BSC 215 1nd Edition Exam 2 Study Guide Chapters 4 7 Chapter 4 Genetics and Cellular Function 4 3 DNA Replication and the Cell Cycle The fundamental steps of the replication process are o The double helix unwinds from the histones o The enzyme called DNA helicase opens up one short segment of the helix at a time The point where the DNA is opened up is called the replication fork o Molecules of the enzyme DNA polymerase move along each strand and arrange marriages with complementary free nucleotides The two separated strands of DNA are copied by separate polymerase molecules proceeding opposite directions On one strand the DNA polymerase moves toward the replication fork and makes the long continuous new strand of DNA to complement the old one On the other strand the DNA polymerase moves away from the replication fork and copies only a short segment of DNA at a time The segments are then joined together by another enzyme called DNA ligase Ultimately from the old parental DNA molecule two new daughter molecules are made Each daughter DNA consists of one new helix conserved from the parental DNA The process is therefore called semiconservative replication o While DNA is synthesized in the nucleus new histones are synthesized in the cytoplasm Millions of histones are transported into the nucleus within a few minutes after DNA replication and each new DNA helix wraps around them to make new nucleosomes Each DNA polymerase works at an impressive rate of about 100 base pairs per second All 46 chromosomes are replicated in a mere 6 to 8 hours To prevent catastrophic damage to the cell there are multiple modes of correcting replication errors Changes in DNA structure called mutations can result from replication errors or from environmental factors such as radiation chemicals and viruses Since 98 of the DNA does not code for proteins the great majority of mutations do not affect protein structure at all Most cells periodically divide into two daughter cells so a cell has a life cycle extending from one division to the next This cell cycle is divided into four main phases G1 S G2 and M o G1 is the first gap phase an interval between cell division and DNA replication During this time a cell synthesizes proteins grows and carries out its preordained tasks for the body o S is the synthesis phase in which a cell makes a duplicate copy of its centrioles and all of its nuclear DNA o G2 the second gap phase is a relatively brief interval 4 6 hours between DNA replication and cell division The cell exhibits further growth makes more organelles finishes replicating its centrioles and synthesizes enzymes that control cell division o M is the mitotic phase in which a cell replicates its nucleus and then pinches in two to form two new daughter cells The M phase takes1 to 2 hours Phases G1 S and G2 are collectively called interphase the time between M phases Stomach and skin cells divide rapidly whereas bone and cartilage cells divide slowly Some cells leave the cell cycle and cease to divide for days years or the rest of one s life mature neurons skeletal muscles cells and adipocytes o Such cells are said to be in the G0 G zero phase Cells divide by two mechanisms called mitosis most cells and meiosis only used for reproductive cells Mitosis serves all the other functions of cell division o Development of an individual composed of some 50 trillion cells and from a one celled fertilized egg o Growth of all tissues and organs after birth o Replacement of cells that die and o Repair of damaged tissues Four phases of mitosis are recognizable prophase metaphase anaphase and telophase o Prophase The chromosomes shorten and thicken The nuclear envelope disintegrates and releases the chromosomes into the cytosol The centrioles begin to sprout spindle fibers o Metaphase The chromosomes are aligned on the cell equator and awaiting a signal that stimulates each of them to split in two at the centromere o Anaphase The two sister chromatids separate at the centromere One daughter chromosome migrates to each pole of the cell with its centromere leading the way o Telophase The chromatids cluster on each side of the cell The rough ER produces a new nuclear envelope around each cluster and the chromatids begin to uncoil and return to chromatin form Telophase is at the end of nuclear division but overlaps with cytokinesis division of the cytoplasm During cytokinesis there is a crease called the cleavage furrow that is created around the equator of the cell 4 4 Chromosomes and Heredity Heredity is the transmission of genetic characteristics from parent to offspring The agent of heredity are the genes and the genes are carried on the chromosomes When all 46 chromosomes are laid out in order by size and other physical features we get a chart called a karyotype Chromosomes appear in 23 pairs the two members of each pair are called homologous chromosomes One member form each pair is inherited from the individual s mother and one from the father Chromosomes X and Y are called the sex chromosomes because they determine the individual s sex all the others are called autosomes Any cell with 23 pairs of chromosomes is said to be diploid Sperm and egg cells are haploid meaning they contain only 23 unpaired chromosomes Sperm and egg cells are called germ cells and all other cells of the body are called somatic cells Genes and Alleles The location of a particular gene on a chromosome is called its locus Homologous chromosomes have the same gene at the same locus although they may carry different forms of that gene called alleles which produce alternative forms of a particular trait o One allele is dominant and the other one recessive o If at least one chromosome carries the dominant allele the corresponding trait is usually detectable in the individual Typically dominant alleles code for a normal functional protein and recessive alleles for a nonfunctional variant of the protein Individuals with two identical alleles such as CC or cc are said to be homozygous for that trait If the homologous chromosomes have different alleles for that gene Cc the individual is heterozygous The paired alleles that an individual possesses for a particular trait constitute the genotype An observable trait such as cleft or uncleft chin is called the phenotype We say that an allele is expressed if it shows in the phenotype of an individual o The only way most recessive alleles can be expressed is for an individual to inherit them from both parents Sex