SBI3U – Grade 11 Biology – Cell Division and Reproduction

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Thanks Hagar

Chapter 4: Cell Division and Reproduction

Does not contain DNA function, RNA and protein synthesis, does not contain selective breeding and cloning (4.3)


4.1 – Cell Division and Genetic Material

  • All new cells come from existing cells so traits = pass on genetic material from parent cell to daughter cell.

Genetics: How the genetic info of living organisms = passed from one generation to next


Cell Theory:

  1. All living things are composed of one or more cells
  2. Cells are the smallest units of living organisms
  3. New cells come only from pre-existing cells by cell division


The Cell Cycle:

  • Cells reproduce through growth + division process called the cell cycle
  • Somatic cells go through cell cycles

Somatic cells: body cells of plants and animals excluding reproductive cells

  • Cell cycle = continuous production of new cells
  • 3 functions of cell division:
  1. Growth of organism
  2. Repair of damaged tissues and organs
  3. Maintenance to replace dying/dead cells
  • Duration of cell cycle depends on type of cell
    • Healthy actively dividing animal cells = 12-24 hrs.


Stages of the Cell Cycle:

  1. Interphase: Cell carries out normal functions, grows, and duplicates genetic material in preparation for next stage of cycle. Longest stage.
  2. Mitosis: Stage when cell’s nucleus + genetic material divide. NUCLEAR DIVISON
  3. Cytokinesis: Begins near end of mitosis, involves division of cell cytoplasm + creation of new cell.


  • Cycle checkpoints monitor growth
    • Ensure that cycle continues when it should
    • If something interferes with signals = uncontrolled growth
    • EX: Cancer = uncontrolled , cancerous cells keep dividing = mass cells = tumour



  • Cell grows, develops into mature, functioning cell, duplicates DNA, and prepares for division.


G1    (Growth 1) 
  • Major growth period of cell
  • Cell synthesize new molecules in preparation for next phase in cell cycle
S       (Synthesis)
  • Cellular DNA = copied / replicated
  • DNA exists in uncondensed fibres called chromatin
  • Cells that complete this phase enter GS
G2    (Growth 2)
  • Final phase of interphase
  • Cell synthesizes more molecules prior to mitosis/cell division




  • Cell’s copied genetic material separates cell prepares to split into 2
  • Key idea of mitosis = separate cell’s replicated DNA accurately
    • Allow cell’s genetic information to be passed into new cells intact = 2 identical cells




  • Cell chromatin condense into chromosomes which contain DNA
    • Each DNA contains 2 copies (because of replication in interphase)
  • Nuclear membrane breaks down
  • Nucleolus disappears
  • Spindle fibres form at centrosomes and move to opposite sides of cell.


  • Spindle fibres guide chromosomes to equator of cell.
  • Spindle fibres reach opposite sides of cell, attach to centromere of each chromosome


  • Centromere splits apart, sister chromatids separate from each other
  • Separated sister chromatids now = chromosomes
  • Spindle fibres shorten, pull the chromosomes to opposite sides
  • @ end of anaphase, one complete set of chromosomes = gathered @ each pole of cell


  • Chromosomes have reached opposite poles of cell
  • Chromosomes start to unwind into less visible chromatin
  • Spindle fibres break down
  • Nuclear membrane forms around new set of chromosomes
  • Nucleolus reforms within each nucleus.







  • Division of cytoplasm
  • Indentation forms in cell membrane along equator of cell.
    • Indentation deepens until cell = pinched in 2
    • Cytoplasm divides equally btwn 2 halves of cell
    • Ends with separation of 2 genetically identical daughter cells
    • Daughter cells are now in G1 of interphase

Animal cells:  Cytokinesis = by microfilaments that pinch cytoplasm

Plant cells:  Plant cell = rigid cell wall covering membrane, can’t be pinched, too strong. Instead, new structure called cell plate forms btwn 2 daughter nuclei. Cell walls then form on both sides of cell plate. When new cell wall = complete = 2 genetically identical plant cells.

Prokaryotic cells:  No nucleus, do cell division with binary fission. When DNA = duplicated, both copies attach to the membrane. When cell membrane grows, attached DNA molecules = pulled apart.


The Structures of Genetic Material:



  • = deoxyribonucleic acid
  • Nucleic acid containing sugar caller deoxyribose = 5 carbon atoms
  • Made up of long stands that form shape called double helix
  • During most of cell cycle DNA = strands of chromatin fibre
  • When mitosis begins, chromatic condenses= distinct chromosomes
  • A polymer, made of monomers = nucleotides
  • Individual units of each strand of DNA = nucleotides


  1. Sugar molecule: deoxyribose
  2. Nitrogen base: A,T,C,G
  3. Phosphate group



  • Sugar + phosphate always same but base changes
  • Since S and P = constant, they form backbone

for bases to hang on. = Sugar phosphate backbone

  • Most living cells, DNA = double-stranded molecule, 2 strands = parallel



  • 2 categories:
    • Pyrimidines = 1 ring of atoms
      • C –  cytosine
      • T – thymine
    • Purines = 2 rings of atoms
      • A-  ademine
      • G- guanine
  • Certain nitrogen bases pair up with others:
    • A always with T
    • C  always with G
  • Pyrimidines always with purines = complimentary base pairs
  • If DNA mutation / genetic mutation, = change in nucleotide sequence

Genome: Complete DNA sequence in every cell of organism


  • Type of bonding involved in base pairing = hydrogen bonds
  • Weak enough so that molecule can be pulled apart when cell divides
  • Strong enough to hold DNA strands together



  • When DNA = replicated during interphase, double helix unwinds, each strand of DNA = template for new strand
  • When DNA = copied, each new double-strand of DNA molecules contains one original strand + one new
  • = Semi conservative because DNA molecule conserves half of original DNA.


Chromosomes are Paired:

  • # of individual chromosomes in cell varies btwn species
  • # doesn’t reflect complexity of organ
  • Human somatic cells = 46 chromosomes = 23 pairs
    • Each pair = 1 from mom, 1 from dad
    • 23rd chromosome pair = sex chromosomes

Sex Chromosomes:  an X or Y chromosome, determines the genetic sex of an organism

Human Female = XX

Human Male = XY

  • Remaining 22 = autosomes

Autosomes: Chromosome that is not involved in determining sex of organism


  • Chromosomes = paired based on sharing similar characteristics



Homologous Chromosomes Contain Alleles:

Homologous Chromosomes: Pairs of chromosomes that appear similar in terms of length, centromere location, + banding pattern

  • Not identical with each other, carry different genes for same traits (hair color, eye color etc.)
  • = Allele

Allele: Different form of the same gene.

Gene: Sections of DNA that contain genetic information for the inheritance of specific traits


Examining Chromosomes: The Karyotype:

Karyotype: Particular set of chromosomes that an individual has

  • To prepare, cell sample = collected + treated to stop cell division during metaphase of mitosis
  • Sample = stained, makes banding pattern on chromosomes visible
  • Chromosomes = sorted + paired
  • Autosomes = numbered 1 – 22, sex chromosomes labelled as X or Y
  • Y chromosome = smaller than X chromosome.


4.2 Sexual Reproduction:

  • When somatic cells = reproduced by mitosis, new daughter cells have same genetic info as parents

Asexual Reproduction: Reproduction that only needs 1 parent (e.g. bacteria)

  • If mitosis = only way of make more cells, we would be exact clones of ourselves
  • Except for identical twins, no person = exact genetic copy of other

Sexual Reproduction: Reproduction that needs 2 parents and produces genetically distinct offspring


Haploid and Diploid Cells in Sexual Reproduction:

  • Sexual Repro = fusion of male repro cell + female repro cell

Gametes: Male/female reproductive cells. Male gamete = sperm. Female gamete = ovum

Zygote:  Result of when the 2 gametes fuse

Fertilization: In humans, the joining of male and female haploid gametes


  • Because 2 gametes = zygote, gametes must have half # of chromosomes as parent cells = haploid

Haploid: Cell that has half number of chromosomes as parent cell. Haploid number of chromosomes in species =

(n also describes the pairs of chromosomes in an organism)

Diploid: Cells that contain pairs of chromosomes (includes all somatic cells)


  • Human gametes = haploid → n = 23 chromosomes
  • After fertilization zygote cell = diploid → 2n chromosomes  ( n from female, n from male)
  • Diploid number in humans = 46
  • When 2 human gametes combine = 23 pairs of homologous chromosomes
  • See page 169 figure 4.12


Meiosis – Producing Haploid Gametes:

  • Process that produces gametes with haploid # of chromosomes = meiosis

Meiosis: Cellular process that produces cells containing half the # of chromosomes as the parent cell

  • 2 outcomes:
    1. Genetic Reduction: Produces daughter cells with half the # of chromosomes of parent cell
    2. Genetic Recombination: Different alleles = combined = offspring that are genetically different from one another / parents = genetic variation in population



  • Cells that will divide by meiosis go through growth/synthesis phase before divide
  • = replication of chromosomes
  • @ start of meiosis, cell contains duplicated chromosomes



  • Prophase, metaphase, anaphase, Telophase X 2
  • 2 cycles of 4 phases → meiosis I, meiosis II





  • Each pair of homologous chromosomes (1 from each parent) lines up side by side) = synapsis

Synapsis: The aligning of homologous chromosomes

  • @ synapsis, homologous chromosomes = held tightly together along their lengths
  • Exchange genetic information = genetic diversity
  • Centrosomes move to poles of cell, spindle apparatus forms


  • Pairs of homo chromosomes line up along equator of cell (they have now exchanged genes)
  • Spindle fibres attach to centromere of each homologous chromosome


  • Homologous chromosomes separate, move to opposite pose of cell
  • Sister chromatids = still held together so centromeres don’t split like in mitosis
  • Result = single chromosome (2 sister chromatids) from each homologous pair moves to each pole of cell.
    • Homologous pair = 2 chromosomes
  • Chromosome number = 2n (diploid ) to n (haploid)


  • Homologous chromosomes begin to uncoil
  • Spindle fibres disappear
  • Cytokinesis takes place
  • Nuclear membrane forms around each group of homologous chromosomes = 2 cells
  • Each new cell = haploid



  • Phases = similar to phases of mitosis
  • Difference = cell that undergoes division in meiosis II = haploid instead of diploid


  • Haploid # of chromosomes line up @ equator


  • Sister chromatids = pulled apart @ centromeres by spindle fibres, chromosomes move to opposite sides of cell


  • Chromosomes reach poles
  • Nuclear membrane + nuclei reform


At the end of meiosis II, Cytokinesis occurs = 4 haploid cells with n # of chromosomes

See page 171 figure 4.13 for diagram

Comparison of Mitosis and Meiosis:

  • Mitosis = 1 set of division phases = 2 diploid daughter

cells that are identical

  • Meiosis = 2 sets of divisions = 4 haploid daughter

cells that are not identical

  • Meiosis = important for humans because = genetic

variation = genetic diversity



Gamete Formation in Animals:

Spermatogenesis: the process of producing male

male gametes (sperm) in mammals

Oogenesis: process of producing female gametes (eggs)

in mammals



  • Most male animals, meiosis takes place in testes
  • Process starts with diploid cell called


  • @ puberty, spermatogonia reproduce by mitosis
  • These cells go through meiosis make 4 haploid cells
  • After meiosis 2, cell develops into mature sperm
  • Nucleus + other molecules = organized into “head”

of sperm

  • Midsection holds mitochondria
  • Tail = flagellum for locomotion
  • See page 173 figure 4.15










  • Most female animals, meiosis = ovaries
  • Oogenesis stars with diploid cell called oogonium
  • Before birth, oogonia reproduce by mitosis, start meiosis, but stop @ prophase 1
  • Meiosis I will continue for 1 cell each month @ beginning of puberty
  • Oogenesis involves unequal division of cytoplasm
    • Cell that gets more cytoplasm after 1st division continues through meiosis I and II = viable egg
    • =  more nutrients will support zygote after fertilization
    • Other smaller cell  = polar body, will degenerate
  • Final stages of meiosis II not completed until fertilization by sperm cell occurs
  • When meiosis II = complete, mature egg + another polar body = produced.
  • Haploid nucleus of the egg cell fuses with haploid nucleus of sperm cell to complete fertilization
    • = diploid zygote


Multiple Births:

  • If more than 1 egg = released = more than one  baby @ once

Fraternal Twins: 2 eggs released, both are fertilized = twins that are like siblings

Identical Twins: Single zygote divides into 2 separate bodies in first few days of development = genetically identical twins


The Importance of Meiosis for Genetic Variation:

  • Outcome of meiosis forms genetically distinct haploid gametes (mitosis = identical)
  • Each diploid cell has 2 copies of each chromosome (one from mom, one from dad)
  • Genetic variation:
    • creation of gametes that carry different combinations of maternal / paternal chromosomes = independent assortment
    • exchange of genetic material between maternal and paternal chromosomes in crossing over


Independent Assortment:

  • @ metaphase 1, homologous pairs line up along equator of cell
  • Chromosomes from mom/dad = oriented on random sides of pole, some facing this way, some that
  • Orientation of chromosomes = independent of orientation of others
  • Depending on how chromosomes line up, number of different combinations of chromosomes = found in gametes

See figure 4.18 on page 175

  • Number of genetically distinct gametes produced from diploid cell = 2n
  • (n = # of chromosome pairs in diploid cell)
  • Humans produce 223 or 8388608 genetically distinct gametes


Crossing Over:

  • During prophase 1 (synapsis), non-sister chromatids of homologous chromosomes exchange pieces of chromosome

Crossing over: exchange of chromosomal segments between a pair of homologous chromosomes

  • can occurs @ several points along non-sister chromatids
  • section of chromosome that = crossed may have thousands of genes
  • Result = individual chromosomes have some stuff from mom and some from dad
  • = genetic diversity of gametes produced


Errors During Meiosis:

  • 2 processes that = genetic variation, (independent assor, + crossing) = potential chromosomal abnormalities
  • Errors that occur during meiosis = gametes that don’t survive
  • If survive + fertilized = zygote
  • Every cell in offspring = from one zygote cell, all cells in offspring will contain the error
  • 2 types of chromosomal errors in meiosis:
    1. Changes in chromosome structure
    2. Changes to chromosome number


Errors Caused by Changes in Chromosome Structure:

  • When cross over, chemical bonds that hold DNA in chromosome = broken + reformed
  • Sometimes chromosome no reform proper
  • Or non-homologous pairs cross over = chromosome that has genes no normally on that chromosome



Error in Chromosome Structure Description Example of Genetic Disorder
Deletion Piece of chromosome = deleted Cri du Chat Deletion in chromosome 5.
Duplication Section of chromosome appears 2 or more times in a row. Charcot-Marie-Tooth DiseaseDuplication on chromosome 17
Inversion A section of a chromosome = inverted  (upside down) FG SyndromeInversion of X chromosome
Translocation A segment of one chromosome becomes attached to a different chromosome Chronic Myelogenous LeukemiaTranslocation between chromosome 9 and 22



Errors Caused by Changes in Chromosome Number:

  • When homologous chromosome pairs / sister chromatids don’t separate proper = non-disjunction

Non-disjunction: Failure of homologous chromosome pairs / sister chromatids to separate during meiosis

  • occurs in anaphase I or II of meiosis
  • Anaphase 1: occurs when homologous chromosome pairs do not separate to opposite poles
    • Instead 1 whole pair = pulled toward same pole
  • Anaphase 2: occurs when sister chromatids no separate to opposite poles
    • both sister chromatids = pulled to same pole
    • produces gametes that have too few / too many chromosomes







Genetic Disorders Associated with Chromosome Number:

  • Many genetic disorders = incorrect # of chromosomes
  • Down Syndrome = extra piece on chromosome 21
  • This incidence o non-disjunction increases with maternal age


Trisomies and Monosomies:

Monosomy: Loss of chromosome due to non-disjunction (missing one chromosome of a homologous pair)

Trisomy: Gain of extra chromosome due to non-disjunction (most common in 21, 18, 13, sex chromo)


Conditions Syndrome Number of Live Births
Trisomy 21 Down 1 in 800
Trisomy 18 Edward 1 in 18 000
Trisomy 13 Patau 1 in 15 000
XXY Klinefelter 1 in 1000 males
XYY Jacobs 1 in 1000 males
XXX Triple X 1 in 1500 females
X Turner 1 in 5000 females



Prenatal Genetic Testing:

  • = test performed on fetus (baby developing in womb)to find genetic-based abnormalities
  • Before only for women in hi-risk situations
    • women 35+ yrs
    • women with family history of genetic disorders
  • Now, doctor can let any women go
  • Ethical dilemmas: could end pregnancy, discrimination against ppl with disabilities


Prenatal Testing Procedures:

  • Blood tests + ultrasound can give info about potential physical + chromosomal abnormalities = non-invasive
  • Non-invasive because do not require direct sampling of fetal cells
  • Invasive test = collect DNA from sample of fetus (EX: Amniocentesis, Chorionic Villus Sampling)


4.3 Reproductive Strategies and Technologies:

Reproductive Strategies in Agriculture:



Artificial Insemination: Sperm is collected and concentrated before being introduced into the female’s reproductive system.
  • Makes semen from high-quality males more widely available through breeders and online sources
  • Farmers can choose desirable traits for male parent
  • Instead of shipping the whole cow, just ship semen




Embryo Transfer: Process where an egg that has been fertilized = artificially transferred into a recipient female’s uterus.
  • Embryos = shipped easily (don’t need to ship the whole cow)
  • Ensured quality  of embryos
  • Animals born and raised in native environment do better than those that are imported



Reproductive Technologies for Humans:

  • Assisted Reproductive Technologies (ART) = for couples who not able to conceive child
  • Artificial Insemination: Sperm collected + concentrated from women’s partner / sperm bank, introduced into woman’s reproductive system


In Vitro Fertilization:

  • for women who have blocked Fallopian tubes
  1. Immature egg = received from the woman
  2. Eggs = combined with sperm in laboratory glassware
  3. After fertilization, developing embryo = placed in uterus
  4. = Test tube babies


Other Terms:

Chromosome: A structure in the nucleus that contains DNA

Sister Chromatid: one of the 2 chromosomes that are genetically identical and are held together at the centromere

Centromere: The region where the 2 sister chromatids are held together in a chromosome

Spindle fibre: a microtubule structure that facilitates the movement of chromosomes within a cell

Centrosome: A structure that helps to form the spindle fibres