Unit 8 Notes- Meiosis (Chapter 11)

Chromosome Number
o Organisms have tens of thousands of genes that determine individual traits.
o The more closely related two organisms are, the more genes they’ll have in common
o Genes are lined up on chromosomes that can hold thousands of genes.
o In body cells of animals and most plants, chromosomes occur in pairs.
o One chromosome in the pair came from the male parent and one came from the
female parent.
o These pairs are called homologous chromosomes – each pair has genes for the same traits
o A cell with two of each kind of chromosome is called diploid (2n).
o Organisms produce gametes (sex cells) that contain one of each kind of chromosome.
o A cell with only one of each kind of chromosome is called haploid (n).
o Sex cells have one of each kind of chromosome so that when they combine (as egg and sperm do during
fertilization), the resulting cell is diploid.
o Each species has a s
o pecific number of chromosomes.
 Humans have 23 pairs (46 total)
 Fruit Flies have 4 pairs (8 total)
 Dogs have 39 pairs (78 total)

Human Chromosomes
o Humans have 23 pairs of chromosomes(46 total)
 22 pairs of autosomes
 1 pair of sex chromosomes
o Half of each pair came from one parent and half came from the other parent

Meiosis: Why?
o Mitosis divides one diploid cell to form two diploid cells
o For example: A human cell with 46 chromosomes divides to form two cells with 46 chromosomes.
o If each parent were to pass on a diploid cell to the offspring, that offspring would then have 4 copies of each
chromosome
o 46 chromosomes from each parent would yield a 92 chromosome offspring
o Meiosis allows for two divisions to divide one diploid cell into four haploid cells.

Meiosis: Where and Who?
o Meiosis takes place in the gonads (sexual organs)
o For humans, these are the ovaries and testes
o The process of meiosis produces egg and sperm cells
o Two gametes come together by fertilization
o The haploid sperm and egg join to form a diploid zygote

Meiosis Phases
o Meiosis I
1.____________________________________ 2.___________________________________
3.____________________________________ 4.___________________________________
o Meiosis II
1.____________________________________ 2.___________________________________
3.____________________________________ 4.___________________________________

Interphase
o Before Meiosis (just like before Mitosis) the cell must prepare for division:
o Cells increase in size
o DNA is replicated
o Necessary proteins and RNA are synthesized
o During this phase, chromosomes are not yet visible.

Meiosis: Prophase I
o Chromosomes become visible
o Nuclear envelope disappears
o Centrioles head to opposite poles and spindle forms
o Homologous chromosomes (one pair of sister chromatids from the mother and one from the father) pair up
to form a tetrad
o The tetrad pairs up so tightly that crossing over occurs

Crossing Over

Meiosis: Metaphase I
o Spindle fibers attach to the centromeres
o Tetrads line up along the equator (or middle of the cell)
o Note that homologous chromosomes line up together along the equator in Meiosis where in Mitosis, they
lined up independently to one another.

Meiosis: Anaphase I
o Homologous chromosomes separate and head to opposite ends of the cell
o Centromeres DO NOT split – Sister chromatids will stay together until the next division

Meiosis: Telophase I and Cytokinesis
o Spindle is broken down
o Chromosomes uncoil
o Cytoplasm divides into two cells

Meiosis: Prophase II
o Chromosomes become visible
o Spindle forms
o
If nuclear membrane reformed after Telophase I, it will break down now

Meiosis: Metaphase II
o Spindle pulls the sister chromatids to the middle of the cell where they line up along the equator in random
order (just as they did during Mitosis)

Meiosis: Anaphase II
o Centromere of each sister chromosome splits and each sister chromatid heads for an opposite pole

Meiosis: Telophase II and Cytokinesis
o Nuclei reform (nuclear envelope reappears)
o Spindle breaks down
o Chromosomes uncoil
o Cytoplasm divides into a total of four haploid cells that will become gametes
o Each cell contains one chromosome from each homologous pair

Variability
o Meiosis has a large role in maintaining variability in a species.
o Through sexual reproduction, offspring are not simply replicas of one organism but a genetic combination
of two organisms
o Crossing over during Prophase I insures that a parent organism can pass on different gametes each time it
reproduces, creating a variety of offspring.

Chromosomal Mutations
o Chromosomal mutations can happen when chromosomes break and do not repair correctly.
o Errors can also occur during Meiosis.
o Sometimes the homologous chromosomes do not separate properly – this is called nondisjunction
o This results in gametes with either an extra copy of a chromosome or no copy at all.

Types of Nondisjunction
o Remember: In normal fertilization, a zygote would get one copy of a chromosome from each parent
resulting in one pair of each type of chromosome (humans: 23 pairs)
o Monosomy – when the zygote gets a copy of a chromosome from only one parent so it is missing one
chromosome
 Most zygotes with monosomy do not survive
 One exception is the case of Turner’s Syndrome
 Females have only one X chromosome instead of two
 These people will still have female sexual characteristics but they will generally be
underdeveloped
o Trisomy – In this case, the zygote gets one copy of a chromosome from one parent and two copies from the
other parent resulting in three copies rather than the normal two copies.
o Down Syndrome (Trisomy 21) – This person has three copies of the 21st chromosome. This can lead to
mental retardation, susceptibility to certain illness or diseases, and a shorter life span
o Klinefelter's syndrome (XXY) – This person has two copies of the X chromosome as well as a copy of the Y
chromosome. This person will be male but may suffer from underdeveloped testicles and infertility.

Identifying Chromosomal Disorders
o To determine whether or not an organism has the proper number of each chromosome, one can look at a
karyotype
o To make a karyotype a photograph is taken of the paired chromosomes during metaphase
o These pairs are cut out and arranged in a chart according to length and location of centromere
o Once arranged, it is easy to see if there are any extra or missing chromosomes