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Chapter 13: Meiosis and Sexual Life Cycles. Essential Knowledge. 3.a.2 – In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis, or meiosis plus fertilization (13.1-13.3).

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Chapter 13: Meiosis and Sexual Life Cycles

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essential knowledge
Essential Knowledge

3.a.2 – In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis, or meiosis plus fertilization (13.1-13.3).

3.c.2 – Biological systems have multiple processes that increase genetic variation (13.4).

  • The transmission of traits from parents to offspring
  • Comment - Humans have been aware of heredity for thousands of years
  • Also known as inheritance
  • The scientific study of heredity
  • Comment - Genetics is only about 150 years old
    • NEW/MODERN field of science
    • DNA was only discovered in 1950s
      • Only 60 years ago!
chromosome review
Chromosome Review
  • Chromosomes (23 pairs=46)
    • Inherit one chromosome of each pair from each parent
    • Total of 46 (23 maternal, 23 paternal)
      • 1 pair/2 = Sex chromosomes
      • 22 pairs/44 = Autosomes
  • The DNA for a trait
  • Locus - the physical location of a gene in a chromosome
  • Genes program traits
    • How? Program cells (through protein synthesis) to make specific proteins
    • These proteins become “visible” to us through outside traits
  • A method of copying genes to pass them on to offspring
  • Fertilization: male/female gametes unite during reproduction
    • Results in zygote (fertilized egg)
  • Two main types:
    • Asexual reproduction
    • Sexual reproduction
asexual reproduction
Asexual Reproduction
  • Parent passes all of its genes to its offspring
    • Offspring are exact copies of parents
    • Also known as cloning (clones)
  • Uses mitosis
  • Often called budding
  • Comment - many organisms reproduce this way
    • Ex: jellyfish, hydra, sponges
advantages of asexual
Advantages of Asexual
  • Only need 1 parent
  • Offspring are identical to the parent
  • Good genetic traits are conserved and reproduced
  • Usually requires less energy
    • Why?
disadvantages of asexual
Disadvantages of Asexual
  • No new DNA combinations for evolution to work on
    • Only genetic differences come from DNA mutations (during DNA replication, etc)
  • Clones may become extinct if attacked by a disease or pest
sexual reproduction
Sexual Reproduction
  • Two parents contribute DNA to an offspring
  • Comment - most organisms reproduce this way, but it hasn’t been proven in some fungi and a few others
sex chromosomes
Sex Chromosomes
  • Homologous?
    • Mostly no! Only a SMALL part of these 2 are homologous
  • Males: One X chrom, One Y chrom
    • Y chromosomes are smaller (with more genetic disorders)
  • Females: Two X chromosomes
    • X chromosomes are bigger (usually contain more genes)
egg vs sperm
Egg vs. Sperm
  • Eggs contain ONLY X chromosomes
  • Sperm contain EITHER X or Y chromosomes
    • Why?
      • Because boys (when they are made during fertilization) get an X chromo from mom and a Y chromo from dad – making them a MALE!
      • If the cell were to get two X chromos, the fertilized cell would become a FEMALE!
who determines sex of baby
Who determines sex of baby?
  • Males determine what sex the offspring will be
    • Why?
      • Their sperm have the “option” of having X or Y
        • X from egg and Y from sperm = XY (male)
        • X from egg and X from sperm = XX (female)
advantages of sexual
Advantages of Sexual
  • Offspring has a unique combination of DNA which may be an improvement over both parents
  • New combination of DNA for evolution to work with
disadvantages of sexual
Disadvantages of Sexual
  • Need two parents (requires time and energy to look/find the right mate)
  • Good gene combinations can be lost
  • Offspring may not be an improvement over the parents
Question ?
  • Do parents give their whole DNA copy to each offspring?
  • What would happen to the chromosome number if they did?
chromosome number
Chromosome Number
  • Is usually constant for a species
  • Two of these are usually sex chromosomes
    • XY or XX in humans
  • Examples:
    • Humans – 46 (pair #23 is sex)
    • Corn - 20
    • Onions - 16
    • Dogs - 72
two options for life cycle
Two options for life cycle…

1) Mitotic cell division continues life

2) Meiosis cell division continues life

WHICH way does life reproduce?

result of life cycle if mitosis
Result of Life cycle: if Mitosis
  • 46 in egg + 46 in sperm=

92 total chromosomes

  • Chromosome number would double each generation.
  • Need a method to reduce the chromosome number.
result of life cycle if meiosis
Result of life cycle: if Meiosis
  • Egg 23 + sperm 23 =

46 total chromosomes

  • Chromosome number will remain the same with each sexual reproduction event.
  • Meiosis is used to produce the gametes or sex cells.
    • 46 reduced to 23 through meiosis


meiosis purpose
Meiosis - Purpose
  • To reduce the number of chromosomes by half
    • 46 to 23 (in humans)
  • Prevents doubling of chromosome numbers during sexual reproduction
sexual life cycle
Sexual Life Cycle
  • Has alternation of meiosis and fertilization to keep the chromosome numbers constant for a species
    • Fertilization: 23 + 23 = 46 total
    • Mitosis: takes original cell made through fertilization and multiplies it to form organs, etc.
    • Meiosis: 46 reduced to 23 (in sex organs)
  • 2 sets of chromosomes
  • Referred to as 2n
  • Most common number in body or somatic cells
    • Humans 2n = 46
    • Corn 2n = 20
    • Fruit Flies 2n = 8
  • 1 set of chromosomes
  • Number in the gametes or sex cells
    • Humans n = 23
    • Corn n = 10
    • Fruit Flies n = 4
    • Half of diploid #!!!!!!!
  • Multiple sets of chromosomes
  • Examples
    • 3N = triploid
    • 4N = tetraploid
  • Common in plants, but usually fatal in animals
    • Ex: Down Syndrome
    • Ex: Seedless watermelons

Triploid watermelon = no gametes produced (no seeds)

Triploid Daffodil = more stem produced per bulb

meiosis mitosis preview of differences
Meiosis/Mitosis Preview of differences
  • LOOK at pg. 256
  • Meiosis
    • Two cell divisions, not one
    • Four cells produced, not two
    • Synapsis and chiasmata will be observed
meiosis mitosis preview of differences1
Meiosis/Mitosis Preview of differences
  • Meiosis, cont.
    • 1st division separates PAIRS of chromosomes, not duplicate chromatids (of chromosome)
      • Known as homologous pairs of chromosomes
    • Interkinesis is present.

Mitosis vs. Meiosis Animation

  • Has two cell divisions (I and II)
  • Steps follow the names for mitosis
    • A “I” or “II” will be added to label
prophase i
Prophase I
  • Basic steps same as in prophase of Mitosis.
  • Synapsis occurs as the chromosomes condense.
    • Synapsis - homologous chromosomes form bivalents or tetrads
    • Synaptonemalcomplex - proteins which hold chromosomes together
prophase i cont
Prophase I, cont.
  • Chiasmataobserved
    • Spot where chromatids cross-over
    • Held together by this until Anaphase
  • Longest phase of division
    • Cell spends 90% of meiosis in this stage
  • Chromatin condenses, nucleolus disappears, nuclear envelope dissolves
metaphase i
Metaphase I
  • Tetrads or bivalents align on the metaphase plate
    • Attached to kinetochores of opposite poles
  • Centromeres of homologous pairs point toward opposite poles
anaphase i
Anaphase I
  • Homologous PAIRSseparate
    • Uses spindle apparatus
  • Duplicate chromosomes are still attached at the centromeres
  • Maternal and paternal chromosomes are now separated randomly
telophase i
Telophase I
  • Similar to Mitosis
  • Chromosomes may or may not unwind to chromatin
  • Cytokinesis separates cytoplasm and 2 cells are formed
  • NOW:
    • Each cell has a haploid set of chromosomes
      • Each chromosome is STILL comprised of 2 identical sister chromatids!!!
  • No DNA synthesis/copying occurs
  • May last for years, or the cell may go immediately into Meiosis II
  • Can appear similar to Interphase of Mitosis
meiosis ii
Meiosis II
  • Steps are the same as in Mitosis
    • Prophase II = Prophase
    • Metaphase II = Metaphase
    • Anaphase II = Anaphase
    • TelophaseII = Telophase
meiosis results
Meiosis - Results

Meiosis animation

  • 4 cells produced
    • Each cell has ONE sister chromatid
    • Remember: Started with homologous chromosome PAIR
  • Chromosome number reduced
  • Gametes (sex cells) made
  • Genetic variation increased
sexual sources of genetic variation
Sexual Sources of Genetic Variation
  • Independent Assortment of

Chromosomes during Meiosis

2. Random Fertilization

3. Crossing Over

independent assortment
Independent Assortment
  • There are 23 pairs of chromosomes in humans
  • The chance to inherit a single chromosome (maternal or paternal) of each pair is 1/2
    • Random arrangement of homologous pairs during metaphase I
    • This arrangement is totally random!
gamete possibilities
Gamete Possibilities
  • With 23 pairs of chromosomes, the number of combinations of chromosome types (paternal and maternal) are:

223or 8,388,608

(w/ each offspring!)

random fertilization
Random Fertilization
  • Choice of which sperm fuses with which egg (random choice)
  • With 8,388,608 kinds of sperm and 8,388,608 kinds of eggs, the possible combos of offspring is over 64 million (for EACH set of parents)
crossing over
  • Crossing over
  • The exchange of sister chromatid material during synapsis
  • Occurs ONLY in Prophase I
  • Produces recombinant chromosomes
    • Chromosomes with DNA from both parents
  • Chiasmata:
    • The point of contact where two chromosomes are crossing-over
  • Breaks old linkage groups
  • Creates new linkage groups increases genetic variation
  • Very common during meiosis
  • Frequency can be used to map the position of genes on chromosomes
  • Offspring can never be 100% like a parent if sexual reproduction is used
  • Multiple cross-overs are common, especially on large chromosomes
  • Genes near the centromere do not cross-over very often
  • Recognize the general relationships between genes, DNA, and chromosomes.
  • Identify characteristics, advantages, and limitations of asexual and sexual reproduction.
  • Recognize several sexual life cycles differing in the timing of meiosis and fertilization.
  • Recognize the stages and characteristics of the meiosis cell division process.
  • Contrast and compare meiosis to mitosis.
  • Identify sexual sources of genetic variation.
  • Use Chapter 46 to see differences in male/female meiosis (pgs. 1006-1007)