Cell Division Biology Lecture Notes PDF

BIOL S103F ESSENTIAL BIOLOGY CELL DIVISION Week 7 Cell Division 2    Key characteristic of life, playing important roles in the lives of organisms Ability of organisms to reproduce their own kind  Reproduction at the cellular level  Produces 2 “daughter” cells that are genetically identical to...

BIOL S103F ESSENTIAL BIOLOGY CELL DIVISION Week 7 Cell Division 2    Key characteristic of life, playing important roles in the lives of organisms Ability of organisms to reproduce their own kind  Reproduction at the cellular level  Produces 2 “daughter” cells that are genetically identical to  Each other  Original “parent” cell  Requires duplication of chromosomes (contain most of the cell’s DNA)  Sorts new sets of chromosomes into the resulting pair of daughter cells Cell division is used for 1. Reproduction of single-celled organisms 2. Growth of multicellular organisms from a fertilized egg into an adult 3. Cell renewal & repair 4. Production of sperms & eggs @2015 Pearson Education, Inc. Living Organisms Reproduce by 2 Methods 3   Asexual reproduction  Produces offspring identical to the original cell or organism  Inheritance of all genes from 1 parent Sexual reproduction  Produces offspring similar to the parents  Variations in traits  Inheritance of unique sets of genes from 2 parents Prokaryotes Reproduce by Binary Fission 4      Prokaryotes (single-celled bacteria & archaea) reproduce by binary fission Chromosome of a prokaryote is typically 1 Chromosome  Single circular DNA molecule replication associated with proteins begins.  Much smaller than those of eukaryotes Chromosome replicates  Beginning at origin of replication  Cell elongates while chromosome is replicating 2 daughter chromosomes actively move apart Plasma membrane pinches inward, dividing the cell into 2 Origin of replication E. coli cell Two copies of origin 2 One copy of the origin is now at each end of the cell. Meanwhile, the cell elongates. Origin Cell wall Plasma membrane Bacterial chromosome Origin 3 Replication finishes. The plasma membrane pinches inward. 4 Two daughter cells result. @2015 Pearson Education Ltd Classification of Eukaryotic Cells based on Division mechanism 5   Somatic cells  Non-reproductive cells  2 sets of chromosomes  Diploid (2n)  Division: Mitosis Gametes  Reproductive cells: sperms & eggs  Half as many chromosomes as somatic cells  Haploid (n)  Division: Meiosis Large, Complex Chromosomes of Eukaryotes Duplicate with Each Cell Division 6    Eukaryotic cells  More complex & larger than prokaryotic cells  More genes  Store most of their genes on multiple chromosomes within the nucleus  Each eukaryotic species has a characteristic number of chromosomes in each cell nucleus Eukaryotic chromosomes are composed of chromatin consisting of  One long DNA molecule  Proteins  Help maintain the chromosome structure  Control gene activity To prepare for division, chromatin becomes  Highly compact  Visible with a microscope @2015 Pearson Education Ltd Large, Complex Chromosomes of Eukaryotes Duplicate with Each Cell Division 7    Before a eukaryotic cell begins to divide, it duplicates all of its chromosomes, resulting in Sister chromatids 2 copies (sister chromatids) The sister chromatids are joined together along their lengths by cohesins & are cinched especially tightly at the centromere When a cell divides, the sister chromatids  Separate from each other (chromosomes)  Sort into separate daughter cells Chromosomes Chromosomal DNA molecules Chromosome duplication Sister chromatids Centromere Separation of sister chromatids and distribution into 2 daughter cells @2015 Pearson Education Ltd Cell Cycle Includes Growing & Division Phases 8  Cell cycle  Ordered sequence of events that extends from the time a cell is first formed from a dividing parent cell until its own division  2 stages 1. Interphase  Preparation for cell division  Cell growth + duplication of chromosome 2. Mitotic (M) phase  Division  Mitosis -- division of nucleus  Cytokinesis -- division of cytoplasm Cell Division is a Continuum of Dynamic Changes 9   Mitosis  Division of nucleus  Progresses through a series of stages 1. Prophase 2. Prometaphase 3. Metaphase 4. Anaphase 5. Telophase Cytokinesis  Division of cytoplasm  Completes the mitotic phase  Often overlaps telophase @2015 Pearson Education Ltd 10 10 μm 11 G2 of Interphase Centrosomes (with centriole pairs) Nucleolus Chromosomes (duplicated, uncondensed) Nuclear envelope Plasma membrane Prophase Early mitotic spindle Aster Centromere Two sister chromatids of one chromosome Prometaphase Fragments of nuclear envelope Kinetochore Nonkinetochore microtubules Kinetochore microtubule @2015 Pearson Education Ltd 10 μm 12 Metaphase Anaphase Metaphase plate Daughter chromosomes Spindle Centrosome at one spindle pole Telophase & Cytokinesis Cleavage furrow Nuclear envelope forming Nucleolus forming @2015 Pearson Education Ltd Meiosis & Crossing Over 13  Gametes are made by meiosis in ovaries & testes  Meiosis reduces the chromosome number by half (single set of chromosomes) MEIOSIS I INTERPHASE A pair of homologous chromosomes in a diploid Sister chromatids parent cell A pair of duplicated homologous chromosomes 1 2 Chromosomes duplicate MEIOSIS II Homologous chromosomes separate Diploid cell with duplicated chromosomes 3 Sister chromatids separate Haploid cells with Haploid cells with duplicated chromosomes unduplicated chromosomes @2015 Pearson Education Ltd Chromosomes are Matched in Homologous Pairs 14       Humans somatic cells  46 chromosomes forming 23 pairs of homologous chromosomes Homologous chromosomes are matched in  Length  Centromere position  Staining pattern Locus A locus (plural, loci) -- position of a gene Sister chromatids Pair of homologous duplicated chromosomes Centromere One duplicated chromosome Different versions of a gene may be found at the same locus on the 2 chromosomes of a homologous pair Human sex chromosomes (X & Y)  Differ in size & genetic composition Autosomes  The other 22 pairs of chromosomes  Each with the same size & genetic composition @2015 Pearson Education Ltd Homologous Chromosome 15 Life Cycle 16       An organism’s life cycle is the sequence of stages leading from adults of one generation to adults of the next Humans, many animals & plants are diploid  All somatic cells contain pairs of homologous chromosomes Gametes  Eggs & sperm  Haploid  Each cell has a single set of chromosomes Human life cycle begins when a haploid sperm fuses with a haploid egg in fertilization Zygote  Formed by fertilization, is now diploid Mitosis of the zygote & its descendants generates all somatic cells into the adult form @2015 Pearson Education Ltd Meiosis Reduces the Chromosome Number from Diploid to Haploid 17  Meiosis  A type of cell division producing haploid gametes in diploid organisms  2 haploid gametes may then combine in fertilization to restore the diploid state in zygote Mitosis Meiosis Both are preceded by duplication of chromosomes  Followed by only 1 cell division  Followed by 2 consecutive cell divisions (Meiosis I & Meiosis II)  2 daughter cells, each with a diploid set of chromosomes which are identical  4 daughter cells, each with a haploid set of chromosomes which are unique MEIOSIS MEIOSISI I MITOSIS MITOSIS Chromosomes Chromosomes are duplicated are duplicated 18 Prophase Prophase Parent cell Parent cell 2n = 4 2n = 4 ChromosomesChromosomes are duplicatedare duplicated Prophase II Prophase Homologous Homologous chromosomes chromosomes pair up pair up Crossing over Crossing over Homologous Homologous chromosomes chromosomes remain separate remain separate Metaphase II Metaphase Metaphase Metaphase Pairs ofof Pairs homologous homologous chromosomes chromosomes line up at the metaphase line up atplate the metaphase plate Chromosomes Chromosomes the line line up up at at the metaphase plate metaphase plate Anaphase Anaphase Telophase Telophase 2n 2n Sister 2n chromatids 2n Sister are separated chromatids are separated nn == 22 nn == 22 MEIOSIS II n 1 division of nucleus & cytoplasm Result: 2 genetically identical somatic diploid cells Used for: Growth, tissue repair, asexual reproduction Anaphase II Anaphase Telophase II Telophase Homologous Homologous chromosomes chromosomes are separated are separated Sister Sisterchromatids chromatids remain attached remain attached n n n Sister chromatids are separated 2 divisions of nucleus & cytoplasm Result: 4 genetically unique haploid gametes Used for: Sexual reproduction @2015 Pearson Education Ltd Origins of Genetic Variation among Offspring 19   The behavior of chromosomes during meiosis & fertilization is responsible for most of the variation that arises in each generation 3 mechanisms contribute to genetic variation 1. Independent assortment of chromosomes 2. Crossing over 3. Random fertilization  Adds to genetic variation because any sperm can fuse with any ovum (unfertilized egg)  Fusion of 2 gametes (each with 8.4 million possible chromosome combinations from independent assortment) produces a zygote with any of ~70 trillion diploid combinations  Crossing over adds even more variation  Each zygote has a unique genetic identity Independent Assortment of Chromosomes 20    Each pair of chromosomes independently aligns at the metaphase plate Each pair of chromosomes sorts maternal & paternal homologs into daughter cells independently of the other pairs Number of combinations for chromosomes packaged into gametes is 2n Possibility A (n  haploid number of Two equally probable chromosomes) arrangements of chromosomes at  For humans (n = 23), metaphase I there are > 8 million (223) possible Metaphase II combinations of chromosomes Possibility B Gametes Combination 1 Combination 2 Combination@2015 3 Combination 4 Pearson Education Ltd Crossing Over 21   Genetic recombination Adapted from  Production of new combinations of genes (different from those carried by the original parental chromosomes) due to crossing over Crossing over  Exchange of corresponding segments between nonsister chromatids of homologous chromosomes  Nonsister chromatids join at a chiasma (plural, chiasmata)  Site of attachment & crossing over  Corresponding amounts of genetic material are exchanged between maternal & paternal (nonsister) chromatids Tetrad  In meiosis in human, an average of 1-3 crossover events occur per chromosome pair  Further increases genetic variability Chiasma Sister chromatids @2015 Pearson Education Ltd Video Clip: Meiosis 22 Nondisjunction 23    Failure of chromosomes or chromatids to separate normally during meiosis Can happen during  Meiosis I -- if both members of a homologous pair go to one pole  Meiosis II -- if both sister chromatids go to one pole Fertilization after nondisjunction  zygotes with altered chromosome numbers Meiosis I Nondisjunction Meiosis II Normal meiosis II Gametes Number of n+1 n+1 n−1 n−1 chromosomes Abnormal gametes Meiosis I Normal meiosis I Meiosis II Nondisjunction n+1 n−1 Abnormal gametes n n Normal gametes @2015 Pearson Education Ltd Trisomy 21 24  Involves the inheritance of 3 copies of chromosome 21  The most common human chromosome abnormality  A person with trisomy 21 has a condition called Down syndrome, which produces a characteristic set of symptoms, including  Characteristic facial features  Short stature  Heart defects  Susceptibility to respiratory infections, leukemia, & Alzheimer’s disease  Varying degrees of developmental disabilities Incidence increases with the age of the mother Trisomy 21 A person with Down syndrome Infants with Down syndrome (per 1,000 births)  90 80 70 60 50 40 30 20 10 0 20 25 30 35 40 45 Age of mother @2015 Pearson Education Ltd Abnormal Numbers of Sex Chromosomes Do Not Usually Affect Survival 25   Genetic balance upset due to Sex chromosome abnormalities seem to be lower than an unusual number of autosomes  Small size of the Y chromosome  X chromosome inactivation The most common human sex chromosome abnormalities  A single Y chromosome is enough to produce “maleness,” even in combination with several X chromosomes  Absence of a Y chromosome yields “femaleness” Alterations of Chromosome Structure Can Cause Birth Defects & Cancer 26  Chromosome breakage can lead to rearrangements that can produce genetic disorders or, if changes occur in somatic cells, cancer 4 types of alternations in chromosome structure Inversion Deletion   Loss of a chromosome segment Duplication  Repeat of a chromosome segment Homologous chromosomes  Reversal of a chromosome segment Reciprocal translocation  Attachment of a segment to a nonhomologous chromosome. A translocation may be reciprocal Nonhomologous chromosomes @2015 Pearson Education Ltd