Learning Mitosis/Meiosis: Objectives
1. 2. Differentiate between haploid (n) and diploid cells (2n) Compare the number and type of chromosomes in a cell in G1, S, G2, and then after M phase. Define homologues and sister chromatids, when do you see each. Recognize how the number and type of chromosomes differs in a gamete (sperm and eggs) compared to a somatic (body) cell. Describe how chromosomes are separated in meiosis and how this differs from mitosis. Apply the knowledge of how chromosomes separate during meiosis and the rules of probability to predict the likelihood that offspring from two parents would inherit a specific combination of chromosomes and the genes they contain.
3.
4. 5.
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Mitosis
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Chromosomes, so many!
l Distinct
size, shape, staining pattern l Number of different chromosomes = (n) also called the haploid number. l Many organisms have pairs of each chromosome (2n) = diploid pairs are called homologues (similar but not identical) l Humans n = 23 2n = 46
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Humans & Most Animals Diploid
l2
copies of each chromosome l called homologues not identical, but variants of each other l 46 total chromosomes
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Clicker Question 1
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In this baggie, I am using pasta to represent chromosomes in a cell. Each different shape represents a different chromosome. What is n for your cell? A. 2 B. 4 C. 6 D. 8
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Clicker Question 2
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In this bag of pasta what would represent a pair of homologous chromosomes?
A
B.
C.
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S-Phase: Replication
Chromosomes copied (replication) called Sister chromatids that are separated during M phase. Identical pastas
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Clicker Question 3
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If you investigated cells from a lab mouse with 40 chromosomes you would find __ sister chromatids first after the ___ stage. A. 10: G1 B. 20: G2 C. 40: M D. 80: S
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Clicker Question 4
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In the bag of pasta what would represent a pair of sister chromatids?
A.
B.
C
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Clicker Question 5
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I'll arrange the pasta shapes on a sheet of paper to show a cell with 2n=8. How many total chromosomes would this cell have?
A. B. C. D.
10
2 4 8 16
Clicker Question 6
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After this cell goes through replication to create identical sister chromatids. How many sister chromatids would be present in total? A. 2 B. 4 C. 8 D. 16
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II. M- Phase (Cell Division)
l Two
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parts: l Nuclear Division (Mitosis) sister chromatids separate l Cytoplasmic Division (Cytokinesis) l Mitosis must finish before cytokinesis can begin
Four Stages of Mitosis
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Group Activity:
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Take a sheet of paper and using two colored pencils, one for each homologue like we used different colored pasta, diagram what the chromosomes look like in a cell with 2n =4. Be sure to label the sister chromatids and chromosomes at: G1 S Metaphase of mitosis After cytokinesis
The Case:
l Jack
and Lisa Nash's daughter Molly was born with a rare, incurable genetic condition called Fanconi anemia which rendered her body unable to produce enough blood cells...
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Matching Organs: HLA
Class I
Class II
l Finding
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a correct match for tissue transplantation depends on matching a specific group of proteins found on the surface of white blood cells that are used to distinguish one's own cells from foreign cells, called HLA, for human leukocyte antigen. l Two major HLA proteins: Class I and Class II each composed of several polypeptide chains.
Matches for Organ Donation
l Human
leukocyte antigen (HLA) proteins encoded by several genes on chromosome 6. lMany variants of gene = mutations that create differences (alleles)
HLA Class I Gene Alleles HLA Class II
6
A 697
B 1,109
C 381
E 9
F
G
21 36
Alleles
1,012
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HLA Proteins Present Foreign Antigens
foreign cell
Macrophage
Lysosome digests proteins
Peptides displayed by HLA proteins
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HLA: Inherited from Parents
Mrs. Nash diploid = two homologues of chromosome 6
HLA-A1 HLA-B35
HLA-A2 HLA-B44
A1
A2
B35 B44
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A1
A2 B44
Review: Mitosis
l Asexual
B35
S phase
Reproduction Sister chromatids Sister chromatids A1 A1 l Occurs in Somatic (body cells) for B35 B35 growth and division l Creates genetically identical cells A2 A2 B44 B44 l How did genetic variation occur in the mitosis first place? l Need new method: sexual A1 A2 A1 A2 B35 B44 B35 B44 reproduction
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Why do diploid organisms need to have specialized sex cells?
l Sex
cells (gametes) allow traits to be combined from two organisms l Can't just fuse any two random cells
2n (46)
A1 B35 A2 B44
+
2n (46)
A3 B41 A26 B35
4n = 92 too many 21
Sexual Reproduction
l Meiosis
= specialized cell division so you have only one of each chromosome, called l Gametes: (n) made only in gonad (testis, ovary)
n (23)
A2 B44
+
2n = 46
n (23)
A26 B44
A26 B35
A2 B35
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Mitosis versus Meiosis
MITOSIS Diploid MEIOSIS Diploid
somatic cell
gamete precursor duplication division
Diploid
Haploid
division
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Meiosis, 2 divisions, Meiosis I and Meiosis II
A1 B35
A2 B44
A1 B35
A2 B44
MitosisSister chromatids
A1 B35 A1 B35 A2 B44 A2 B44
Meiosis I Sister chromatids
Homologues Pair
A1 B35 A1 A2 B35B44
mitosis
meiosis I
A1
A2 B44
A1 B35
A2 B44
A1 B35
A1 B35
A2 B44
A2 B44
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B35
Clicker Question 7
At the end of meiosis I, all of the following have occurred, EXCEPT: A. Sister chromatids have been separated into two different cells. B. Each new cell has only one of each different chromosome (n, haploid) C. Homologous chromosomes have been separated into different cells, the sister chromatids are still together.
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Meiosis II: sister chromatids separate
A1 B35 A1 B35 A2 B44 A2 B44
meiosis II
meiosis II
A1 B35
A1 B35
A2 B44
A2 B44
l End
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result: four haploid cells total
Group Activity:
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On your same sheet of paper with the colored pencils and your cell showing (2n = 4) and labeling the sister chromatids and chromosomes Diagram what the cell looks like during: Metaphase I of meiosis After cytokinesis I Metaphase II of meiosis After cytokinesis II
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Clicker Question 8
A. B. C. D. E.
When Mrs. Nash produces eggs, they would have which combinations of the HLA-A and HLA-B genes and in what proportions? 100% B35-A1 A1 A2 50% B35-B44 & 50% A1-A2 B35 B44 50% B35-A1 & 50% B44-A2 100% B44-A2 Mrs. Nash's 50% B35-A2, & 50% B44-A1 chromosome 6s
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Clicker Question 9
A. B. C. D. E.
When Mr. Nash produces sperm, they would have which combinations of the HLA-A and HLA-B genes and in what proportions? 100% B5-A26 A3 A26 50% B35-A3 & 50% B41-A26 B41 B35 50% B35-B41 & 50% A3-A26 100% B44-A2 B35-A26 Mr. Nash's 50% B35-A26, & 50% B41-A3 chromosome 6s
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Clicker Question 10
A. B. C. D.
Which of the following shows one combination of HLA-A and HLA-B genes expected in an offspring of Lisa and Jack Nash? B35, B35, A1, A2 A1 A2 A3 A26 B35, B44, A2, A3 B35 B44 B41 B35 B44, B35, A2, A26 B41, B44, A26, A2 Mrs. Nash Mr. Nash
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Clicker Question 11
Is it possible for any child born to this couple to be identical in both HLA-A and HLA-B with either parent? A. Yes A1 A2 A3 A26 B. No
B35 B44 B41 B35
Mrs. Nash
Mr. Nash
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Clicker Question 12
A. B. C. D.
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E.
Using the proportion of each type of sperm and egg that you calculated in questions I and II, what is the likelihood that Molly's sibling would inherit the same combination of HLA genes and thus be a good organ donor for Molly? No chance 25% Chromosome 6s for A26 A2 50% Molly Nash B35 B44 75% 100%
Summary
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So, clearly Molly's best chance of finding a bone marrow match was with her sibling. Unfortunately, Molly was an only child. The Nashes had always wanted to have more children, but because Fanconi anemia is an inherited condition they knew that if they had another child, that child had a chance of getting the disease just as Molly had. But, neither Jack nor Lisa had the disease because the mutation is recessive. In order to have Fanconi anemia like Molly, both copies of the FANCC gene would have to have this recessive mutation, and they only had one.
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Punnett Squares: Show All Possible Combinations of Gametes
FANCC Normal
Father
meiosis
FANCC
Normal
normal Normal FANCC Normal
FANCC
normal
normal
meiosis
FANCC FANCC FANCC normal FANCC
Mother 34
Pre-Implantation Genetic Screening
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When they discovered that they could conceive a baby that was free from Fanconi Anemia, Jack and Lisa Nash underwent in-vitro fertilization followed by a procedure called pre-implantation genetic screening to choose an embryo that would have HLA proteins (B44, B35, A2, A26) that matched Molly, and to choose an embryo that would also be free of Fanconi anemia. Ethics: this is the first example of the use of preimplantation genetic screening to select for a baby solely as a treatment for its sibling
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Update on the Nashes
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After four In Vitro Fertilization attempts, Lisa Nash gave birth to a baby boy, Adam, on August 29, 2000. Adam's placenta was gathered immediately and all the cord blood saved. Molly started chemotherapy to destroy her bone marrow and received a transfusion of the cord blood cells a month later. Today both Molly, Adam, and new little sister Delaine are doing well. The transplant cured Molly's bone marrow failure, but she still suffers from Fanconi anemia and visits the doctors 35-40 times a year to screen for solid-tumor cancers. A common cold could have dire consequences for her, but her bone marrow is functioning normally.