1
Name: ______________________
Date: ___________________
5B Sexual Reproduction and Meiotic Cell Division
NOTES
Eukaryotic Chromosomes
 DNA binds to proteins called histones
 DNA and histones condense to form
chromatin
 Chromatin is condensed completely to
form chromosomes after the DNA has
replicated and the cell is ready for division.
 Chromosomes make it possible to separate DNA precisely during cell division.
Chromosomes can be seen in two forms:
1. Single-Arm: are composed of a single chromatid
2. Double- Arm (Replicated Form, Duplicated form): are made up of
paired, genetically identical chromatids, called sister chromatids.
 Sister chromatids are joined at the centromere.
 Because the sister chromatids are formed during replication of
DNA, they are identical right down to the nucleotide sequences!
Chromosome Number
Organisms have 2 sets of chromosomes:






One set from the ____________________ parent
One set from the _______________ parent
Example: The fruit fly (Drosophila)
8 chromosomes total
How many from mom? ________ How many from dad? _________
The corresponding chromosomes from mom and dad are called
______________________.
Notes/Summary
2
Homologous Chromosomes





same size
same _____________________ position
same banding pattern
same ______________ in the same locations on each pair
BUT, the nucleotide sequences at the same gene location may or may not be the same
Diploid and Haploid
Diploid- cells containing ______________ homologous chromosomes
 Diploid number of chromosomes is represent by “2N” where “N” is the number of
chromosomes in a single set
 Example: Drosophila have 8 total chromosomes so 2N=8
(N= ______)
Haploid- cells containing just ___________________ of chromosomes
 Gametes are haploid
 “N”
 Example: In Drosophila, N = _______
Human Chromosome Number
 Humans have ________ pairs of chromosomes for a total of
_________.
 1 of each pair came from mom, and 1 of each pair came
from dad
A Karyotype shows a complete ______________ set of
chromosomes, grouped in pairs, arranged in size order.
Notes/Summary
3
Somatic Cells and Sex Cells
Somatic cells- ______________ cells which contain all of the chromosomes.
Sex cells (gametes)- sperm and egg cell which contain ________________ the number of
chromosomes.
 One of each sex cell is needed during sexual reproduction to produce an offspring
Reproduction
- the making of new individuals
Asexual Reproduction Requires only one “parent” cell
 Offspring are genetically ______________________ to the parent
Sexual Reproduction
 Involves fusion of _________ separate parent cells
 Genetic information is inherited from both parents
 Most animals and plants reproduce this way
Comparing Asexual and Sexual Reproduction
Species survive by reproduction
Species better suited to their environment ________________ better, and reproduce more,
passing on their genes
Asexual Reproduction
Fast!
- All organisms _________________.
- This is an advantage if conditions are
favorable and a disadvantage of they
are not.
Notes/Summary
Sexual Reproduction
Requires more time
- Provides genetic _____________
because DNA comes from both parent
cells
- Advantageous in varying
environments.
4
Asexual
Sexual
Number of Parents
Are sex cells needed?
(yes/no)
Genetic Variability of
Offspring (high/low)
Advantage in what kind of
environment?
(Changing/non-changing)
Mitosis vs. Meiosis
Mitosis –a step in cell division in which the nucleus divides into two, genetically identical nuclei
- Occurs during asexual reproduction, growth and repair of cells (somatic cells)
Meiosis –division of nuclear material that produces __________________ (egg & sperm) with
half the number of chromosomes
- Occurs during sexual reproduction to make sex cells
Review of Mitosis
Interphase
 G1, S, G2
Mitosis
 PMAT
Cytokinesis
Meiosis
The process of producing ______________________ (sex cells)
In this process the number of chromosomes is cut in ______________
Why is it important for gametes to have only half the number of chromosomes?
_____________________________________________________________________________
____________________________________________________________________________.
Notes/Summary
5
Meiosis only occurs in cells that will produce sex cells
 In mammals, these cells are located in the ovary or the
testes
The rest of the _____________________ that make up an
organism divide normally by ________________.
Meiosis Involves 2 Separate Divisions
1. Meiosis I: Similar to Mitosis
A. During S-phase of Interphase I the chromosomes
_______________________
B. During Prophase I, the homologous chromosomes
pair up into ________________
C. Crossing over (the exchange of parts of the
homologous chromosomes) occurs at this time
What will crossing over do to the daughter cells that will be produced?
___________________________________________________________________
Notes/Summary
6
Metaphase, Anaphase, Telophase, Cytokinesis (I)
 These stages occur the same way they occur in Mitosis except in Meiosis I, the
_________________________ chromosomes separate and end up in different cells
(segregated)
Anaphase I
RULE OF INDEPENDENT ASSORTMENT
Homologs (chromosomes that are homologous to each other) move in a
_______________________ fashion to either end of the cell.
 The two resulting cells end up with a random _____________________ of the mother
and father’s chromosomes.
 How are the daughter cells resulting from meiosis I different from their parent cell?
 ________________________________________________________________________
 ________________________________________________________________________
________________________________________________________________________
 ________________________________________________________________________
Notes/Summary
7
The Second Meiotic Division
2. Meiosis II: The division of the daughter cells made in meiosis I
- Unlike the first division, chromosomes are
______________________________ during Interphase II
- Prophase II- no tetrads form because homologous
chromosomes have already been separated
Metaphase, Anaphase, Telophase, Cytokinesis (II)
 Similar to the stages of Meiosis I, only ____________________________ are separated
this time
 This gives each daughter only _________ the chromosomes of the original parent cell
o ________ daughters are produced
o The daughters have a _____________ number of single armed chromosomes
Metaphase II
Anaphase II
Telophase II
/Cytokinesis II
The products of Meiosis




The female gamete = _________ cell
The male gamete = ______________ cell
Each is Haploid (has ½ the normal # of chromosomes)
During fertilization, egg and sperm randomly combine making a
Diploid ________________
 The zygote grows into an organism through Mitosis
Notes/Summary
8
Sources of Genetic Variation in Offspring
1. ______________________________
2. ______________________________________________
3. ______________________________________________
These all contribute to why siblings are all different despite having the same parents!
Summary
Meiosis I-First division –diploid number of double-arm chromosomes is reduced to a haploid
number of ______________________________________.
Meiosis II- Second division –haploid number of double-arm chromosomes is divided to
produce cells with a haploid number of __________________________
Why do you think Meiosis is called a reduction division process?
_____________________________________________________________________________
____________________________________________________________________________.
Notes/Summary
9
Egg and Sperm Production
GAMETOGENESIS- the production of ___________________.
The process is slightly different in males and females
1. Oogenesis- production of ________________
2. Spermatogenesis- production of __________________
1. Oogenesis
 In the _______________ of the female
 Produces ________ egg for every parent cell
 Occurs in human females during embryonic development and continues until
menopause.
 Note that the cytoplasm is NOT equally divided between the eggs during this process.
The result is one large, functional egg with a better chance of survival.
2. Spermatogenesis
 in the male _____________
 produces ________ viable
(functioning) sperm for every
beginning cell
Notes/Summary
10
Chromosomal Disorders
Occur mainly due to errors in _________________ (rare)
Nondisjunction- when homologous chromosomes fail to _______________during meiosis I
 Or when sister chromatids fail to separate during meiosis II
 Gametes will be made with the _____________________ of chromosomes
Example: An extra chromosome in a gamete will lead to an offspring with 3 copies of one
chromosome
 This is called __________________
 The most common form of trisomy involves
chromosome 21
 Three copies of chromosome 21 leads to
_________________________
Notes/Summary
11
Other Chromosomal Disorders
Notes/Summary
12
Compare
Notes/Summary