1. DNA and the Genetic Basis of Life: Another Great Divide
All About You
Science Teacher Education Course

2. Bell Ringer
How many chromosomes are in a human somatic (body) cell?

3. What is sexual reproduction?
Sexual reproduction—the process in which genetic material from two parents combines and produces offspring that is genetically different from each parent—is how humans create new life. Children resemble their parents because they inherit half their genetic material from their mother and the other half from their father. In order to ensure the genetic diversity of offspring while keeping the amount of genetic material the same, sexual reproduction depends on meiosis, which is the cell division process that creates specialized sex cells.
Refer to Figure 1. Each parent of a sexually reproducing species contributes a unique sex cell (gamete). Males produce sperm in their testes and females produce eggs in their ovaries. Each sex cell contains a haploid number of chromosomes, or half the number of chromosomes. When a sperm cell successfully fertilizes an egg cell, their nuclei fuse. They produce a diploid somatic cell called a zygote. A zygote is a cell that contains the full number of chromosomes. For humans, the haploid number is n=23, and the diploid number is 2n=46 (or 23 pairs). This zygote inherits half of each pair of chromosomes from the mother, and inherits the other half of each pair of chromosomes from the father. The zygote will undergo countless rounds of cell division, which includes mitosis and cytokinesis, to become a fully developed human being.

4. Interphase Precedes meiosis
The diploid cell can grow by obtaining energy from food, transporting materials into and out of the cell, and preparing for cell division
Originally, there was only one copy of the purple DNA and one copy of the green DNA but now they are duplicated
In early interphase, the nucleolus is still visible
In late interphase, the nucleolus disappears and DNA condenses into chromatin
The nuclear membrane is still intact
Centrioles may also be visible at this time
Cytoplasm
Cell membrane
Chromatin
Centrosome (centrioles)
Nuclear membrane
Nucleus

5. Meiosis I: Prophase I Duplicating homologous chromosomes
Chromatin is now condensed into chromosomes
First, we see a pair of homologous chromosomes of a diploid parent cell
The green chromosome was inherited from the mother and the purple chromosome was inherited from the father
Each pair of homologous chromosomes in the diploid cell is duplicated and the copies are called sister chromatids and they are attached by a protein disk called a centromere
The resulting pair of duplicated homologous chromosomes is called a tetrad (there are four)

6. Meiosis I: Prophase I (continued)
Crossing over
Each pair of duplicated homologous chromosomes (non-sister chromatids) exchanges genetic material
The homologous chromosomes line up so that they only exchange parts of the same genes
This ensures that at the end of meiosis, each sex cell will contain a unique genetic make-up and helps to explains why siblings, with the exception of identical twins, do not look exactly alike
Tetrad
Centromere
Sister chromatids

7. Meiosis I: Prophase I (continued)
The chromosomes are found in between the two poles of the cell
The centrioles are now at either poles of the cell
Spindle microtubules extend from the centrioles towards the center of the cell
Cytoplasm
Cell membrane
Spindle microtubules
Chromosomes
Centrosome (centrioles)

8. Centrosome (centrioles)
Meiosis I: Metaphase I
Tetrads (duplicated homologous chromosomes) line up along the middle of the cell
Chromosomes face opposite poles of the cell
Spindle microtubules extend towards the center of the cell and some attach to the centromeres while other spindle microtubules hang into the cytoplasm
Cell membrane
Cytoplasm
Chromosomes
Spindle microtubules
Centrosome (centrioles)

9. Centrosome (centrioles)
Meiosis I: Anaphase I
The spindle microtubules attached to the centromere of each homologous chromosome separate them to opposite ends of the cell (note that the sister chromatids stay together)
Cell membrane
Chromosome
Cytoplasm
Chromosome
Spindle microtubules
Centrosome (centrioles)

10. Meiosis I: Telophase I and Cytokinesis
Each half of the cell contains one copy of the chromosomes, thus setting up the correct environment for the cells to become haploid
Spindle microtubules disintegrate and cytokinesis begins
Cell membrane and cytoplasm pinch inward until two new haploid cells are produced
Afterwards, meiosis II can begin
Chromosome
Cell membrane
Daughter cells
Cleavage furrow
Cytoplasm
Chromosome
Centrosome (Centrioles)

11. Meiosis II: Prophase II
Spindle microtubules begin to form at the poles of the two haploid cells
Chromosomes, consisting of two sister chromatids each, migrate towards the center of the cell (recall that due to crossing over, the sister chromatids are no longer genetically identical to one another)
Cytoplasm
Centrosome (Centrioles)
Cell membrane
Spindle microtubule
Chromosome

12. Meiosis II: Metaphase II
Chromosomes line up in the middle of the cell
Each sister chromatid faces the opposite end of the cell
Spindle microtubules extend towards the center of the cell and some attach to the centromeres, while others hang into the cytoplasm
Chromosome
Cell membrane
Centrosome (Centrioles)
Cytoplasm
Spindle microtubules

13. Meiosis II: Anaphase II
The spindle microtubules attach to the centromere of each sister chromatid and separate them to opposite ends of the cell
Sister chromatid
Sister chromatid
Cell membrane
Centrosome (Centrioles)
Cytoplasm
Spindle microtubule

14. Meiosis II: Telophase II and Cytokinesis
Sister chromatids reach the opposite end of each cell and nuclear membranes form around them
Spindle microtubules disintegrate
Cell membrane and cytoplasm pinches inward to create new cells
The result is four genetically different haploid cells that can be sperm or egg cells
Cytoplasm
Cell membrane
Nucleus
Nuclear membrane
Chromatin

15. Karyotype Centromere Sister chromatids Pair of homologous chromosomes5
Sex chromosomes 15

16. Down Syndrome
Trisomy 21, called Down syndrome, produces a characteristic set of symptoms, which include:
mental retardation,
characteristic facial features,
short stature,
heart defects,
susceptibility to respiratory infections, leukemia, and Alzheimer’s disease, and
shortened life span.
The incidence increases with the age of the mother.

17. Figure 8.19A
Figure 8.19A A karyotype showing trisomy 21, and an individual with Down syndrome
Trisomy 21 17

18. Accidents Occur in Meiosis
Nondisjunction is when chromosomes or chromatids do not separate normally during meiosis. This can happen during
meiosis I, if both members of a homologous pair go to one pole or
meiosis II if both sister chromatids go to one pole.
Fertilization after nondisjunction yields zygotes with altered numbers of chromosomes.

19. Multicellular diploid adults (2n  46) Diploid stage (2n)
Figure 8.12A
Haploid gametes (n  23) n
Egg cell n
Sperm cell
Meiosis
Fertilization
Ovary
Testis
Figure 8.12A The human life cycle
Diploid
zygote
(2n  46) 2n
Key
Mitosis
Haploid stage (n)
Multicellular diploid
adults (2n  46)
Diploid stage (2n) 19

20. MEIOSIS I Nondisjunction MEIOSIS II Normal meiosis II Gametes
Figure 8.20A_s3
MEIOSIS I
Nondisjunction
MEIOSIS II
Normal
meiosis II
Figure 8.20A_s3 Nondisjunction in meiosis I (step 3)
Gametes
Number of
chromosomes
n  1
n  1
n  1
n  1
Abnormal gametes 20

21. MEIOSIS I Normal meiosis I MEIOSIS II Nondisjunction n  1 n  1 n n
Figure 8.20B_s3
MEIOSIS I
Normal
meiosis I
MEIOSIS II
Nondisjunction
Figure 8.20B_s3 Nondisjunction in meiosis II (step 3)
n  1
n  1 n n
Abnormal gametes
Normal gametes 21

22. Mistakes with Sex Chromosomes
Sex chromosome abnormalities tend to be less severe, perhaps because of
The Y-chromosome being very small and carrying very little genetic information or
Extra X-chromosomes becoming inactivated and the organism operating with one functioning one X- chromosome

23. Errors in Cell Division Can Make New Species
Errors in mitosis or meiosis may produce polyploid species, with more than two chromosome sets.
The formation of polyploid species is
widely observed in many plant species but
less frequently found in animals.
Gray Tree Frog
Tetraploid organism (4n)
Scientists still do not understand how polyploidy can cause organisms to so different from their diploid ancestors

24. Alterations to a Chromosome’s Structure Can Cause Birth Defects and Cancer
Deletion- the loss of a chromosome segment
Duplication- the repeat of a chromosome segment,
Inversion- the reversal of a chromosome segment
Translocation- the attachment of a segment to a nonhomologous chromosome that can be reciprocal

25. Reciprocal translocation
Figure 8.23A
Deletion
Inversion
Duplication
Reciprocal translocation
Homologous
chromosomes
Nonhomologous
chromosomes
Figure 8.23A Alterations of chromosome structure 25