This document contains notes on meiosis, detailing processes such as crossing over, non-disjunction, and the creation of gametes. It explains the significance of meiotic divisions in reducing chromosome numbers and promoting genetic variation, along with methods for obtaining cells for karyotype analysis. The notes also highlight the implications of non-disjunction, including its links to chromosomal abnormalities like Down syndrome and the influence of parental age.

1. IB Biology Chapter 3 Notes: Meiosis (3.3) NAME:
Word Definition
Crossing over When portions of non-identical homologous chromatids are exchanged during
prophase I of meiosis in order to increase variation
Somatic cell Body cells – Diploid in chromosome number
Gamete Sex cells (egg and sperm) – Haploid in chromosome number
Non-disjunction When chromosomes fail to separate during meiosis, either during Anaphase I or
Anaphase II. This causes too many or too few chromosomes in the sperm or egg.
Meiosis The process of cell division that creates gametes. It is a reductive division (reduces
the cells from diploid to haploid) and it creates variation in the gametes.
Amniocentesis The process of removing a small amount of amniotic fluid from around the embryo /
fetus. The fluid contains embryonic cells that can be used to make a karyogram
Chorionic Villus
Sampling
The process of removing a small amount of the placenta. The placenta contains
embryonic cells and can be used to create a karyogram
Bivalent A pairing of homologous chromosomes that occurs in Prophase I of meiosis

2. 3.3.1 One diploid nucleus
divides by meiosis to
produce four haploid
nuclei.
3.3.2 The halving of the
chromosome number
allows a sexual life cycle
with fusion of gametes.
3.3.3 DNA is replicated
before meiosis so that all
chromosomes consist of
two sister chromatids.
3.3.4 The early stages of
meiosis involve pairing of
homologous chromosomes
and crossing overfollowed
by condensation.
3.3.5 Orientation of pairs of
homologous chromosomes
prior to separation is
random.
3.3.6 Separation of pairs of
homologous chromosomes
in the first division of
meiosis halves the
chromosome number.
3.3.10 Methods used to
obtain cells for karyotype
analysis (chorionic villus
sampling and
amniocentesis) and the
associated risks.
3.3.7 Crossing over and
random orientation
promotes genetic variation.
3.3.8 Fusion of gametes
from different parents
promotes genetic variation.
Why must gametes have a half-set of chromosomes?
What is a ‘bivalent?’
What occurs in synapsis?
What occurs in crossing over?
Amniocentesis Chorionic Villus Sampling
Where are cells
sampled from?
How are cells
obtained?
Risk of
miscarriage?
Crossing Over:
Random Orientation of Bivalents:

3. 3.3.9 Non-disjunction can
cause Down syndrome and
other chromosomal
abnormalities. Studies
showing age of parents
influences chances of non-
disjunction.
3.3.11 Draw diagrams to
showthe stages of meiosis
resulting in the formation
of four haploid cells.
What is Non-disjunction and how does it lead to chromosomal abnormalities?
What abnormality is the cause of Downs Syndrome?
Describe the effects of Down Syndrome:
What is the relationship between parental age and chance of non-disjunction occurring?
Phase Description Drawing
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II