genetic diversity  Examples: sperm + egg = zygote  seed plants  Asexual reproduction: 1 parent and one or many offspring which are an exact copy of the parent.  Some organisms reproduce both ways!

 Some vegetables such as sweet potatoes which grow sprouts can reproduce asexually.  Strawberries form runners  Some organisms such as hydra and sponges can reproduce through a process called budding. 

 Sea stars, Planarians, Lizards can regrow a body part called regeneration. regeneration  Bacteria and amoeba reproduce via binary fission. Bacteria

 Meiosis: a process that produces 4 sex cells (haploid) which have 23 chromosomes. Sperm or egg.  Meiosis has two phases, meiosis 1 and meiosis 2. During meiosis 1 two cells form and during meiosis 2 four cells form.  Often there are mistakes during meiosis and a cell can have too few or too many chromosomes. I.e. Downs syndrome

 In 1952 Rosalind Franklin observed the structure of DNA by x-rays.  In 1953 Watson & Crick DNA model

 DNA: deoxyribonucleic acid is made sugar – phosphate back bones and N-base pairs in the rungs.  The N bases are:  Adenine (A) : Thymine (T)  Guanine (G) : Cytosine (C)

 1. Dna polymerase separates the DNA sides and the molecule unzips.  2. New bases pair with bases on original DNA.  3. Two new identical DNA molecules are produced.

 The instructions for making proteins are found on genes which are sections of DNA on a chromosome. Genes are found in the nucleus, but proteins are carried from the nucleus to the ribosomes by another type of nucleic acid called ribonucleic acid, or rna.

 RNA: ribonucleic acid is a single strand structure, it contains ribose sugars and instead of thymine has the nitrogen base of uracil (U). There are three main types:  Messenger RNA: copies the instructions from the DNA.  Ribosomal RNA: along with proteins forms the ribosomes which is the site for protein synthesis.  Transfer RNA: carries amino acids and adds them to the growing protein.

 Mistakes in copying DNA can occur either by adding extra chromosomes, missing chromosomes, shifting.  Mutations can be either harmful, helpful, or neutral.