Chromosomes, Reproduction, Meiosis, & Karyotypes

Chromosomes, Reproduction, Meiosis, & Karyotypes Notes
To Review:
•Cells divide to stay small
•The process of cell division is called mitosis
•Mitosis is a very small part of the cell’s life cycle
•Most of the cell’s life is spent in interphase
•Interphase is divided into G1, S, and G2
•G1 is the period when the cell is doing what it was designed to do
•S is DNA replication so there will be 2 copies when it is ready to divide
•G2 is the period when the cell is making more organelles and other materials needed for mitosis
•Mitosis is divided into prophase, metaphase, anaphase, and telophase
•Mitosis is generally followed by cytokinesis
•When the cell cycle is out of control, cancer can result
To Preview:
•Some cells in the reproductive structures of organisms do not divide by mitosis
•Some cells in the reproductive structures of organisms divide by meiosis
•The reproductive structures in animals are typically called testes in males and ovaries in females
•The cells in testes called primary spermatocytes divide by meiosis to become sperm
•The cells in ovaries called primary oocytes divide by meiosis to become eggs
•The cells in organisms that divide by mitosis are called somatic (soma = body) cells
•The cells in organisms that divide by meiosis are called gametes (sex cells; eggs and sperm are gametes)
•Both mitosis and meiosis involve chromosomes
•Scientists can analyze chromosomes using a karyotype
•Sometimes meiosis (or mitosis, actually) doesn’t go perfectly and chromosomal disorders can result
•One type of chromosomal disorder that can result from an error in meiosis (or mitosis) is called a
nondisjunction disorder, such as Down syndrome
Part 1: Chromosomes
Vocabulary Words
Copy, Define, and Illustrate the following terms:
–Sex Chromosomes
Differences Among Species
•Each organism has a characteristic number of chromosomes
•The number is constant with the species
•Potatoes, plums, and chimpanzees all have 48 chromosomes
–Units of information
–Segment of DNA that codes for a protein or RNA molecule (more later)
–Composed of DNA and proteins (chromatin)
–Consist of 2 identical CHROMATIDS attached by a CENTROMERE when cell is ready to divide
(after S of interphase)
•Before division, chromatids separate to ensure identical DNA in both
Somatic Cells
•Any cell other than a sperm or egg cell
•Have 46 chromosomes in humans
–Differ in size, shape, & genes
–23 pairs
•2 HOMOLOGOUS chromosomes that
are similar in size, shape, & genes
•One comes from each parent
•Diploid (2n)
–Contains 2 sets of
–In humans, 2n=46
•Sperm or egg cell
•Contain 1 set of chromosomes
–23 in humans
•Haploid (n)
•In humans n=23
•Fusion of 2 gametes = FERTILIZATION
–Fertilized egg cell is a ZYGOTE
–23 chromosomes + 23 chromosomes = 46 chromosomes
–n + n = 2n
–22 of the 23 pairs of chromosomes in somatic cells (44 of 46 chromosomes)
Sex chromosomes
–1 of the 23 pairs (2 of the 46 chromosomes)
–Contain genes that will help determine the sex of the organism (plus others…)
–X and Y chromosomes in many organisms
–In most organisms, if there is a Y it is male
•XY in humans is male
–If there is no Y, it is a female
•XX is a normal female in humans
 Who determines the sex of the organism?
XO – O refers to no chromosome
•birds and butterflies have an XO system
•In humans, XO is a genetic disorder called Turner’s (all individuals with Turner’s are female
since there is no Y).
This is a photograph of the chromosomes of a cell after DNA replication has occurred. This is
typically when scientists try to photograph a squash of the nucleus, which hopefully results in
the chromosomes being separated out.
If a karyotype is going to be constructed, a computer can sort the chromosomes, line them up by
size, and look for any abnormalities. It can (and used to be) also be done “by hand”: the
photograph taken through the microscope would be enlarged and printed, then the
chromosomes would be cut apart with scissors and sorted and ordered by hand.
Karyotype - photograph of chromosomes grouped in order from largest to
smallest in pairs; used to analyze chromosomes
This is a human karyotype.
 How many chromosomes are shown?
Was this made from the nucleus of a
somatic cell or a gamete?
How many autosomes are shown?
How many sex chromosomes?
Is this from a male or a female?
How do you know?
Part 2:Reproduction
•Asexual – reproduction resulting from mitosis (or a similar process) that involves only one parent; the
offspring are genetically identical to the parent (a clone)
•Sexual – reproduction resulting from an exchange of genetic material; in most organisms this involves
the fusion of gametes (formed during meiosis)
Ways of Reproducing Asexually
Binary Fission – separation of a parent into 2 or more individuals, ex. bacteria
Fragmentation-- body breaks into several pieces, ex. worms
Budding-- new individuals split off from existing ones. Bud may break off or remain attached to parent,
ex. jellyfish, corals, yeast
Regeneration– renewal/regrowth of an organism from a part of that organism; ex. starfish, planaria
DNA varies little between organisms, which may make organisms not be able to adapt to a
changing environment
Produce many offspring in short period of time without using energy to produce gametes or to
find a mate
Offspring are perfectly adapted to current environment, therefore often used in stable
Sexual Reproduction
Sexual reproduction uses a lot of metabolic energy in the development
and maintenance of gametes, as well as a lot of biochemical resources.
Sexual reproduction has several advantages over asexual reproduction.
One of those advantages is the frequent production of new combinations
of genes. This flexibility in the gene pool of a population helps insure the
survival of a species, especially if there is rapid or sudden change in the environment. This is
important in the process of natural selection. (more later…)
Part 3: Meiosis
Vocabulary Words
Copy, Define, and Illustrate the following terms:
independent assortment
sexual reproduction
random fertilization
crossing over
genetic variation
Meiosis – the formation of gametes
* source of genetic variation through
* crossing over – during prophase I pieces of homologous chromosomes are exchanged
* independent assortment – during metaphase I and II which chromosomes go to which
cell is random
Sexual reproduction – two parents required
* source of genetic variation through
* random fertilization – which egg and which sperm are involved is random
Nondisjunction – source of some chromosomal disorders
Karyotypes – picture of all of an organism’s chromosomes
What is Meiosis?
•Meiosis is a form of cell division that produces sex cells (gametes)Used in fertilization.
How is Meiosis Different?
•There are 2 divisions in meiosis
–Meiosis I and meiosis II
•The result is 4 cells instead of 2
•In meiosis II, the DNA is not replicated again. (No interphase)
•The ending number of chromosomes is 23 in humans (egg has 23 & sperm has 23)This is haploid (n).
Mitosis vs. Meiosis
•1 division=2 cells
•Daughter cells identical
•Diploid cells (2n)=46
•Body/Somatic cells
2 divisions = 4 cells
Daughter cells different
•Haploid cells (n)=23
•Gametes (sex cells—eggs or sperm)
Things that can occur that lead to genetic variation…
•Independent assortment—homologous chromosomes are randomly sorted/distributed during
•Crossing-over—the exchange of genes that can occur between homologous chromosomes
during Prophase I
•Random-fertilization—the fertilization of an egg and sperm is random
Crossing over
Independent Assortment
Sexual Reproduction This is how many new organisms are made!!
»Fertilization –syngamy (word parts!)
Sexual Reproduction Life Cycle for Animals
(23 pair)
Part 4: Karyotypes, Nondisjunction, & Chromosomal Disorders
Vocabulary Words
Copy, Define, and Illustrate the following terms:
Chromosomal disorder
A karyotype is a photograph of all of an organism’s chromosomes
•Humans have 23 pairs of chromosomes.
•The last pair (#23) is the sex chromosomes. (XX-female, XY-male)
•All others are called autosomes.
•A karyotype allows you to study differences in chromosome shape, structure, and size.
By looking at a karyotype, you should be able to:
1.determine the sex/gender of the organism,
2.determine if the organism is “normal” or has a chromosomal disorder,
3.identify where the disorder is located and what type of disorder (either monosomy, trisomy,
or malformation of a chromosome) and possibly the specific name (like Turner’s, Down, etc.),
4.determine the number of autosomes and sex chromosomes present.
What do you mean, abnormal?
What kind of disorder?
How does this happen?
Sometimes during meiosis when gametes are made, the chromosomes fail to separate
correctly at anaphase I or anaphase II. This failure to separate is called nondisjunction.
Most nondisjunction disorders are lethal to the developing organism. However, there are a few
that do allow the organism to continue developing with varying degrees of effect.
Nondisjunction could result in one of the zygotes (once egg & sperm unite) formed having only
one copy of the affected chromosome. This is called monosomy.
Another zygote could have 3 copies of one chromosome. This is called trisomy.
Nondisjunction disorders can often be diagnosed with a karyotype.
 A karyotype is created from a picture of a cell during mitosis when the chromosomes are
condensed and look like little Xs. Then a computer sorts the chromosomes into identical pairs
and arranges them from largest to smallest (in length) and puts the 2 sex chromosomes at the
end (pair 23).
A doctor or other professional then analyzes the information shown for errors or abnormalities and
uses them for an initial diagnosis.
Common Nondisjunction Disorders (information below that is in italics will not be tested)
Klinefelter's Syndrome: Trisomy+ of sex chromosomes
•One or more extra sex chromosomes (i.e., XXY or
•1 out of every 500-1,000 newborn males
•Presence of Y chromosome directs development
into a male
•Underdevelopment of testes; usually infertile; taller
than average; less hair than average
•Although some lower scores on standardized tests
have been reported, this is not necessarily the case.
•Can be treatable
Turner’s Syndrome: Monosomy of the sex
•One sex chromosome, XO
•1 in 2500 female births
•Absence of Y chromosome à develops into female
•Abnormal development of ovaries and secondary
sexual characteristics during puberty; often
infertile; shorter than normal
•“Webbing" of the skin of the neck (extra folds of
skin extending from the tops of the shoulders to the
sides of the neck)
Down Syndrome: Autosomal disorder
•Trisomy 21, most common birth defect
•1 per 800 to 1,000 births (risk goes up as mother’s
age goes up)
•Mild to moderate learning disabilities (sometimes
•Eyes that slant upward and small ears that may fold
over slightly at the top. Their mouth may be small,
making the tongue appear large. Their nose also may
be small, with a flattened nasal bridge
•Where did nondisjunction occur? (circle it in the
•What is the sex of this person? ________
Patau Syndrome: Autosomal disorder,
•Trisomy 13, rarely live past infancy
•Extra fingers or toes (polydactyly)
•Deformed feet, known as rocker-bottom
•Neurological problems such as small head
(microcephaly), failure of the brain to divide
into halves during gestation
(holoprosencephaly), severe mental
•Facial defects such as small eyes
(microphthalmia), absent or malformed
nose, cleft lip and/or cleft palate
•Heart defects (80% of individuals)
•Kidney defects
Edward’s Syndrome: Autosomal disorder
Trisomy 18, 30% babies die by 1 mo
•Moderate to severe learning
•Small head (microcephaly), small
and wide-set eyes, small lower jaw
•Congenital heart defects (90% of
individuals) such as ventricular
septal defect and valve defects
•Clenched hands with 2nd and 5th
fingers on top of the others, and
other defects of the hands and feet
•Malformations of the digestive tract,
the urinary tract, and genitals
Practice with Karyotypes: