1. PowerPoint Presentation Materials to accompany
Genetics: Analysis and Principles
Robert J. Brooker
CHAPTER 3
REPRODUCTION AND CHROMOSOME TRANSMISSION
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2. Objectives: To study reproduction at the cellular level.
To review the inheritance of chromosomes.
To consider how cell distribute their chromosomes during the process of cell division called mitosis.
To explore a second form of cell division: meiosis.
To understand how the transmission of chromosomes accounts for the inheritance patterns observed by Mendel.

3. Introduction
chromosomes- contain the genetic material, genes, a long segment of DNA and proteins
chromatin – complex between DNA and proteins
Prokaryotes – bacteria and archaea, prenucleus, single chromosome, not membrane bound nucleus
Nucleoid – region in a cytoplasm with the chromosome
Eukaryotes- true-nucleus, compartamentalization
Nucleus- two membranes, localization of chromosomes
Mitochondria, chloroplast – contains extranuclear or extrachromosomal DNA

5. Introduction
Cytogenetics – field of genetics that involve microscopic examination of the chromosomes
Somatic cell- cell of the body that is not a gamete or a precursor to a gamete
Germ cell- gametes (sperm and egg, or their precursors)
Karyotype – organized representation of the chromosomes
Diploid – each chromosome is in pair, two sets of them
Homolog- the member of each par or chromosomes in diploid cells
Locus – physical location of a gene in a chromosome
Best stage – condensed, shortens, increase their diameter
Disctintive shapes and numbers, visible with a light microscope
Changes in number, shapes, abnormalities
Bands..

7. The physical location of a gene on a chromosome is called its locus.
Figure 3.3
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8. MITOSIS
Cell division in eukaryotes requires a replication and sorting process that is more complicated than simple binary fission
Cell cycle: a series of stages observed in eukaryotic cells that are destined to divide (grow) Refer to Figure 3.5
Common outcome of eukaryotic cell division is to produce two daughter cell with the same number and types of chromosomes as the original mother
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9. 3-24 The Cell Cycle Synthesis Gap 1 Gap 2 Figure 3.5
This diagram shows the progression of a cell through mitosis to produce two daughter cells. The original diploid cell had three pair of chromosomes, for a total of six individual chromosomes.
During S phase, these have replicated to yield 12 chromatids found in six pairs of sister chromatids.
After mitosis and cytokinesis are completed, each of the two daughter cells contains six individual chromosomes, just like the mother cell.
The chromosomes in G0, 1, S and G2 phases are no condensed. IN this drawing, they are shown partially condensed so they can be easily counted.
Figure 3.5
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10. MITOSIS
In actively dividing cells, G1, S and G2 are collectively know as interphase
A cell may remain for long periods of time in the G0 phase
A cell in this phase has
Either postponed making a decision to divide
Or made the decision to never divide again
Terminally differentiated cells (e.g. nerve cells)
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11. MITOSIS G1 phase: the cell prepares to divide
The cell reaches a restriction point and is committed on a pathway to cell division
Then the cell advances to the S phase, where chromosomes are replicated
The two copies of a replicated chromosome are termed chromatids
They are joined at the centromere to form a pair of sister chromatids
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12. 3-27 Figure 3.6 (b) Cuantos cromosomas hay aqui?
Cuantas cromatidas? Each chromatid is a distinct unit
Figure 3.6 (b)
3-27
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13. Therefore the term chromosome is relative
At the end of S phase: a cell has twice as many chromatids as there are chromosomes in the G1 phase
A human cell for example has
46 distinct chromosomes in G1 phase
46 pairs of sister chromatids in S phase
Therefore the term chromosome is relative
In G1 and late in the M phase, it refers to the equivalent of one chromatid
In G2 and early in the M phase, it refers to a pair of sister chromatids
G1 y M tarde – un cromosoma es una cromatida
G2 y M temprana – un cromosoma es un par de cromatidas hnas
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14. It then progresses into the M phase of the cycle where mitosis occurs
G2 phase: the cell accumulates the materials that are necessary for nuclear and cell division
It then progresses into the M phase of the cycle where mitosis occurs
The primary purpose of mitosis is to distribute the replicated chromosomes to the two daughter cells
In humans for example,
The 46 pairs of sister chromatids are separated and sorted
Each daughter cell thus receives 46 chromosomes
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15. The process of mitosis is shown in Figure 3.7
Mitosis was first observed microscopically in the 1870s by the German biologist, Walter Flemming
He coined the term mitosis
From the Greek mitos, meaning thread
The process of mitosis is shown in Figure 3.7
The original mother cell is diploid (2n)
It contains a total of six chromosomes
Three per set (n = 3)
One set is shown in blue and the homologous set in red
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16. Mitosis is subdivided into five phases Prophase Prometaphase Metaphase
Anaphase
Telophase
Refer to Figure 3.7
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17. Chromosomes are decondensed
By the end of this phase, the chromosomes have already replicated
But the six pairs of sister chromatids are not seen until prophase
The centrosome divides (MTOC)
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18. Nuclear envelope dissociates into smaller vesicles
Centrosomes separate to opposite poles
The mitotic spindle apparatus is formed
Composed of mircotubules (MTs)
Spindle
MTOCs
Centrosomas
centriolos
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19. Microtubules are formed by rapid polymerization of tubulin proteins
There are three types of spindle microtubules
1. Aster microtubules
Important for positioning of the spindle apparatus
2. Polar microtubules
Help to “push” the poles away from each other
3. Kinetochore microtubules
Attach to the kinetochore , which is bound to the centromere of each individual chromosome
Refer to Figure 3.8
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20. 3-35 Figure 3.8 Contacts the centromere Contacts the other two
Contacts the kinetochore microtubule
Figure 3.8
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21. Spindle fibers interact with the sister chromatids
Kinetochore microtubules grow from the two poles
If they make contact with a kinetochore, the sister chromatid is “captured”
If not, the microtubule depolymerizes and retracts to the centrosome
The two kinetochores on a pair of sister chromatids are attached to kinetochore MTs on opposite poles
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22. Pairs of sister chromatids align themselves along a plane called the metaphase plate
Each pair of chromatids is attached to both poles by kinetochore microtubules
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23. The connection holding the sister chromatids together is broken
Each chromatid, now an individual chromosome, is linked to only one pole
As anaphase proceeds
Kinetochore MTs shorten
Chromosomes move to opposite poles
Polar MTs lengthen
Poles themselves move further away from each other
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24. Chromosomes reach their respective poles and decondense
Nuclear membrane reforms to form two separate nuclei
In most cases, mitosis is quickly followed by cytokinesis
In animals
Formation of a cleavage furrow
In plants
Formation of a cell plate
Refer to Figure 3.9
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25. The two daughter cells are genetically identical to each other
Mitosis and cytokinesis ultimately produce two daughter cells having the same number of chromosomes as the mother cell
The two daughter cells are genetically identical to each other
Barring rare mutations
Thus, mitosis ensures genetic consistency from one cell to the next
The development of multicellularity relies on the repeated process of mitosis and cytokinesis
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26. Practica de laboratorio:
Observación de filminas de mitosis
Observarlas, dibujarlas, rotularlas
Estar preparados para identificarlas
Preparación de laminillas de Mitosis usando raices de cebolla.
Protocolo a continuación.

27. Cells division: Mitosis
Preparing Chromosome Squashes:
1. Place the onion root tip in 1 M HCl for 4 minutes.
2. Transfer the tip to Carnoy’s fixative** for 4 minutes.
3. Remove the slide from the Coplin jar containing 70% ethanol, dry with a scientific cleaning wipe, and label it.

28. Cells division: Mitosis
4. Place the onion tip on the slide, and cut off the distal 2 mm portion of the tip; discard the remainder of the tip.
5. Cover the root tip piece with carbol-fuschin stain for 2 minutes.
6. Blot off excess stain and cover tip with 1–2 drops of H2O.

29. Cells division: Mitosis
7. Gently tease the root tip apart with dissecting probes or needles. Place the cover slip over the root tip and cover the cover slip with a scientific cleaning wipe.
8. Firmly press down on the cover slip with your thumb or with the eraser end of a pencil. Do not twist the slide.

30. Cells division: Mitosis
Counting Cells and Analyzing Data:
1. Observe the cells at high magnification (400–500 X).
2. Look for well-stained, distinct cells.
3. Within the field of view, count the cells in each phase. Repeat the counts in two other field areas. Identification of these stages is prerequisite knowledge.

31. Cells division: Mitosis
4. Collect your group data, and prepare your laboratory report.
** Carnoy’s fixative: 125 mL glacial acetic acid mixed with 375 mL 95% ethanol
 Copyright © College Board, Media Service. Investigation 7 Cell Division: Mitosis and Meiosis, Genetics and Information Transfer.