BIO 1103 Exam II Readings 02 21 2013 14 3 How DNA is Replicated 1 The hydrogen bonds connecting the two strands of the DNA molecule are broken as the two strands are unwound by special proteins 2 Each strand is then used as a template for the construction of a new strand of DNA 3 When this process is completes there are two identical copies of the original DNA molecule each with the same sequence of bases Each copy is composed of a template strand of DNA from the original DNA molecule and one newly synthesized strand of DNA Those mode of replication is known as semiconservative replication because the old strand the template strand is retained or conserved in each new double helix o the key enzyme in DNA replication and repair is know known as DNA Polymerase o despite copying billions of nucleotides an hour cells make remarkably few mistakes when they copy DNA o 14 4 Repairing Replication Errors and Damaged DNA cells can make errors by aligning the incorrect proteins in the transcription phase of the DNA replication because 99 of cells repair their errors almost instantly the chances of error in DNA replication is about 1 in a billion When an error does occur in the alignment of proteins it is then replicated in every DNA replication after that Mutation a change to the sequence of bases in an organisms DNA replication o Also refers to changes in the DNA sequence that are created by chromosomal abnormalities o Mutations result the formation of new alleles o some can be beneficial most are either neutral or harmful o harmful alleles cause cancer and other health disorders such as cycle cell anemia or Huntington s disease Mutagen substances or energy sources that alter DNA Changes in the chromosomal organization results in changes of many genes o This leads to adding deleting or rearranging the nucleotide sequence Causes of cell damage Radiation or heat Collision with other molecules in the cell Attacks by viruses Random chemical accidents result of normal metabolic process or pollutants DNA Repair steps 1 Recognized 2 Removed 3 Replaced o mutations are the consequences of failure in the steps of DNA repair 10 1 Why Cells Divide cells divide for two basic reasons to reproduce organisms to grow and repair a multicellular organism cell division generates daughter cells from a parent cell with the transfer of genetic information in the form of DNA form the parent to the daughter cells all types of organisms need cell division to create the next veneration of individuals new individuals which receive DNA from their parents are call offspring asexual reproduction clones offspring that are identical to parent cells sexual reproduction offspring that take information from two individuals of opposite mating types both propagate the species but sexual reproduction increases diversity in the population sexual reproduction more likely to develop resistance to new viruses asexual is more cost effective no need to search for a mate many bacteria use binary fission for asexual reproduction binary fission prokaryotic single celled organisms split in two to replicate steps of binary fission duplication of the DNA giving rise to two DNA molecules cell expands and a partition appears roughly though the center of the cell partition consists of plasma membrane and usually cell wall material segregates the two DNA molecules into separate cytoplasmic compartments each compartment expands until the two separate creating two daughter cells from the parent cell binary fission is a form of asexual reproduction because it produces two genetically identical daughter cells identical to the parent cell mitotic division the process that generates two genetically identical daughter cells from a single parent cell in eukaryotes mitosis the division of the nucleus cytokinesis the splitting of the original cytoplasm into two daughter cells one complete copy of DNA is delivered to opposite sides of the original cells all multicellular organisms rely on mitotic divisions for the growth of tissues and organs and the body as a whole and for repairing injured tissue and replacing worn out cells meiosis specialized type of cell division that makes reproduction possible in animals male body s generate sperm cells while females produce daughter cells which matures into eggs These are called gametes reduces the amount of genetic information in the cell by half the single set of genetic information is called the haploid set represented by n somatic cells non sex cells in multicellular organisms twice as much genetic information this is known as the diploid set represented by 2n zygote the merging of the egg and sperm cells 2n in animals the zygote divides mitotically to create a mass of developing cells known as the embryo embryo fetus juvenile adult germ line cells create gametocytes which undergo meiosis and create gametes egg cells in females and sperm cells in males 10 2 The Cell Cycle interphase the longest stage in the cell cycle divided into three main intervals G1 S and G2 G1 cell has grown and is ready to replicate its DNA S DNA replication happens here G2 checks are made to ensure that conditions are suitable for cell division Gzero many cells in the human body exit the cell cycle and remain in a resting stage for some time Cell division the replicated DNA is separated with precision Mitosis and cytokinesis M DNA molecules are divided equally between the two daughter cells Runaway cell division can turn into a cancer Cancer cells develop by the creation of tumors Tumors are the result of unregulated cell division in which cells begin to multiply rapidly creating a group of rogue cells Growth factors hormones and proteins in human cells that influence the commitment to divide When the signal to divide is given special cell cycle regulatory proteins are activated to control division and prevent tumors o These regulatory proteins give the cell the green light to pass onto the next stage of the cell cycle past the checkpoints between G1 S and between G2 M Apoptosis a form of programmed cell death begins with mitochondrial damage followed by the activation of protein destroying enzymes caspases that digest the cell from the inside Cell shrinks and the DNA is broken into fragments as the cell begins to die Cell remnants are typically engulfed by phagocytes the immune systems cleanup crew Stages of Mitosis and Cytokinesis from one cell to two identical cells Interphase DNA is replicated during the S phase before mitosis