C4.4 - Cell Cycle and Cellular Division

Overview

• all cells come from pre-existing cells
  • conditions no longer favourable for abiogenesis
• two processes: mitosis and meiosis
• Reasons for Division
  • healing
  • tissue repair
  • growth
  • reproduction

The Cell Cycle

• life of cell divided into stages
• cell cycle divided into interphase and division
• interphase: phase involving cell growth, duplication of chromosomes, prep. for cell division

Interphase

• cell may spend as much as 90% of cell cycle in interphase
• cell carries out metabolic processes and performs its reg. cellular activities
• G₁ phase
  • cell prep. for division by
    • increasing protein supply
    • creating more cytoplasmic organelles
    • growing in size
• S phase
  • S stands for DNA synthesis
  • DNA of chromosomes duplicated
  • chromatin: mass of DNA and protein
  • chromatid: condensed chromatin fibers
  • sister chromatid: two identical joined copies of chromatids
  • centromere: region where 2 chromatids are joined tightly together
• G₂ Phase
  • G stands for gap
  • cell “double checks” duplicated chromosomes for error
  • makes any needed repairs

Mitosis (M Phase)

• mitosis: the stage of which the cell divides asexually
• asexual reproduction: reproduction of cells from one source
• Mother cell divides into two identical daughter cells
• Process found in unicellular and multicellular organisms
  • somatic cells: body cells
• involves division of nuclear material
• nuclear materials contains chromosomes
• chromosomes: condensed DNA
• chromosomes contain 2 identical sister chromatids held together w/ a centromere

Prophase

Early prophase, both images

• chromosomes shorten and thicken, becoming visible
• sister chromatids still connected
• nuclear membrane dissolves
• nucleolus disappears
• centriole: organelle in animal cell responsible for producing and handling spindle fibers
• centrioles migrate to opposite poles
  • only in animal cells
• spindle fibres (a.k.a. microtubules) start to form
  • sister chromatids attach to spindle fibers

Late prophase

Metaphase

• spindle fibers fully attached to centromeres
• spindle fibers pull on chromosomes
• chromosomes line up in middle of cell (equator)

Anaphase

• spindle fibers pull chromosomes apart
• centromere splits and attached spindle fibers shorten
• pulls centromeres to opposite cell poles

Telophase

• chromatids reach opposite ends of cell
• chromosomes begin to uncoil and turn back into chromatin
• nuclear membrane reforms
• cytokinesis begins (usually w/ telophase)

Cytokinesis (not part of M)

• cytokinesis: division of cytoplasm and organelles
• in animals, occurs along cleavage furrow
  • ring of protein under plasma membrane contracts
  • …making cleavage furrow deeper
  • cleavage furrow: region between 2 splitting animal cells (refer to above diagram)
• in plants, cell wall forms via cell plate between 2 daughter cells

Errors in Mitosis

• mutations can disrupt normal process of mitosis
• mutations may arise through contact w/ carcinogens
• carcinogen: agents that cause cancer
  • may incl. toxic chemicals
  • radiation
  • viruses
• oncogene: gene w/ potential to cause cancer
• if certain oncogenes are altered, mitosis spirals out control resulting in tumour that can be…
  • benign tumour: non-cancerous tumour
  • malignant tumor: cancerous tumour
• oncogenes may also cause protein synthesis to increase unregulated

Meiosis

• sexual reproduction: process in which 2 parents provide genetic material in order to produce offspring
• meiosis: form of cell division that produces 4 cells, each containing 1/2 the chromosomes as parent cell
• karyotype: display of all the chromosomes in a cell / individual (p. 108)
• many multicellular organisms, including humans reproduce sexually resulting in a combination of traits
• sex cells divide differently so that when a male and a female sex cell fuse,
  • they have the correct number of chromosomes

Sex Cells (Gametes)

• gametes: male and female sex cells
  • $n$ or haploid, 23 in humans
  • $n=23$
• zygote: fused cell (sperm + egg)
  • $2n$ or diploid, 46 in humans
  • $2n=46$
• homologous chromosomes (a.k.a. homologs): chromosomes of each matching pair inherited from mother and father
  • each member of pair carries same series of genes controlling same inherited traits
• diploid: two sets of chromosomes ($2n$)
• haploid: single set of chromosomes ($n$)
• 2 cell divisions in meiosis
  • Meiosis I
  • Meiosis II

Meiosis I (Reduction)

Interphase

Similar to mitosis

Prophase I

• most lengthy and complex phase
• homologous chromosomes line up
  • each duplicated chromosome pairs w/ corresponding homologous chromosome
  • result: tetrad
  • tetrad: 4 chromatids
• homologs contain same genes but are not identical
  • they may contain different alleles
• homologs then do genetic recombination
  • genetic recombination: exchange of DNA segments between homologs
• independent assortment of originally maternal and paternal DNA
• independent assortment: way in which chromosomes distributes during meiosis (random)

Crossing Over

• crossing over: exchange of genetic material between homologs
• crossing over occurs contributing to genetic variation
• chiasmata: visual manifestations of crossovers
  • Greek: crosses

Metaphase I

• spindle fibers attach to centromere
• homologous pairs pulled to equator where they line up together side by side

Anaphase I

• homologous chromosomes separate from tetrad and moves to opposite poles
• centromere does not divide as in mitosis
• sister chromatids remain attached at centromeres
• one chromosome from each pair now at each pole

Telophase I

• chromosomes uncoil
• spindle fibres disappear
• cytoplasm divides
• nuclear membrane reforms
• daughter cells diploid ($2n$)
• does not occur in all cells
  • if not, cell goes directly into meiosis II

Meiosis II (Division)

Interkinesis

• period of rest, some growth
• chromosomal duplication does not occur

Meiosis II

• identical to mitosis
• daughter cells divide again
• 4 haploid cells ($n$) results (gametes)

Errors in Meiosis

• nondisjunction: failure of chromosomes to separate properly during anaphase I or II
  • result: chromosome may be added or deleted from a gamete
  • can result in trisomy
  • cause not fully known
• trisomy: zygote has extra chromosome (3 copies of particular chromosome)
  • i.e. trisomy 21 or Down Syndrome
  • Down syndrome: condition which individual carries extra copy of chromosome 21
    • mild to severe developmental delays
    • characteristic facial features
    • below avg. height
    • may also incl. heart defects and impaired immune sys.
• polyploidy: organisms contain >2 sets of chromosomes
  • rare in animals
  • i.e. seedless grapes and watermelon

The chance of having a baby with Down syndrome increases w/ the mother’s age

Changes in Chromosome Structure

• mutation: any change to DNA of cell
• may spontaneously occur at any stage of development
• may or may not produce viable offspring depending on degree or severity

Deletion of Deficiency

• deletion: portion of chromosome deleted
• causes: viruses, radiations, mutagens
• i.e. Cri du chat disorder
  • seg. of chromosome 5 missing
  • cancer cells often show chromosomal deletions

Duplication

• duplication: gene sequence repeats in one or several chromosomes
• i.e. Fragile X Syndrome
  • duplication of genes in X chromosome
  • results in mental impairment and autism-like symptoms
  • affects 1/4,000 males and 1/8,000 females
  • leading cause of inherited mental impairment

Inversion

• inversion: reversal of fragment of original chromosome
• 2 breaks occur in chromosome
• gene segment separates from chromosome and reinserts itself in reverse order
• generally viable and show no phenotypic abnormalities

Translocation

• translocation: fragment of one homolog attaches to nonhomologous chromosome
• 2 nonhomologous chromosomes exchange parts
• most common are reciprocal translocations
• one segment is exchanged directly with another
• i.e. semisterility
  • half of gametes lethal to zygote
• i.e. Robertsonian translocation
  • 5% of all Down syndrome individuals
    • translocation Down syndrome
    • translocation between chr. 21 and a14
  • translocation of part of chromosome 14 and 21

Sources

• https://cdn-academy.pressidium.com/academy/wp-content/uploads/2021/12/Stages-of-Mitosis.png