Download Gene – Sequence of DNA that codes for a particular protein or trait

Chromatid
Gene – Sequence of DNA that codes for a particular
protein or trait.
Ex. Gene for pea plant flower color
Allele – Different form of a particular gene
Ex. P = purple, p = white
Locus (loci) – the physical location of a gene on a
chromosome
Ex. Closer/farther from the centromere (colored
bands on the chromosomes above)
Different species have different numbers of chromosomes
in their nucleus.
Human body cells (somatic cells) contain 46 chromosomes.
 44 autosomal chromosomes, 2 sex chromsomes
 Chromosomes easily visible during metaphase of
Mitosis
o For each replicated autosomal chromosome,
there is another that is identical in length, genes
and centromere location.
 How many pairs of chromosomes are in
each cell? 22
Homologous Chromosomes (Homologous Pairs)
Homologous Chromosomes – Pairs of chromosomes, one
from a man and one from a woman, that carry the same
genes at the same locations.
Allele F
Allele F
Humans have 23 pairs of chromosomes
 22 homologous pairs of chromosomes are called
autosomal chromosomes (autosomes)
o Non-sex chromosomes
 Women have one homologous pair of sex
chromosomes (XX)
 Men have one pair of sex chromosomes (XY)
o Homologous only at some locations
We inherit one chromosome of each set (123) from our
biological mothers and the other (123) from our
biological fathers.
Cells and Chromosome Number
Di- or Dipl- two/double
Diploid Cells are cell with two sets of chromosomes, one
from each biological parent(123 + 123).

Most cells contain homologous pairs
o (1 and 1, 2 and 2, 3 and 3 etc)
Ex. Body cells
 The Diploid number is the total number of
chromosomes in the cell
o Abbreviated “2n”
 What is the human diploid number?
2n = 46
Hapl- simple (think half)
Haploid Cells are cells with one set of chromosomes
 These cells do not contain homologous pairs
o (1, 2, 3 etc)
Ex. Sex cells / Gametes/ Eggs and Sperm
 The Haploid number is half the Diploid number
o Abbreviated “n”
 What is the human haploid number? n = 23
Haploid
Egg (n)
+
Sperm (n)

Zygote (2n)
Diploid
Fertilization
What type of cell division forms gametes?
Meiosis
Why do cells undergo this type of division to form
gametes?
Keeps the chromosome number from doubling in each
generation
Zygote undergoes mitosis to form the multicellular adult
Common Name
Genus and Species
Diploid Chromosome
Number
Buffalo
Bison bison
60
Cat
Felis catus
38
Cattle
Bos taurus, B. indicus
60
Dog
Canis familiaris
78
Donkey
E. asinus
62
Goat
Capra hircus
60
Horse
Equus caballus
64
Human
Homo sapiens
46
Pig
Sus scrofa
38
Sheep
Ovis aries
54
Chimpanzees, Gorillas and Orangutans have 48 chromosomes
Meiosis
Meiosis I
XX
Meiosis II
X
X
I
I
I
I
MEIOSIS
Meiosis – a type of cell division that produces 4 haploid
cells from 1 diploid cell.
 Cells undergo Interphase prior to Meiosis
 Two nuclear divisions (and two cellular divisions)
o Meiosis I
o Meiosis II
 Purpose/Use – to produce sex cells/ gametes/ eggs
and sperm for sexual reproduction with half the
genetic material as the parent cell.
(Haploid = 1 set of chromosomes)
 Location – Ovaries and testes
Meiosis I
Interphase
 Same events: G1, S, G2
Meiosis I
 Prophase I – Most complex and time consuming (90%)
o Replicated chromatin supercoils into replicated
chromosomes; Centromere visible
o Homologous chromosomes pair – process called
synapsis.
 Pair called a tetrad
 Crossing over can take place
o Nucleoli disappear
o Centrosomes visible and move towards the poles
o Spindle fibers begin to form
o Nuclear envelope breaks down
o Some spindle microtubules attach to a protein
structure called the kinetochore located at the
centromere of each replicated chromosome
o Other spindle microtubules connect to microtubules
from the other centrosome
 Metaphase I – Middle – Homologous Pairs
o Homologous Chromosome line-up along the
metaphase plate
 Imaginary plane between the poles
 Kinetochores face the centrosome poles
o Random Assortment/Independent Assortment
 Anaphase I – Separate – Homologous Pairs
o Homologous Chromosomes separate
 Pulled by shortening microtubules to opposite
poles
 Spindle/Spindle microtubule interactions
elongate the cell
 Telophase I– reverse prophase
o
o
o
o
Nuclear envelopes reform around genetic material
Chromosomes uncoil to chromatin
Nucleoli reform
Meiotic spindles disappear
Cytokinesis - Division of cytoplasm to both new cells
 Usually begins during telophase I
Some cells enter an interphase after cytokinesis, other continue
right into Meiosis II
Questions:
Are the daughter cells at the end of Meiosis I diploid or
haploid?
Haploid
How are homologous pairs represented in the picture
above?
Size
Are there homologous pairs in the daughter cells at the end
of Meiosis I?
No
Are the daughter cells at the end of Meiosis I identical?
No
Meiosis I
Meiosis II (“Mitosis Lite” – low calorie mitosis)
Meiosis II
 Prophase II
o Replicated chromatin supercoils into replicated
chromosomes; Centromere visible
o Nucleoli disappear
o Centrosomes visible and move towards the poles
o Spindle fibers begin to form
o Nuclear envelope breaks down
o Some spindle microtubules attach to a protein
structure called the kinetochore located at the
centromere of each replicated chromosome
o Other spindle microtubules connect to microtubules
from the other centrosome
 Metaphase II - Middle
o Chromosomes line-up along the metaphase plate
 Imaginary plane between the poles
 Kinetochores face the centrosome poles
 Anaphase II – Separate – Sister chromatids separated
o Centromere separates
o Sister chromatids pulled by microtubules to opposite
poles
o Now each an unreplicated chromosome
o Spindle/Spindle microtubule interactions elongate
the cell
 Telophase II – reverse prophase
o
o
o
o
Nuclear envelopes reform around genetic material
Chromosomes uncoil to chromatin
Nucleoli reform
Meiotic spindles disappear
Cytokinesis - Division of cytoplasm to both new cells
 Usually begins during telophase
Result:
 4 haploid daughter cells
 Not identical to each other
 Not identical to parent cell
 Used for sexual reproduction (eggs and sperm)
Are cells diploid during Anaphase II? No
Were there Homologous Chromosomes in Mitosis? Yes
Male Meiosis
Female Meiosis
Meiosis I
XX
Meiosis II
X
X
I
I
I
Meiosis I
Meiosis II
I
What is “wrong” with this diagram?
Homologous Chromosomes – Pairs of chromosomes, one
from a man and one from a woman, that carry the same
genes at the same locations.
Should be
more pink
Sources of Variation
Alleles and Crossing Over
Alleles – Different versions of the same gene.
 Gene for flower color
o Purple allele and White allele
Crossing Over
 Takes places during Prophase I of Meiosis
 Homologous chromosomes in tetrad
o Chromosomes exchange corresponding segments
of their chromatids (sections that carry the same
genes) – 1-3 locations per chromosome pair
 Chiasmata (chiasma-singular) are sites
where homologous chromatids are attached
Tetrad
Independent/Random Assortment
Offspring produced from sexual reproduction are highly
varied
 Not identical to each other (except identical twins)
 Not identical to parents
What explains this variation?
Question:
How many different number combinations can you make
from the following numbers: 1, 1, 2, 2?
 No combinations with the same number (ex. 11)
 Must use one of each number
 21 and 12 are the same thing
4:
12, 12, 12, 12
Question:
How many different number combinations can you make
from the following numbers: 1, 1, 2, 2, 3, 3?
 Same rules
8: 123, 123, 123, 123, 123, 123, 123, 123
Question:
How many different number combinations can you make
from the following numbers: 1, 1, 2, 2, 3, 3, 4, 4?
 Same rules
16: 1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234,
1234, 1234, 1234, 1234, 1234, 1234, 1234, 1234
What is the pattern?
2n
 2 = number of different colors (number of sets)
 n = amount of different numbers of each color
(Number of chromosomes/set)
o 22 = 4
o 23 = 8
o 24 = 16
How does this relate to cells?
Homologous pairs line up along the metaphase plate during
Meiosis I
 The way one pair lines up is random or independent of
the way other pairs line up.
o XX or XX
o XX
xx
or
XX
xx
How many possible combinations for humans?
Number chromosome sets
=
n = number of chromosomes in each set
=
2
(23)
223 = about 8 million different combinations for each parent
 8 million X 8 million = 64 trillion combinations in the
zygote
What is wrong with the image below?
Cells should be
“n” at the end of
Meiosis I
Karyotypes
A picture/display of homologous chromosomes
 Arranged by size (largest to smallest*)
o 23rd pair the sex chromosomes
 Use images of chromosomes from Metaphase of
Mitosis
Why?
Shows:
Sex of organism
Women will have 23 homologous pairs on their karyotype
Men will have 22 homologous pairs on their karyotype and
one “X” and one “Y”
Chromosomal abnormalities
 Number
 Size/Structure
Does this karyotype belong to a man or a woman? How
can you tell?
Man: 23rd pair is one X and one Y chromosome
Are these chromosomes replicated or unreplicated? Why?
Replicated: picture was taken during metaphase of
Mitosis. Genetic material replicated
Process:
 White blood cells are separated from a blood sample
 Chemicals applied to arrest/freeze cell division at
metaphase
o Chromosomes replicated and supercoiled
 Chromosomes stained to see unique banding pattern
o Allows homologous chromosomes to be paired in
the picture/display
Chromosomal Abnormalities: Number and Structure
Most embryos with an abnormal number of chromosomes
are not viable (not able to survive)
Some number abnormalities have consequences less severe
than death
Trisomy – Three copies of a specific chromosome
Trisomy 21 – 3 copies of the 21st chromosome leads to
Down Syndrome
Causes of Trisomy
Nondisjunction during Meiosis
 Homologous chromosomes or sister chromatids fail to
separate during Anaphase I or II of meiosis, respectively.
o Risks increase with mother’s age
Normal division with the wrong chromosome number
Examples:
Patau Syndrome - The result of an extra copy of
chromosome 13. These people have serious eye, brain,
circulatory defects as well as cleft palate. Children rarely
live more than a few months.
Edward's Syndrome - The result of an extra copy of
chromosome 18. The child has almost every organ system
affected. Children rarely live more than a few months.
Nondisjunction Of The Sex Chromosomes - These
disorders can be fatal, but many people are fine.
 The Y chromosome carries few genes
 Only one X chromosome functions in each cell
Klinefelter Syndrome (XXY) – Small testes, sterile,
some female body characteristics (ex. breast
enlargement)
Also XX+Y+: XXYY, XXXY, XXXXY
XYY – No defined syndrome, taller than average
Trisomy X (XXX) – Function normally, identified only
by karyotype
Turner Syndrome (Monosomy X)
The result of only a single X karyotype in women.
Women with Turner's have only 45 chromosomes
(normal is 46). These people are genetically female but
they do not mature sexually during puberty and are
sterile. 98% percent of these fetuses die before birth.
Abnormal Chromosomes
Breaks in chromosomes can alter structure/function and
cause disorders
 Deletion – loss of a chromosomal fragment
 Duplication – second copy of a section of
chromosome from a sister chromatid.
 Inversion – reversal of fragment to original
chromosome
 Translocation – attachment of a chromosomal
fragment to a nonhomologous chromosome.
o Reciprocal – exchange segments
Which might be least harmful?
Inversion?
Growth and cell size