M4

• Deoxyribonucleic acid or DNA

  • A polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group 
  • Chargaff’s rules (by Erwin Chargaff)
  • The number of A and T bases are equal, and the number of G and C bases are equal
• • In 1953, James Watson and Francis Crick introduced a double-helical model structure based on Rosalind Franklin’s X-ray crystallography of the DNA molecule
  • Watson and Crick determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C) 
  • The Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C
• The Basic Principle: base pairing to a template strand

  • Since the complementary DNA strands act as a template for building a new strand in replication
  • The parent module unwinds, and two new daughter strands are built based on base-pairing rules
  • Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand and one newly made strand
• • DNA replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble.” 
  • A eukaryotic chromosome may have hundreds or even thousands of origins of replication
  • Replication proceeds in both directions from each origin, until the entire molecule is copied
• • Replication fork - a y-shaped region at the end of each replication bubble where new DNA strands are elongating
  • Helicases - enzymes that untwist the double helix at the replication forks
  • Single-strand binding proteins bind to and stabilize single-stranded DNA
  • Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
  • RNA primer - initiate synthesis of DNA polynucleotide
• Antiparallel elongation

  • The antiparallel structure of the double helix affects the replication
  • DNA polymerases add nucleotides only to the free 3’ end of a growing strand; therefore, a new DNA strand can elongate only in the 5’ to 3’ direction
  • Leading strand [5’-3’]
  • Lagging strand [3’-5’]
  • Okazaki fragments
• Replicating the Ends of DNA molecules

  • DNA polymerase create problems for the linear DNA of eukaryotic chromosomes
  • The usual replication machinery provides no way to complete the 5’ ends, so repeated rounds of replication produce shorter DNA molecules with uneven ends
  • This is not a problem for prokaryotes, most of which have circular chromosomes
• Telomeres

  • Special nucleotide sequences at the eukaryotic chromosomal DNA ends
  • Do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules
  • It has been proposed that the shortening of telomeres is connected to aging
  • In unicellular organisms, the division of one cell reproduces the entire organism
  • multicellular eukaryotes depend on cell division for 
  • Development from a fertilized cell
  • Growth
  • Repair
  • Cell division is an integral part of the cell cycle
• Distribution of chromosomes during cell division

  • In preparation for cell division, DNA is replicated, and the end chromosomes condense
  • Each duplicated chromosome has two sister chromatids
  • The centromere is where the two chromatids are most closely attached
  • Once separate, the chromatids are called chromosomes
• Phases of the cell cycle

  • Mitotic (M) phase (mitosis and cytokinesis)
  • Interphase (cell growth and copying of chromosomes in preparation for cell division)
  • G1 phase (first gap)
  • S phase (synthesis)
  • G2 phase (Second gap)
  • The cell grows during all three phases but chromosomes are duplicated only during the S phase
• The mitotic spindle

  • Structure made of microtubules that control chromosome movement during mitosis
  • Assembly of spindle microtubules begins the centrosome 
  • The spindle includes the: 
  • Centrosomes
  • Spindle microtubules
  • Asters
  • Kinetochores are protein complexes associated with centromeres
• Living organisms are distinguished by their ability to reproduce their own kind

  • Heredity is the transmission of traits from one generation to the next
  • Variation is demonstrated by the differences in appearance that offspring show from parents and siblings
  • Genetics is the scientific study of heredity and variation
• Offspring acquire genes from parents by inheriting chromosomes

  • Children do not inherit particular physical traits from their parents
  • It is genes that are actually inherited
• Comparison of asexual and sexual reproduction

  • Asexual reproduction
  • A single individual passes all of its genes to its offspring without the fusion of gametes
  • A clone is a group of genetically identical individuals from the same parent
  • Sexual reproduction
  • Two parents give rise to offspring that have unique combinations of genes inherited from them
• Karyotype

  • An ordered display of the pairs of chromosomes from a cell
  • Humans somatic cells have 23 pairs of chromosomes
  • Homologous chromosomes or homologs – the two chromosomes in each pair
  • Chromosomes in a homologous pair of genes controlling the same inherited characters
• Sex chromosomes

  • Determine the sex of the individual
  • Human females
  • Homologous pair of X chromosomes XX
  • Human males
  • One x and one y XY
• Autosomes

  • The remaining 22 pairs of chromosomes
• Each pair of homologous chromosomes includes one chromosome from each parent

  • A diploid cell (2n) has two sets of chromosomes
  • For humans, 46 (2n = 46)
  • A gamete (sperm or egg) contains a single set of chromosomes and is haploid (n)
  • For humans 23 (n = 23)
• Behavior of chromosome sets in the human life cycle

  • Fertilization
  • The union of gametes (sperm and egg)
  • Zygote
  • The fertilized egg
  • Has one set of chromosomes from each parent
  • Produces somatic cells by mitosis and develops into an adult
• At sexual maturity, the ovaries and testes produce haploid gametes

  • Gametes are produced by meiosis
  • Meiosis results in one set of chromosomes in each gamete
  • Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number
• Meiosis reduces the number of chromosome sets from diploid to haploid

  • Meiosis is preceded by the replication of chromosomes
  • Meiosis takes place in two consecutive cell divisions, meiosis 1 and meiosis II
  • The two cell divisions result in four daughter cells
  • Each daughter cell has many chromosomes as the parent cell
• Prophase I

  • Each chromosome pairs with its homolog, and crossing over occurs
  • X-shaped regions called chiasmata are sites of crossover
• Metaphase I

  • Pairs of homologs line up at the metaphase plate, with one chromosome facing each pole
  • Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad
  • Microtubules from the other pole are attached to the kinetochore of the other chromosome
• Anaphase I

  • Pairs of homologous chromosomes separate
  • One chromosome of each pair moves toward opposite poles, guided by the spindle fibers
  • Sister chromatids remain attached at the centromere and move as one unit toward the pole
• Telophase I and cytokinesis

  • Each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids
  • Cytokinesis usually occurs, simultaneously forming two haploid daughter cells
  • A cleavage furrow forms
  • No chromosome replication occurs between meiosis I and meiosis II because the chromosomes are already replicated
• Prophase II

  • A spindle apparatus forms
  • Chromosomes move toward the metaphase plate
• Metaphase II

  • The sister chromatids (that have crossed over) are arranged at the metaphase plate
  • Due to crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical
• Anaphase II

  • The sister chromatids separate
  • The sister chromatids of each chromosome now move as two newly individual chromosomes toward opposite poles
• Telophase II and cytokinesis

  • The chromosomes arrive at opposite poles
  • Nuclei form and chromosomes decondensed
  • Cytokinesis separates the cytoplasm
  • At the end of meiosis, there are four daughter cells
  • Each daughter cell is genetically distinct
• A comparison of mitosis and meiosis

  • Mitosis conserves a number of chromosome sets, producing cells that are genetically identical to the parent cell
  • Meiosis reduces the number of chromosome sets from 2n to n, producing cells that differ genetically from each other and the parent cell
• What principles account for the passing of traits from parents to offspring?

  • The “blending” hypothesis is the idea that genetic material from the two parents blends together (like blue and yellow paint blend to make green)
  • The “particulate’ hypothesis is the idea that parents pass on discrete heritable units (genes)
  • Mendel documented a particulate mechanism through his experiments with garden peas
• Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

  • Character: a heritable feature that varies among individuals, such as flower color
  • Trait: each variant for a character, such as purple or white color for flowers
  • Other advantages of using peas
  • Short generation time
  • Large numbers of offspring
  • Mating could be controlled
  • Plants could be allowed to be self–pollinated
• Mendel mated two contrasting, true-breeding varieties, a process called hybridization

  • The true-breeding parents are the P generation
  • The hybrid offspring of the P generation are called the F1 generation
  • When F1 individuals self-pollinate or cross-pollinate with other F1 hybrids, the f2 generation is produced
• Punnet square - can show possible combinations of sperm and egg

  • The capital letter represents a dominant allele
  • The lowercase letter represents a recessive allele
• Homozygous - an organism with two identical alleles for a character
• Heterozygous - an organism that has two different alleles for the gene controlling that character
• An organism’s traits do not always reveal its genetic composition due to the different effects of dominant and recessive alleles

  • Phenotype - physical appearance
  • Genotype - genetic makeup
• The Testcross

  • Used to determine the genotype 
  • A dominant phenotype could be either homozygous dominant or heterozygous
  • If any offspring display the recessive phenotype, the mystery parent must be heterozygous
  • Monohybrid cross - a cross between heterozygous following one character
• The law of independent assortment

  • Developed by Mendel using a dihybrid cross (following two characters at the same time) 
  • It states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
  • Applies only to genes on different, non-homologous chromosomes or those far apart on the same chromosome
  • Genes located near each other on the same chromosome tend to be inherited together
• Crossing two true-breeding parents differing in two characters

  • Produces dihybrids in the f1 generation
  • Heterozygous for both characters