Bio 1010
Dr. Bonnie A. Bain
CHAPTER 8
Cell Reproduction
Part 2
Assignments
See handout
10 points each
Complete 4 of these by the end of the semester
APA format
Standard citation format used in science writing
Assignments
APA format examples:
Author. Year. Title. Journal name, volume, pages
Karcher, C. A. (1986). Censorship, American style: The case of Lydia Maria Child. Studies in the American Renaissance, 9, 283­303.
MITOSIS: SIMPLE CELL DIVISION Cells make exact replicas of themselves, including chromosome number
MEIOSIS: NOT­SO­SIMPLE CELL DIVISION Function: forms gametes (egg, sperm)
Meiosis involves both nuclear and cellular division and is more complicated than mitosis
Unnumbered Figure 08_UN141c
Why Meiosis??
1. Allows: Maple trees to make more maple trees
Goldfish to make more goldfish
Humans to make more humans
etc.
When sperm and egg combine, the new individual is not an exact copy of either parent
Figure 8.12
Why Meiosis??
2. Meiosis maintains the constancy of the chromosome
number for a species
Any deviation from this number can result in a pathological condition
Example: Down's Syndrome (Trisomy 21)
Figure 8.22
Chromosomes
Humans: 23 pairs of homologous chromosomes or 46 individual ones
Since sperm and egg are going to combine at fertilization, each needs half the chromosome number so that the zygote will have the normal number of chromosomes (23 pairs)
Some Definitions:
Somatic cell: all cells except the germ cells
Germ cell:
Cells which make gametes (eggs or sperm)
Some More Definitions:
Karyotype:
A picture of all chromosomes in a cell
Taken during Metaphase
Homologous chromosomes:
Chromosomes occur in pairs
One is from Mom and one is from Dad
Figure 8.13
Homologous chromosomes:
Each carry versions of the same gene
Example: an eye color gene
The chromosome from Mom has the gene
for blue eyes
The chromosome from Dad has the gene for brown eyes
At Metaphase, the homologous pairs of
chromosomes line up on the spindle fibers
Humans have 23 pairs of chromosomes
22 of these are called autosomes
Pair #23 are the sex chromosomes (next slide)
SEX CHROMOSOMES Females have an identical pair of these:
XX
Males have one X and one little tiny Y:
XY
Figure 8.13
Extra Photo 08.13x4
Extra Photo 08.13x2
More definitions
2n = Diploid number (normal number)
n = Haploid number (half the normal number)
4n, 6n, 8n, etc. = Polyploid
Plants are OK with this, animals not so OK
In humans:
2n = 23 pairs of chromosomes (46 total)
n = 23 individual chromosomes
ALL BODY CELLS EXCEPT GAMETES ARE DIPLOID
GAMETES ARE HAPLOID
Figure 8.14
Question: How can we produce large quantities of gametes which will have only half the normal chromosome number?
Answer: through Meiosis
Eggs are produced in the female ovaries
Sperm are produced in the male testes
Fertilization: egg and sperm join together
Zygote: the developing embryo
Figure 8.14
COMPARISON OF MITOSIS AND MEIOSIS
Mitosis: The chromosomes replicate once
and
The nucleus divides once
and
the cytoplasm divides once
Result:
2 new daughter cells with the same chromosome number (2n or diploid)
Unnumbered Figure 08_UN141c
COMPARISON OF MITOSIS AND MEIOSIS
Meiosis:
Chromosomes replicate once
and
the nucleus divides twice
(cytoplasm also divides several times)
Result:
4 new daughter cells, each has half the normal chromosome number or n (haploid)
Figure 8.15
One pair (red, blue) of homologous chromosomes:
Meiosis I
Prophase I
Metaphase I
Anaphase I
Telophase I
End Result: 2 haploid cells
Meiosis II
Prophase II
Metaphase II
Anaphase II
Telophase II
End Result: 4 haploid cells
Meiosis I
Prophase I
Chromatin condenses to form chromosomes
Each chromosome made of 2 chromatids joined by centromere
Homologous chromosomes form pairs (tetrads)
Crossing over occurs here Centrioles produce spindle fibers
Nuclear envelope disintegrates
Meiosis I
Meiosis I
Metaphase I
Tetrads line up on equatorial plate of cell
Their centromeres are attached to the spindle fibers
Anaphase I: Homologous chromosomes pull apart and migrate to opposite poles
Telophase I
New nuclear envelopes form around the chromosomes
Cytokinesis (cytoplasmic division) occurs
Result: 2 haploid cells
Meiosis I
Meiosis II
Prophase II
Each chromosome has 2 chromatids
New mitotic spindle forms
Nuclear envelopes disintegrate again
Metaphase II
Chromosomes align on equatorial plane (again)
Meiosis II
Meiosis II
Anaphase II
Centromeres divide
Sister chromatids migrate to opposite poles
Each chromatid is now a single strand (no longer double)
Meiosis II
Meiosis II
Telophase II
New nuclear envelopes form
Chromosomes unwind and “disappear”
Cytoplasm divides again
End Result: 4 haploid cells
Spermatogenesis
Oogenesis
Origins of Genetic Variation
Independent Assortment of Chromosomes
Which chromosomes go into each gamete is a random process (see next slide)
Crossing Over
Chromosomes have sticky ends
When they are lined up, the ends stick to each other and sometimes switch places
Figure 8.18
Figure 8.19
Crossing Over
Chiasma: the place where
crossing over occurs
Recombinant Chromosomes:
These are the result of the genetic recombination from
the crossing over
Accidents during Meiosis
Usually, Meiosis is accident­free, but sometimes not
Nondisjunction: An accident where the members of the chromosome pair fail to separate at Anaphase
Can occur in either Meiosis I or Meiosis II
Result: gametes with abnormal chromosome
numbers (p. 138­139 in text)
Figure 8.20
Figure 8.21
Table 8.1
Klinefelter's Syndrome (XXY)
Male sex organs present, but testes abnormally small (he is sterile due to this)
Often has breast enlargements and female body contours
Some have XXYY, XXXY, or XXXXY
Many of these have developmental disabilities as well as the above symptoms
XYY Male
No defined syndrome, appears normal, but taller than average
XXX Female
No defined syndrome, can only tell by looking at karyotype
Turner's Syndrome (XO)
Female
Have a short stature, web of skin between neck and shoulders, sex organs do not fully mature (making them sterile) Normal intelligence
Only non­fatal condition where a human has only 45 chromosomes