1. Lecture 19: Dividing the Cell II - Meiosis
Bio 93: DNA to Organisms
Lecture 19: Dividing the Cell II - Meiosis
What’s wrong with this karyotype?

2. Lecture 19: Learning Objectives
19.1 – Describe the features of chromosomes and diploid and haploid cells 19.2 – Explain what happens in each step of meiosis and how it reduces chromosome number 19.3 – Explain how meiosis contributes to genetic variation 19.4 – Predict how abnormal chromosome number or alterations in chromosome structure can lead to disorders

3. Lecture 19: Key terms Heredity Haploid Synapsis Variation Meiosis
Chiasmata
Genetics
Zygote
Independent assortment
Genes
Fertilization
Gametes
Meiosis I
Random fertilization
Nondisjunction
Somatic cells
Meiosis II
Locus
Prophase I
Aneuploidy
Monosomy
Asexual reproduction
Metaphase I
Trisomy
Sexual reproduction
Anaphase I
Karyotype
Telophase I
Polyploidy
Deletion
Homologous chromosomes
Prophase II
Duplication
Metaphase II
Sex chromosomes
Anaphase II
Inversion
Autosomes
Translocation
Telophase II
Diploid
Crossing over

4. Down syndrome results from an extra copy of chromosome 21
How could a cell get an extra copy of a chromosome?

5. 1 Which statement is TRUE?
A – Skin cells go through both mitosis and meiosis
B – Skin cells go through mitosis but not meiosis
C – Skin cells go through meiosis but not mitosis
19.1

6. replication replication division division division “division”
“reduction division”
Bio 93 (Summer 2017), Copyright © 2017, Dr. Salme Taagepera. All rights reserved.

7. 2 What is true about this cell? Select all that apply.
A – The cell is 2n = 4
B – The cell is haploid
C – The cell has gone through S phase
D – The cell has two pairs of homologous chromosomes
E – The cell is a gamete 2
Pair of homologous chromosomes
Diploid – two sets of chromosomes (one from mom AND one from dad)
Haploid – one set of chromosomes
(one from mom OR one from dad)
n = number chromosomes in a single set
Sister
chromatid
mom
dad

8. 1 Is each cell diploid or haploid?
19.2
Is each cell diploid or haploid?
How many haploid cells are on the screen? Click in a number!

9. How meiosis reduces chromosome number
19.2
Figure 13.7

10. Figure 13.10a Figure 13.10 MITOSIS MEIOSIS Prophase Duplicated
chromosome
Metaphase
Anaphase
Telophase 2n
Daughter cells
of mitosis
Sister
chromatids
separate.
Individual
chromosomes
line up.
Chromosome
duplication
2n = 6
Parent cell
Chiasma
MEIOSIS I
Prophase I
Homologous
pair
Metaphase I
Anaphase I
Telophase I II
Pairs of
homologous
Homologs
chroma-
tids
Daughter cells of meiosis II
Daughter
cells of
meiosis I n
Figure 13.10a
Figure 13.10a A comparison of mitosis and meiosis (part 1: mitosis vs. meiosis art)
Figure 13.10

11. Meiosis promotes genetic variation through:1
1. Independent assortment
2. Crossing over
Figure 13.11
3. Random fertilization
19.3

12. Breakdown of nuclear envelope
What happens if the mitotic spindle does not form properly during meiosis?
Breakdown of nuclear envelope
Prophase I
Metaphase I
Two groups of mice, A and B.
One is healthy, one is abnormal
What can you conclude?
A – The spindle is not forming properly in mice A
B – The spindle is not forming properly in mice B
Mice A
Mice B 2
19.2

13. A normal mouse oocyte has 20 chromosomes
A normal mouse oocyte has 20 chromosomes. As shown in the graph, an error could lead to a mouse oocyte with 21 chromosomes. If this oocyte is fertilized by a normal sperm, the zygote would have 41 chromosomes. Which of the following best explains what happened to result in this zygote? 2
19.4
Gray – wild type
Green – mutant
A – An inversion occurred leading to monosomy of a chromosome
B – A non-disjunction occurred leading to trisomy of a chromosome
C – A duplication occurred leading to trisomy of a chromosome
D – A deletion occurred leading to monosomy of a chromosome

14. homologous chromosomes
Meiosis I
Meiosis II
Nondisjunction
Non- disjunction
Gametes
Number of chromosomes
n + 1 n
n − 1
Figure 15.13
All gametes abnormal
Half gametes abnormal
Nondisjunction of
homologous chromosomes
in meiosis I
(b) Nondisjunction of
sister chromatids
in meiosis II
19.4

15. Meiosis is an error-prone process
Trisomy 21
Trisomy 13
Trisomy 18
Roughly 5% of all conceptuses are trisomic or monosomic.
% of trisomies and monosomies result from non-disjunction at maternal M I.
Analysis of human gametes: 20% oocytes and 3% of sperm have incorrect number of chromosomes.
The most common types of autosomal trisomy that survive to birth in humans are:
Trisomy 21 (Down's syndrome) = birth defects, intellectual disabilities, characteristic facial features. Often involves heart defects, visual and hearing impairments.
Trisomy 18 (Edwards syndrome) = 50% of babies who are carried to term will be born alive, 10%+ may survive to their first birthdays.
Trisomy 13 (Patau syndrome) = 20% of children with Patau syndrome survive to their first birthday.
D103 lecture 24 © Grant MacGregor & Christine Sütterlin – All rights reserved

16. Summary Number of chromosomes
Bio 93 (Summer 2017), Copyright © 2017, Dr. Salme Taagepera. All rights reserved.