Transcript ChromoSocks Lesson 1

ChromoSocks
Lesson 1
What is a chromosome?
• A chromosome is a piece of DNA that is
maintained inside the cell.
• It replicates or copies itself and is segregated
with each cell division.
• Different chromsomes contain different
numbers of genes.
• DNA is tightly coiled in the nucleus in long
strands called chromotin.
What are homologous chromosomes?
• Humans typically have 46 chromosomes,
organized into 23 pairs.
• Out of each pair, one is inherited from the
mother, and the other is inherited from the
father.
• The two copies of each pair are called
homologs.
• .
What are homologous chromosomes?
• Homologs are the same size , shape and have
genes located in the same place on the
chromosome.
• Although they are similar, they are NOT
identical.
• For example, a person can inherit a gene for
curly hair from one parent and straight hair
from the other parent.
What are replicated chromosomes?
• Before cells divide, the DNA is replicated or
copied, and condensed even more.
• The two replicated copies of each
chromosome are called sister chromatids.
• They are held together at a specific location
called the centromere.
What is Meiosis?
• Meiosis is a type of cell division that occurs
only in the production of sex cells.
• The purpose of meiosis is to reduce the
chromosome number (46) by half (23) that
will be present in the mature egg or sperm.
• The result of this is that when the egg and
sperm unite, the embryo will have the correct
number of chromosomes (46).
What is Meiosis?
• NOTE: 46 is the chromosome number for
humans. Other organisms have different
chromosome numbers.
• For humans, 46 is called the diploid number,
and 23 is called the haploid number.
• During meiosis, the chromosome number is
reduced, going from 2 copies of each
chromosome (diploid) to 1 single copy
(haploid).
What is Meiosis?
• At the end of meiosis, 4 haploid cells are
produced, known as gametes.
• In males, 4 functional sperm are produced
while in females, one large egg and 3 polar
bodies are produced. (Polar bodies are nonfunctional and store extra chromsomes)
What are the stages of Meiosis?
• S Phase: DNA within the cell is replicated.
(this is not a stage of meiosis)
• During this phase, the physical amount of DNA
is doubled, but the cell remains diploid.
MEIOSIS 1
• There are 4 stages in Meiosis 1. You can
remember them in order by remembering the
phrase PMAT.
• 1. Prophase: Chromosomes condense and
become visible. Each chromosome forms a
pair with its homolog.
Prophase 1
• Crossing over may occur: equal portions of
DNA from maternal and paternal homologs
are exchanged.
• Crossing over ensures genetic variation!
Metaphase 1
• Homologous chromosomes line up side by
side in the middle of the cell.
Anaphase 1
• The spindle fibers shorten which pulls the
homologous chromosomes to opposite poles
of the cells.
• Each pair segregates randomly and this
contributes to genetic diversity.
Telophase 1
• The centrioles and spindle fibers break down
and the nuclear membrane separately reforms
around two distinct sets of chromosomes.
Cytokinesis
• The remainder of the cell divides around the
two new nuclei.
• This results in 2 haploid cells, each with 23
chromosomes (in humans)
Meiosis 2
• The second cell division of meiosis is called
Meiosis 2. It begins with Prophase 2
Prophase 2
• Prophase 2: centrioles and spindle fibers form
and attach to the centromere of each
chromosome.
• The centromeres of sister chromatids act
independently with a spindle fiber from one
pole attaching to the centromere of 1 sister
chromatid and a spindle from the other pole
attaching to the other sister chromatid.
Prophase 2
Metaphase 2
• Chromosomes line up along the spindle of the
cell.
Anaphase 2
• The sister chromatids are pulled to opposite
ends of the cell due to the shortening of the
spindle fibers.
Telophase 2
• The centrioles and spindle fibers break down
and the nuclear membrane reforms.
Cytokinesis
• In cytokinesis, the cytoplasm divides and the cell
membrane pinches the cell in two.
• This occurs for both of the cells produced at the end
of Meiosis 1 and results in 4 genetically diverse,
haploid cells.
Errors in Meiosis
• The most frequent cause for meiotic errors is a
process called nondisjunction.
• In nondisjunction, something doesn’t separate like it
should.
• If a pair of homologous chromosomes fails to
separate in Meiosis 1, this would result in two sex
cells with an extra copy of that chromosome and two
sex cells that lack any copy of it.
Nondisjunction
• If sister chromatids fail to separate in
Meiosis 2, one sex cell would have an extra
copy of that chromosome and one cell would
lack it. The other two would have the correct
number of chromsomes.
Trisomy: having an extra chromosome
Monosomy: Missing a chromosome
Monosomy
Polyploidy
• If the cell completely fails to divide, it would
result in a sex cell with an extra copy of all
chromosomes. This is called polyploidy.
• In females, nondisjunction occurs in about
20% (1 in 5) female meiotic events. The rate is
lover in males.
Trisomy 21 (Down syndrome)
• Individuals have 3 copies of chromosome 21.
• Causes a range of physical and developmental
differences such as facial features, low muscle
tone, heart and intestinal abnormalities, and y
some level of intellectual and developmental
delays.
Down Syndrome
Trisomy 18 (Edward Syndrome) and
Trisomy 13 (Patau syndrome)
• These are more severe than Trisomy 13.
• Individuals with Edwards syndrome have three
copies of chromosome 18, while those with
Patau syndrome have three copies of
chromosome 13.
• Characterized by severe delays in growth and
development and multiple skeletal and organ
abnormalities. Most do not survive birth and
if they do, they usually pass in the 1st year.
Edward Syndrome
Patau Syndrome
Monosomy X (Turner Syndrome)
• Individuals with Turner Syndrome are females
with one X chromosome instead of the typical
two X chromosomes.
• They are usually shorter than average, have
characteristic facial features, and often have
heart and kidney abnormalities.
• These females also do not have ovarian
function and are infertile.
Turner Syndrome
XXY (Klinefelter syndrome)
Individuals with Klinefelter syndrome are males
with an extra X chromosome.
They are tall, with long arms and legs, have low
muscle tone, and less than typical hair on the
body.
They also have an increased chance for learning
disabilities and are infertile.
Klinefelter Syndrome
XYY
• Individuals with XYY are males with an extra Y
chromosome.
• They are taller than average, have an
increased risk of learning disabilities and have
minor behavior problems.
XYY