Ch 13: Meiosis and Sexual Life
Cycles
Chapter 13: Meiosis
From Topic 3.1Essential idea: Every living organism inherits a blueprint for life from its parents.
Understandings:
• A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic.
• A gene occupies a specific position on a chromosome.
From Topic 3.2
Essential idea: Chromosomes carry genes in a linear sequence that is shared by members of a species.
Nature of science: Developments in research follow improvements in techniques—autoradiography was used to establish the length of DNA molecules in chromosomes (1.8).
Understandings:
•In a eukaryote species there are different chromosomes that carry different genes.
• Homologous chromosomes carry the same sequence of genes but not necessarily the same alleles of those genes.
• Diploid nuclei have pairs of homologous chromosomes. • Haploid nuclei have one chromosome of each pair.
• The number of chromosomes is a characteristic feature of members of a species.
• A karyogram shows the chromosomes of an organism in homologous pairs of decreasing length.
• Sex is determined by sex chromosomes and autosomes are chromosomes that do not determine sex.
Applications and skills:
• Application: Cairns’ technique for measuring the length of DNA molecules by autoradiography.
• Application: Comparison of diploid chromosome numbers of Homo sapiens, Pan troglodytes, Canis familiaris, Oryza sativa, Parascaris equorum..
• Application: Use of karyograms to deduce sex and diagnose Down syndrome in humans.
Guidance:
• The terms karyotype and karyogram have different meanings. Karyotype is a property of a cell—the number and type of
chromosomes present in the nucleus, not a photograph or diagram of them.
• Genome size is the total length of DNA in an organism. The examples of genome and chromosome number have been selected to allow points of interest to be raised.
• The two DNA molecules formed by DNA replication prior to cell division are considered to be sister chromatids until the splitting of the centromere at the start of anaphase. After this, they are individual chromosomes.
• Aim 6: Staining root tip squashes and microscope examination of chromosomes is recommended but not obligatory
From Topic 6.6 (further discussed in the Sexual Reproduction Mini-Unit of HL 1)
Understandings:
Chapter 13: Meiosis
From Topic 3.3Essential idea: Alleles segregate during meiosis allowing new combinations to be formed by the fusion of gametes.
Nature of science: Making careful observations—meiosis was
discovered by microscope examination of dividing germ-line cells (1.8).
Understandings:
• One diploid nucleus divides by meiosis to produce four haploid nuclei.
• The halving of the chromosome number allows a sexual life cycle with fusion of gametes.
• DNA is replicated before meiosis so that all chromosomes consist of two sister chromatids.
• The early stages of meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.
• Orientation of pairs of homologous chromosomes prior to separation is random.
• Separation of pairs of homologous chromosomes in the first division of meiosis halves the chromosome number.
Crossing over and random orientation promotes genetic variation. • Fusion of gametes from different parents promotes genetic variation.
Applications and skills:
Skill: Drawing diagrams to show the stages of meiosis resulting in the formation of four haploid cells.
Guidance:
• Preparation of microscope slides showing meiosis is challenging and permanent slides should be available in case no cells in meiosis are visible in temporary mounts.
• Drawings of the stages of meiosis do not need to include chiasmata. • The process of chiasmata formation need not be explained.
From Topic 3.4 Understandings:
• Gametes are haploid so contain only one allele of each gene.
• Fusion of gametes results in diploid zygotes with two alleles of each gene that may be the same allele or different alleles.
• The two alleles of each gene separate into different haploid daughter nuclei during meiosis.
From Topic 3.5 (further discussed in the Biotech Mini-Unit of HL 1 and reinforced in HL 2)
Understandings:
• Clones are groups of genetically identical organisms, derived from a single original parent cell.
• Many plant species and some animal species have natural methods of cloning.
• Animals can be cloned at the embryo stage by breaking up the embryo into more than one group of cells.
• Methods have been developed for cloning adult animals using differentiated cells.
Chapter 13: Meiosis
From Topic 10.1Essential idea: Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells.
Understandings:
• Chromosomes replicate in interphase before meiosis. • Homologous chromosomes separate in meiosis I. • Sister chromatids separate in meiosis II.
• Crossing over is the exchange of DNA material between non-sister homologous chromatids.
• Crossing over produces new combinations of alleles on the chromosomes of the haploid cells.
• Chiasmata formation between non-sister chromatids can result in an exchange of alleles.
• Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I.
Applications and skills:
• Skill: Drawing diagrams to show chiasmata formed by crossing over.
Guidance:
• Diagrams of chiasmata should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma was formed.
• Aim 6: Staining of lily anthers or other tissue containing germ-line cells and microscope examination to observe cells in meiosis are possible activities.
From Topic 10.3 (introduced in HL 1 but reinforced in HL 2)
Heredity and Variation
•
Heredity- Passing of traits from one generation
to another
•
Variation- Inherited Differences among
individuals of the same species.
•
Genetics- The study of heredity and hereditary
Heredity Review
•
Offspring acquire genes from parents by inheriting
chromosomes.
•
Allele
-one specific form of a gene, differing only be a few
bases and occupying the same locus.
•
Genes
- The length of DNA that codes for a protein.
•
Chromosomes
- Sections of DNA that contain Genes and
other Associated DNA.
Types of Reproduction
•
Asexual
•
single parent
•
genetically
identical (
clone
)
•
very rarely
generate any
differences; if so,
from a mutation
•
Sexual
•
Two parents
•
each gives half
•
unique combinations
Chromosomes
•
Somatic cells have 46 chromosomes (23 from mom and 23
from dad).
•
The two chromosomes that compose of the same length,
centromere position, and staining pattern are called
Karyogram
•
Homologous autosomes
•
1-22
•
Same loci
•
Sex Chromosomes
•
23
rd
pair
•
Not same loci (males)
•
Diploid (2n): 46
•
Two sets of chromosomes
•
Haploid (n): 23
•
One set
•
Gamete – haploid
reproductive cell.
•
Karyogram
:
Human Life Cycles
•
Fertilization:
•
Union of two gametes (n)
•
Zygote is formed (2n)
•
Gametes
(sex cells) are produced
in gonads (ovaries and testes).
•
Meiosis
is the process by which
Meiosis Overview: Has 2 parts
Meiosis overview:
http://highered.mheducation.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html
http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.swf
Meiosis I
Meiosis I
•
Interphase I
•
Growing, Replication,
Prepration
•
Prophase I
(approx 90% of
time)
•
Synapsis occurs
•
Tetrad forms
•
Crossing over occurs
(
chiasmata
)
•
Centrioles move apart
•
Nuclear membrane and
nucleoli disappear
Meiosis I
•
Metaphase I
•
Chromosomes move to
midline
•
Independent
assortment
happens
(Mendel)
•
Segregation
happens
(Mendel)
•
Anaphase I
•
Homologous
chromosomes (2
chromatids) move to
opposite sides at the
Law of Independent Assortment
•
Independent assortment:
Mendel states that allele pairs separate
independently during the formation of gametes.
Law of Independent Assortment
2n = ___
Therefore
n = ___
Fruit Fly
Spermatogonium
Or
Oogonium
Predict how many
different way’s the
homologous pairs.
Can independently
assort during
Option 1
Option 2
How many different
gametes can be
ultimately produced?
2
n
Law of Segregation
•
Law of Segregation:
Mendel
proposed that allele pairs separate
during gamete formation.
Meiosis I
•
Telophase one
•
Cell either pinches forming a cleavage furrow or new cell plate
forms
Meiosis Overview2
Meiosis overview:
http://highered.mheducation.com/sites/0072495855/student_view0/chapter28/animation__how_meiosis_works.html
Karyotype/ non disjunction project intro
http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.swf
Meiosis I
Meiosis II
•
Prophase 2
•
Metaphase 2
•
Anaphase 2
•
Telophase 2
•
4 haploid daughter cells formed (
gametes
)
Meiosis Application
Random fertilization: Genetic Variations!
•
A zygote produced by mating of a woman and man has a unique
genetic identity.
•
An ovum is one of approximately 8 million possible chromosome
combinations (actually 2
23
).
•
The successful sperm represents one of 8 million different possibilities
(actually 2
23).
•
The resulting zygote is composed of 1 in 70.4 trillion (2
23x 2
23) possible
combinations of chromosomes.
Genetic Variations!
Errors in Chromosomal Inheritance
•
Alteration of chromosome number
•
Aneuploidy: abnormal number of a
specific chromosome
•
If there is three, it is said to be trisomic.
•
If it is missing one it is said to be monosomic.
•
If there is an error early in development all
others will have same problem.
http://www.sumanasinc.com/webcontent/animations/content/mistakesmeiosis/mistakesmeiosis.swf
•
Polyploidy: having more than two
complete chromosome sets (3n or 4n
instead of 2n)
•
All due to non-disjunction
•
Either in meiosis one or two
•
Down Syndrome: Trisomy 21
•
Patau Syndrome: Trisomy 13
•
Edwards Syndrome: Trisomy 18
•
Kleinfelter Syndrome: XXY (Not so normal male)
•
Extra Y: XYY (Normal Male)
•
Triple X: XXX (Normal Female)
•
Turners: XO (Not so normal female)
Examples of Aneuploidy
Sperm
cell
n
+ 1
n
(normal)
Zygote
2
n
+ 1
Egg
•
Fetal Screening Test:
•
Blood test.
This blood test measures the levels of pregnancy-associated plasma
protein-A (PAPP-A) and the pregnancy hormone known as human chorionic
gonadotropin (HCG). Abnormal levels of PAPP-A and HCG may indicate a problem
with the baby.
•
Ultrasound.
Ultrasound is used to measure a specific area on the back of your
baby's neck known as a nuchal translucency screening test. When abnormalities are
present, more fluid than usual tends to collect in this neck tissue.
•
Fetal Diagnosis Tests:
•
Amniocentesis:
http://www.doctoroz.com/videos/amniocentesis-animation
•
Chorionic Villus sampling:
http://www.muschealth.com/video/Default.aspx?videoId=10068&cId=34&type=rel
•
Ultrasound
•
Fetoscopy
•
Newborn screening
•
Down Syndrome: After birth, the initial diagnosis of Down syndrome is often based
on the baby's appearance. The doctor will likely order a test called a chromosomal
karyotype. Using a sample of blood, this test analyzes your child's chromosomes. If
there's an extra chromosome 21 present in all or some cells, the diagnosis is Down
syndrome.
Trisonomy 21
•
Trisomy 21, is a genetic syndrome caused by a triplication of
chromosome 21.
•
It occurs in about 1/800 to 1/1000 live births
Trisonomy 13
•
Patau Syndrome Trisomy 13
•
Least common of the live-born
trisomy disorders, with an
incidence of 1 in 10,000
newborns
•
75% of trisomy 13 cases are due
to maternal nondisjunction
•
heart defects, small or poorly
developed eyes,
spinal cord
or
brain abnormalities, cleft lip or
palate, extra toes or fingers, and
decreased muscle tone
•
A chromosome number that has more than two complete set
of chromosomes.
•
Triploidy
•
Tetraploidy
•
Polyploidy is important in plants
- For example, strawberries are octoploid (has 8 sets of
chromosomes (instead of 2) for each of their 7
chromosomes. Some species of strawberries are decaploid.
•
Rare in animals, some mosaics occur but very uncommon.
http://kisdwebs.katyisd.org/campuses/MRHS/teacherweb/hallk/Teacher%20Documents/AP%20Biology%20Materials/Evolution/Speciation%20by%20Changes %20in%20Ploidy/25_A02s.swf
