Reproduction Reproduction and Meiosis Continuing the

							Reproduction and
         Meiosis
     Continuing the species
       Types of Reproduction

Asexual reproduction      Sexual reproduction
 One parent               Usually two parents
 Offspring genetically    Offspring genetically
  identical to parent       unique
  Asexual Reproduction – Binary
             Fission

 Occurs in bacteria (prokaryotic cells)

 Increases size, doubles DNA, and divides in
  half (sounds like mitosis!)
 Produces two identical daughter cells

 Does not involve the combination of genetic
  material, like sexual reproduction
  Asexual Reproduction – Binary
             Fission

Ring = DNA
  Asexual Reproduction – Binary
             Fission
Interesting fact:
 Some bacteria can grow and divide as
  frequently as every 20 minutes!
 In just 48 hours, they would reach a mass
  approximately 4,000 times the mass of Earth!
 Thankfully, in nature, growth is held in check
  by availability of food and production of waste
  products.
       Sexual Reproduction

 Sexual reproduction involves
   Meiosis – gamete (egg, sperm) production
   Fertilization (formation of a zygote) – restores
    diploid number
 Sexual reproduction produces genetic variation
  among offspring (siblings can look similar, but not
  necessarily identical)
            What is Meiosis?

 Meiosis:
   Is a special type of cell division where the
    daughter cells have half the number of
    chromosomes of the parent cell
   Creates gametes (sperm in males and egg in
    females)
   One diploid cell divides into four haploid cells
         Human Chromosomes
 Human body cells (somatic
  cells) have 46 chromosomes
  - Organized into 23 pairs of
     chromosomes
  - One set of 23 from mom, one
     set of 23 from dad
  - Each pair (one from mom, one
     from dad), is considered
     homologous chromosomes
  - Each homologous pair has the
                                   Homologous
     same type of genetic          chromosomes
     information
      Homologous Chromosomes –
           Shoes Analogy
Shoes:                             Homologous Chromosomes:
 You have 23 pairs of shoes        You have 23 pairs of
                                     homologous chromosomes
 46 shoes total                    46 chromosomes total
 Each pair of shoes is             Each pair of homologs is
  different from the other           different from the pairs of
  pairs of shoes                     homologs
 In each pair of shoes, each       In each homologous pair,
  shoe is very similar, but a        each homolog is very similar,
  little different – one for the     but a little different – one
  right foot and one for the         from mom and one from dad
  left
                 Gametes

 Human gametes (sperm for males and egg for
  females) cells have only 23 chromosomes (n)
  while somatic cells (body cells) have 46
  chromosomes (2n)
 What do you think has to happen in order to
  create gametes from somatic cells (body cells)?
       Diploid vs. Haploid Cells
 “n” = number of chromosomes in an organism’s cells

 Body cells (somatic) are DIPLOID (2n)
  - Diploid cells contain the full sets of chromosomes
  - The two corresponding chromosomes = homologous
     chromosomes
  - Each set of chromosomes comes from 1 parent

 Gametes are HAPLOID (1n)
  - Cells with half the set of chromosomes
  - Haploid = Half the normal number
  - Ex. Sperm and egg cells in humans
       Human Chromosomes

In human somatic cells:
 23 pairs of chromosomes (46 chromosomes total)

 Two types of chromosomes:

1. One pair of sex chromosomes, which determine
  gender of human
     XX – Female
     XY – Male

2. The other 22 pairs are autosomes
     Chromosomes Quick Check
Q: How many total chromosomes in this picture?

A: 46 chromosomes

Q: How many different types of

chromosomes?

A: 2 types – sex chromosomes

and autosomes
     Chromosomes Quick Check
Q: How many of each type of chromosome is there?

A: 2 sex chromosomes and 22 autosomes



Q: Are these chromosomes

taken from a haploid (n) or a

diploid (2n) organism?

A: diploid – each chromosome is

shown with its homologous pair
    Please do
“Quick Check”
   worksheet
         The Human Life Cycle

Why do gametes need to be haploid?

 Remember that humans have 46 chromosomes

 The nucleus of a sperm needs to fuse with the nucleus of
  the egg – the chromosomes get added together

Q: What would happen if a nucleus with 46 chromosomes
  fused with a nucleus with 46 chromosomes?

A: You would end up with a cell with 2 times too many
   chromosomes! Instead of having 46, it would have 92!
                      Fertilization
                                      Sperm cell (small cell
                                      in yellow)
 Fertilization – process that
  occurs when the nucleus of a
  sperm fuses (combines) with
  the nucleus of an egg

 Results is a single cell with one
  nucleus, containing 46
  chromosomes

 This cell is referred to as the
  zygote
                                      Egg cell (big spherical
                                      cell in green)
     Please do
“Introduction
   to Meiosis”
    Crossword
        Puzzle
        Big Picture of Meiosis

 Production of 4 haploid gametes from 1 diploid cell

 Cells undergo G1 (growth), S (DNA replication),
  and G2 (organelles duplicate) phases of interphase
 Meiosis begins with duplicate set of chromosomes
  (same as mitosis)
 Cells divide twice – meiosis I, meiosis II
         Homologs in Meiosis
Before cell division begins, all 46 chromosomes make
exact copies of themselves (DNA replication during S
phase) to form replicated chromosomes (X shape)




          From Dad   From Mom
Meiosis Animation
Overview of Meiosis I: Separation
 of Homologous Chromosomes
                       Each homologue in the
 Includes             cell pairs with its partner,

   Prophase I                                        then the partners
                                                      separate
   Metaphase I
   Anaphase I
   Telophase I and cytokinesis
 Involves division of a cell with splitting of
  homologous chromosomes, not of chromatids
 Results in two cells, each with one set of
  homologous chromosomes (n) (still replicated
  chromosomes, though)
                  PROPHASE I
 Duplicated DNA condenses into
    chromosomes (X shapes)
   Nuclear membrane breaks down
   Centrioles move to opposite poles
   Spindle fibers forms
   Crossing-over can take place
                Tetrad




Crossing Over
 Homologues intertwine

 Occurs between replicated
  homologous chromosomes (tetrad)
   Pieces of chromosomes swapped to
     mix up genes

 After crossing over, each
  chromosome contains DNA from
  mom (maternal) and dad (paternal)
                     Metaphase I
 Centrioles are at opposite poles
 Spindle is fully formed
 Homologous pairs randomly line
  up along metaphase plate =
  independent assortment (some
  chromosomes from mom in one
  gamete, some of dad’s in the same
  gamete)
 Chromosomes line up two-by-
  two…not single file!
Metaphase I
                     Anaphase I
 Spindle pulls one of each
  homologous chromosome to
  opposite pole
 Duplicated chromosomes (X
  shapes) still visible – (one X is
  pulled from another X, but the X
  is not split apart)
 Therefore, sister chromatids are
  NOT pulled apart
Anaphase I
                      Telophase I/
                      Cytokinesis
 Nuclear membrane may or may not
   appear
 Cytokinesis occurs
Q: If in prophase a cell has 10
   replicated chromosomes, how many
   are in each daughter cell at this
   stage?
A: 5
Q: Are these chromosomes replicated
   or not?
A: Yes
 End of meiosis I – haploid cells
Telophase I/
Cytokinesis
  Overview of Meiosis II: Separating
         Sister Chromatids
 Starts with two cells from the end of meiosis I
 Includes
   Prophase II
   Metaphase II        one chromosome            two chromosomes
                          (duplicated)              (unduplicated)
   Anaphase II
   Telophase II and cytokinesis
 Involves division of a cell with splitting of sister
  chromatids
 End of meiosis I: haploid cells with duplicated
  chromosomes
 End of meiosis II: haploid cells with single chromosomes
                Prophase II
 If nuclear membrane
 reformed in telophase
 I, then it breaks down
 Spindle forms and the
 centrioles move away
 from each other
             Metaphase II


 Replicated chromosomes
 line up single file along
 metaphase plate (looks
 like metaphase of
 mitosis)
               Anaphase II


 Spindle fibers pull sister
  chromatids to opposite
  poles of cell
                 Telophase II/
                  Cytokinesis
Q: How many cells are made in telophase II for one
  cell that started meiosis?
A: 4
Q: If the prophase I cell had 10 replicated
  chromosomes, how many chromosomes are
  present in each product of telophase II?
A: 5
Q: Are these chromosomes replicated?
A: No
       FOUR PRODUCTS FULLY
             FORMED



 No two are identical due
  to crossing over
 Cells develop
  immediately into
  gametes
Meiosis Animation
         “Meiosis
Sequencing Cards”
 Please do
  “Meiosis
Matching”
worksheet
   Please do
     meiosis
concept map
   Please do
“Exercise 10”
  Worksheet
             Spermatogenesis
 Sperm cells are derived from special cells within the
  testes
 These cells undergo meiosis to produce 4 sperm cells
SPERMATOGENESIS
Sexual Reproduction and Spermatogenesis: Passing on Your DNA
http://www.pbs.org/wgbh/nova/miracle/program.html
                 Oogenesis
 Four cells are produced, but unlike sperm, only
  one forms the mature ovum (egg).
 The other three become polar bodies, which are
  not used for reproduction.
Oogenesis
Spermatogenesis/Oogenesis
Results of Mitosis and Meiosis

Mitosis
  - Two diploid cells produced
  - Each identical to parent

Meiosis
  - Four haploid cells produced
  - Differ from parent and one another
   Please do
“Exercise 11”
  Worksheet