Cell Signalling, Cell Division, Genetics by F2Qe57

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									Cell Signalling, Cell Division,
Genetics
   AP Biology Review
Cell Signalling
  Cells that are close to one another or are far
   away from each other have to communicate
  Most communication is by signal molecules
   called hormones
     Can be water soluble or lipid based
     Can bind to specific sites on cell
      membranes OR
     Can pass through membrane through active
      transport and act inside cells
Nearby cells

    Use a local regulator; 2 types
      Paracrine signalling: signal is secreted
       into extracellular fluid and it acts on a
       nearby cell
      Synaptic signalling: neuron secretes a
       signal (neurotransmitter) into a
       synapse (gap between neurons)
LE 11-4




                                 Local signaling                                          Long-distance signaling


                          Target cell                            Electrical signal      Endocrine cell         Blood
                                                                 along nerve cell                              vessel
                                                                 triggers release of
                                                                 neurotransmitter


                                                                     Neurotransmitter
   Secreting                Secretory                                 diffuses across
   cell                     vesicle                                      synapse                         Hormone travels
                                                                                                         in bloodstream
                                                                                                         to target cells


   Local regulator
   diffuses through                                  Target cell                               Target
   extracellular fluid                               is stimulated                             cell

    Paracrine signaling                     Synaptic signaling



                                                                                        Hormonal signaling
Distant Cells

  Hormones are released into the blood
   stream and travel to the target cells
  Cells must have the proper receptor to
   receive the signal and the right internal
   signaling system to respond to the signal
3 stages in cell signalling

    Stage 1:Signal reception
      Signal (ligand) binds to a receptor protein in
       the cell membrane
      Receptor protein changes shape, becomes
       activated, and can interact with other
       molecules
      If the signal receptor is in the cytoplasm, the
       signal molecules must pass through the
       plasma membrane
LE 11-5_1




  EXTRACELLULAR                                      CYTOPLASM
  FLUID
                         Plasma membrane

             Reception                Transduction

  Receptor




  Signal
  molecule
   Stage 2: Signal transduction
       Change in the receptor starts a process of cellular
        response
       Signal molecule itself is not passed on but the
        information is
       Can activate or deactivate proteins in the cell
       Can create second messengers in the cell (small
        molecules or ions that activate other enzymes in
        the cell)
LE 11-5_2




    EXTRACELLULAR                                           CYTOPLASM
    FLUID
                           Plasma membrane

               Reception                  Transduction

    Receptor




                           Relay molecules in a signal transduction
                                          pathway

    Signal
    molecule
   Stage 3: Cellular response
     Can be any cellular activity
     Can regulate the function of proteins in the
      cytoplasm
     Can regulate transcription of DNA in the
      nucleus by turning genes on or off
LE 11-5_3




  EXTRACELLULAR                                           CYTOPLASM
  FLUID
                         Plasma membrane

             Reception                  Transduction                    Response

  Receptor

                                                                      Activation
                                                                      of cellular
                                                                      response
                         Relay molecules in a signal transduction
                                        pathway

  Signal
  molecule
LE 11-6
          Hormone          EXTRACELLULAR
          (testosterone)   FLUID                  The steroid
                                               hormone testosterone
                                               passes through the
                                               plasma membrane.

                                   Plasma
                                   membrane        Testosterone binds
          Receptor                             to a receptor protein
          protein                              in the cytoplasm,
                                  Hormone-     activating it.
                                  receptor
                                  complex


                                                  The hormone-
                                               receptor complex
                                               enters the nucleus
                                               and binds to specific
                                               genes.
                           DNA

          mRNA                                     The bound protein
                                               stimulates the
                                               transcription of
                                               the gene into mRNA.
          NUCLEUS                New protein

                                                   The mRNA is
                                               translated into a
                                               specific protein.
          CYTOPLASM
LE 11-8
                                      Signal molecule




                  Receptor
                                        Activated relay
                                        molecule


             Inactive
          protein kinase
                1                                       Active
                                                       protein
                                                       kinase
                                                          1

                              Inactive
                           protein kinase        ATP
                                 2                        ADP                Active        P
                                                                            protein
                                                   PP                       kinase
                                            Pi                                 2

                                                Inactive
                                             protein kinase           ATP
                                                                               ADP                Active   P
                                                    3
                                                                                                 protein
                                                                        PP                       kinase
                                                                 Pi                                 3

                                                                       Inactive
                                                                        protein            ATP
                                                                                                    ADP                  P
                                                                                                               Active        Cellular
                                                                                               PP              protein       response
                                                                                      Pi
LE 11-14
                                   Growth factor
                                                        Reception
                                   Receptor




                                   Phosphorylation
                                         cascade
                                                     Transduction




           CYTOPLASM




                  Inactive
                  transcription Active
                  factor        transcription
                                factor                  Response
                                                P

            DNA


                                                     Gene


           NUCLEUS                      mRNA
Cell Division

    2 types
     •   Type 1: produces somatic cells—mitosis—
         maintains the chromosome number; used
         for growth and repair
     •   Type 2: produces gametes—meiosis—
         reduces chromosome number; only in testis
         and ovaries
Cell life cycle

    New cell is produced it spends 90% of
     its life span growing, metabolizing, etc
     signal is received which causes
     replication of the chromosomes cell
     enters mitosis which produces 2 identical
     cells which now grow, etc
The cell cycle

  G1: cell growth and metabolism
  G0: extended metabolism phase
  S: DNA synthesis
  G2: makes large molecules and
   organelles
  M: division of chromosomes and cell
   division
Figure 12.4 The cell cycle
Chromosome structure

  6 feet of DNA per cell are wound around
   protein spools called histones
  These are further wrapped into groups of 8
   called nucleosomes
  Most of the time DNA is in a form called
   chromatin; “spaghetti-like” form in which the
   info in the DNA molecule is accessible
  For accurate cell division chromatin must be
   condensed into chromosomes
LE 19-2a




                                                  2 nm

                  DNA double helix

   His-             Histone
   tones            tails

                                     Histone H1   10 nm



           Linker DNA          Nucleosome
           (“string”)          (“bead”)
     Nucleosomes (10-nm fiber)
LE 19-2b




                                30 nm




                   Nucleosome
     30-nm fiber
LE 19-2d




                            700 nm




                            1,400 nm



     Metaphase chromosome
Figure 12.3 Chromosome duplication and distribution during mitosis
Mitosis

  This process ensures each daughter cell
   gets the right chromosomes; 5 phases
  Prophase
     -Chromatin coils up
     -nucleoli and nuclear membrane disappear
     -centrioles divide, begin to move to opposite
       poles and forms spindles (called the
       centrosome)
   Metaphase
     Spindle fibers attach at the kinetochore
     Chromosomes line up at the equator

     Non-kinetochore fibers cause elongation of
      the cell
    • Anaphase

    -centromeres split
    -chromosomes are pulled apart toward
      opposite poles
   Telophase
       Nuclear membrane and nucleolus reform,
        chromosomes uncoil, spindle disappears, cell plate
        forms in plants
    •   Cytokinesis
       Plasma membrane pinches in as actin
        microfilaments form a ring and pinch the cell in two
    •   Daughter cells enter G1 phase
    •   Results in 2 small cells identical to parent cell
Figure 12.5 The stages of mitotic cell division in an animal cell: G2 phase; prophase;
prometaphase
Figure 12.5 The stages of mitotic cell division in an animal cell: metaphase; anaphase;
telophase and cytokinesis.
Prokaryote reproduction

    Bacteria reproduce by binary fission
      The single, circular chromosome is copied
      Any plasmids present are copied (extra
       piece of DNA that can have genes that help
       a bacteria survive)
      The cell elongates and pinches in two
       forming 2 smaller bacteria
Figure 12.10 Bacterial cell division (binary fission) (Layer 3)
Cell cycle control system
  Molecular signaling system that switches the
   cell on and off
  Checkpoints exist at different phases which
   make sure the cell has the right conditions to
   continue
  Restriction point in G1 is the most important
   checkpoint
        If the cell receives the go-ahead signal, DNA will
         replicate and cell will divide
        If not received, cell enters the G0 phase
        Some cells can re-enter the cycle if conditions
         change
Figure 12.13 Mechanical analogy for the cell cycle control system
Other factors

  Chemical growth factors required for
   cells to divide
  Density-dependent inhibition: cells stop
   growing when they come in contact with
   each other
  Cancer cells don’t respond to normal
   cell-cycle controls (depleted chemicals,
   contact) and stop dividing at random
   points in the cycle
Meiosis

  Reduces chromosome number by half
  Occurs only in reproductive organs
  Separates homologous chromosomes
   (chromosomes that contain info about
   the same traits) so only 1 of each pair
   ends up in the gametes
  Fertilization restores normal
   chromosome number
Figure 13.4 The human life cycle
Figure 13.x3 Human female karyotype shown by bright field G-banding of chromosomes
Figure 13.x5 Human male karyotype shown by bright field G-banding of chromosomes
Stages of meiosis

  In meiosis 1 the homologous
   chromosomes separate
  Prophase 1
      Homologous chromosomes pair up forming
       a tetrad
      Crossing over occurs between non-sister
       chromatids, forming chiasma
      Nuclear membrane, nucleolus breaks down;
       spindle forms from centrioles
Figure 13.10 The results of crossing over during meiosis
   Metaphase 1
     Chromosomes line up on the equator in
      pairs
     It’s purely chance as to which homolog ends
      up on which side of the cell
     Differs from mitosis because pairs line up
      on the equator, not separate chromosomes
   Anaphase
       Homologs separate and move toward
        opposite poles
   Telophase
     Cell division occurs—cytokinesis
     Cells are now haploid


    Short interphase occurs but no DNA
     replication
   In meiosis 2 the sister chromatids
    separate
     Prophase 2: chromosomes condense
     Metaphase 2: chromosome line up on the
      equator
     Anaphase 2: chromatids are pulled apart

     Telophase 2: cytokinesis occurs
Figure 13.7 The stages of meiotic cell division: Meiosis I
Figure 13.7 The stages of meiotic cell division: Meiosis II
Figure 13.6 Overview of meiosis: how meiosis reduces chromosome number
Results

    Meiosis produces 4 haploid cells
        In males—4 sperm cells are produced; occurs
         continuously from birth
        In females—1 large egg cell is formed, along with 3
         polar bodies; meiosis 1 occurs before birth, meiosis
         2 is stalled until puberty
    Non-disjunction: Failure of chromosome pairs
     to separate during meiosis
        Results in gametes with too many or too few
         chromosomes (aneuploidy, polyploidy)
Figure 13.9 The results of alternative arrangements of two homologous chromosome
pairs on the metaphase plate in meiosis I
Figure 13.8 A comparison of mitosis and meiosis
Genetics vocabulary
  Allele: alternate form of a gene;
   symbolized by a letter
  Gene: sequence of nucleotides that
   code for a certain trait
  Phenotype: outward expression of a
   gene
  Genotype: the alleles an organism has
  Homozygous: 2 of the same alleles;
   either dominant or recessive
  Heterozygous: 2 different alleles
Figure 14.3 Alleles, alternative versions of a gene
Inheritance patterns

    Complete dominance: one allele can
     completely mask another; not always the
     most common phenotype
      Dominant allele can hide another; is
       symbolized by a capital letter which is
       chosen for the dominant phenotype
      Recessive allele is hidden by the dominant;
       is symbolized by a lower case letter
Monohybrid crosses

  TT x TT– g:All TT, p: 4/4 tall
  TT x Tt– g: ½ TT ½ Tt, p: 4/4 tall
  TT x tt– g: 4/4 Tt, p: 4/4 tall
  Tt x Tt– g: ¼ TT ½ Tt ¼ tt, p: ¾ tall ¼
   short
  Tt x tt– g: ½ Tt ½ tt, p: ½ tall ½ short
  tt x tt– g: 4/4 tt, p: 4/4 short
Figure 14.2 Mendel tracked heritable characters for three generations
Incomplete dominance

  Heterozygote shows
   a blending of the
   parental traits
  CRCW , RR’, RW
Co-dominance

  Heterozygote shows both parental traits
  Red cattle x white cattle Roan cattle
  CRCW , RR’, RW
Multiple alleles

  More than 2 alleles for a given trait
  ABO blood type
      Pheno. Geno.        Antigen    antibodies
      A     IAIA or IAi         A          B
      B     IBIB or IBi         B          A
      AB    IA IB               A, B       none
      O     ii                  none       A,B
Table 14.1 The Results of Mendel’s F1 Crosses for Seven Characters in Pea Plants
Mendel’s Laws

  Law of segregation: homologous
   chromosomes separate and are
   packaged into different gametes
  Law of independent assortment: each
   pair of homologous chromosomes lines
   up independently of each other during
   meiosis 1
Figure 14.4 Mendel’s law of segregation (Layer 2)
LE 14-8
          P Generation
                                                  YYRR                      yyrr


                                                Gametes YR            yr




                                                                      YyRr
          F1 Generation
                                        Hypothesis of                      Hypothesis of
                                        dependent                           independent
                                        assortment                           assortment

                                                                                                                                             Sperm
                                                                                         1
                                                                                             4   YR       1
                                                                                                              4   Yr       1
                                                                                                                                4   yR   1
                                                                                                                                             4    yr
                                                           Sperm
                                                                           Eggs
                                         1
                                             2   YR    1
                                                           2    yr
                                                                        1           YR
                                                                            4
                          Eggs                                                               YYRR             YYRr         YyRR              YyRr
                           1
                               2   YR
          F2 Generation                      YYRR          YyRr
          (predicted
                                                                        1
                                                                            4       Yr
                                                                                             YYRr             YYrr          YyRr             Yyrr
          offspring)
                           1       yr
                               2
                                                YyRr           yyrr     1           yR
                                                                            4
                                                                                             YyRR             YyRr          yyRR             yyRr
                                        3              1
                                            4              4
                                                                        1           yr
                                                                            4
                                    Phenotypic ratio 3:1                                     YyRr             Yyrr             yyRr          yyrr

                                                                                9                3                     3                 3
                                                                                    16               16                    16                16

                                                                                                 Phenotypic ratio 9:3:3:1
Figure 15.1 The chomosomal basis of Mendel’s laws
Trihybrid crosses and beyond
    AaBbCcDd x AaBbCcDd
      What is the probability of producing
       offspring that is heterozygous for all the
       traits?
      Desired outcome: AaBbCcDd
      Aa x Aa = ½ Aa, Bb x Bb = ½ Bb, Cc x Cc =
       ½Cc, Dd x Dd = ½ Dd
      ½ x ½ x ½ x ½ = 1/16
      What is the probability of producing a
       homozygous recessive offspring?
      ¼ x ¼ x ¼ x ¼ = 1/256
The Testcross
 How can we tell the genotype of an individual with
  the dominant phenotype?
 Such an individual must have one dominant allele,
  but the individual could be either homozygous
  dominant or heterozygous
 The answer is to carry out a testcross: breeding
  the mystery individual with a homozygous
  recessive individual
 If any offspring display the recessive phenotype,
  the mystery parent must be heterozygous
Figure 14.6 A testcross
Pedigree charts

  Squares are male
  Circles are female
  If they are colored in they have the trait
  Sometimes carriers are ½ colored in
  Horizontal lines are mating lines
  Vertical lines are offspring lines
  Allows you to see how traits are passed
   from parents to offspring
LE 14-14a


                                                          First generation
              Ww      ww        ww    Ww                  (grandparents)


                                                         Second generation
                                                         (parents plus aunts
            Ww ww ww Ww         Ww    ww                     and uncles)


                                                               Third
                                                             generation
                           WW   ww                          (two sisters)
                           or
                           Ww




       Widow’s peak                    No widow’s peak

      Dominant trait (widow’s peak)
Epistasis
  A gene at one locus alters the
   phenotypic expression of another gene
   (stands on it)
  Ex: In mice the gene for pigment
   deposition C is epistatic to the gene for
   pigment production
      CC or Cc—melanin can be deposited; cc—
       albino
      BB or Bb—black; bb—brown

      Genes are on separate chromosomes and
       assort independently
Figure 14.11 An example of epistasis
Polygenic inheritance

  Traits are determined by many loci so
   there is a range of phenotypes
  ex: skin pigmentation in humans is
   controlled by 3 genes—A, B, C—which
   show incomplete dominance; the more
   capital letters, the darker the skin color
   (the more melanin is produced)
Figure 14.12 A simplified model for polygenic inheritance of skin color
Sex Chromosomes

  Humans have 23 pairs of chromosomes:
   22 pairs of autosomes and 1 pair of sex
   chromosomes
  Females are XX, males are XY (X and Y
   look different from each other); females
   give all their eggs one X chromosome
   plus cytoplasm and organelles; males
   give either an X or a Y chromosome to
   their sperm
Sex determination
  Males determine the sex of the child since ½
   of their sperm get the X and ½ get the Y
  Any information on the X chromosome will
   appear in males, whether recessive or
   dominant; females require 2 recessive alleles
   to show a recessive trait
  Information solely on the Y chromosome are
   called holandric genes (porcupine quill body
   hair, hairy ear rims, SRY gene); only affect
   men
Barr Bodies
  In body cells of females one X chromosome at
   random is turned off early in development;
   inactivated X is called a Barr body
  All the cells descended from that cell have the
   same X turned off
  If female is heterozygous she becomes a
   mosaic—some areas have the dominant gene
   expressed, some have the recessive
  Ex: calico cats, patches of colorblindness
Sex linkage problems

  It is now important if the offspring is male
   or female so the X and Y have to be
   used along with superscripts to show the
   alleles
  Examples of X-linked disorders:
   colorblindness, hemophilia, muscular
   dystrophy, eye color in fruit flies
Hemophilia

  H-normal, h-hemophiliac
  Phenotype                 Genotype
     Normal female           XHXH
     Carrier female          XHXh
     Hemophiliac female      XhXh
     Normal male             XHY
     Hemophiliac male        XhY
Sample problem
Figure 15.2 Morgan’s first mutant
Crossing over
  The degree of crossing over of any 2 loci is
   proportional to the distance between them.
  In complete linkage only the parental type
   gametes are produced
  In gametes produced by crossing over, two
   new combinations appear (the result of non-
   sister chromatids exchanging segments) along
   with the parental types
  The closer the loci are to each other the less
   recombinants are formed.
Calculating recombinant frequency

    In corn colored kernels (K) are dominant to
     white (k) and green leaves (G) are dominant to
     yellow (g). In a mating of a colored, green corn
     plant to a white, yellow one the following
     results were obtained: colored, green: 88;
     colored yellow: 12; white, green: 8; and white
     yellow: 92. What is the frequency of
     recombination of these 2 genes? How far
     apart are they from each other?
 # of recombinants/total x 100 =
  recombinant frequency
 20/200 x 100= 10% so the genes are 10
  map units apart
 A linkage map is a genetic map of a
  chromosome based on recombination
  frequencies
 Distances between genes can be expressed
  as map units; one map unit, or
  centimorgan, represents a 1%
  recombination frequency
 Map units indicate relative distance and
  order, not precise locations of genes
Example

  Black body, cinnabar eyes and vestigial
   wings are mutations in fruit flies
  Data--crossing over observed:
      Cinnabar, vestigial = 9.5%
      Black, vestigial = 17%

      Black, cinnabar = 9%

    What is the order of the genes?
 Black and vestigial must be farthest
  apart since they have the largest
  recombination frequency
 Black and cinnabar, and cinnabar and
  vestigial are closer together since they
  have lower recombination frequencies
 Since 9 + 9.5 is close to 17, cinnabar
  must be in the middle and the others on
  the sides
Correct order:

								
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