Download Cell Reproduction (continued)

Cell Reproduction
(continued)
Review
• Prokaryote and eukaryote cells reproduce
differently.
• Prokaryotes reproduce by binary fission
(they split in two), which is an asexual
process.
• Eukaryotes reproduce sexually and
asexually.
• Somatic cells in eukaryotes reproduce
asexually by mitosis.
Mitosis: A few more facts
• At the end of mitosis, you end up with two
identical daughter cells.
• Each daughter cell (in a human) has
exactly 46 chromosomes.
• If a human cell has 46 chromosomes, it is
diploid (2n), where n = 23.
• ALL somatic cells are diploid. They
contain 2n chromosomes.
Now, let’s talk about sex…
• In Biology, when you talk about sex, you’re talking about
meiosis.
• The reproductive cells, or gametes, of eukaryotes
reproduce sexually by a process called meiosis.
• Meiosis is very similar to mitosis EXCEPT:
– Meiosis consists of 2 division cycles
– Meiosis starts with pairs of homologous chromosomes (or a
tetrad of chromatids)
– You end up with 4 daughter cells (sperm or egg)
– Each daughter cell is genetically different
– Each daughter cell is haploid (n), which means it contains half
as many chromosomes as somatic cells.
Diploid vs. Haploid
• A diploid cell has a full set
of chromosomes. It is 2n.
– 1 set from Mom (1n)
– 1 set from Dad (1n)
• A cell with a half set of
chromosomes is called a
haploid cell. It is 1n.
• Two gametes (egg and
sperm) combine to form a
diploid zygote.
DNA
Recall the
structure of a
chromosome:
chromatin
chromosome
Homologous Chromosomes
• Because we get 1 set of
chromosomes from mom,
and 1 from dad, we have
2 versions of the same
chromosome (homologs).
• These 2 chromosomes
have genes in the same
places (called loci).
• However, while Mom’s
version of the gene might
code for blue eyes, Dad’s
may code for brown.
• Different versions (here
represented by colors) of
genes are called alleles.
The Sex Chromosomes
• One pair of chromosomes
is unique. These are the
sex chromosomes (X & Y
chromosomes). They
determine whether or not
the baby will be a boy or
a girl.
• Girls have 2 X
chromosomes.
• Boys have 1 X and 1 Y
chromosome.
Chromosomes & Karyotyping
• Your other 22 pairs of
chromosomes are called
autosomes.
• They contain the genes
that code for everything
else. (see Human
Genome Project Poster)
• A karyotype shows all
23 pairs of chromosomes.
It can show whether or
not a person contains
certain chromosomal
abnormalities or
conditions.
Some Chromosomal
Abnormalities/Conditions
• Irregular number of
chromosomes- results in
fetal death, or severe
retardation, malformation,
& short life span.
– Trisomy (3 copies)
– Monosomy (1 copy)
• Ex.: Down Syndromeresults from extra copy of
chromosome 21
Some Chromosomal
Abnormalities/Conditions
• Female Sex
abnormality- Turner
syndrome- can occur
when there is only
one copy of the X
chromosome.
• Symptoms include
webbed neck, widelyspaced underdeveloped breasts,
and short stature.
Some Chromosomal
Abnormalities/Conditions
• Both of the previous disorders were a result of
mechanical errors that occurred during meiosis.
They are called nondisjunction errors.
• Sometimes chromosomal abnormalities only
show up in some cells. This is called
mosaicism. In these cases, the abnormality will
usually be less severe.
• http://www.biology.arizona.edu/human_bio/activi
ties/karyotyping/karyotyping.html
Back to Meiosis: Why is sex good?
• Organisms reproduce sexually because it
provides variation in the species. With
the exception of identical twins, every
single one of us is different, and different
is good, especially when differences give
us an advantage.
Meiosis I & II
• The first division cycle of meiosis is called
Meiosis I. It consists of:
–
–
–
–
Prophase I
Metaphase I
Anaphase I
Telophase I
• The second division cycle of meiosis is called
Meiosis II. It consists of:
–
–
–
–
Prophase II
Metaphase II
Anaphase II
Telophase II
Prophase I
• This is the first stage of
Meiosis I.
• The cell has made extra
chromosomes, and now
contains 4n (humans =
92).
• The nuclear membrane
disappears.
• Homologous
chromosomes (4
chromatids) pair up. This
is called synapsis.
• Sometimes, they cross
legs and switch parts.
This is called crossing
over.
Metaphase I
• Homologous
chromosomes line up
in the middle of the
cell, attached to the
spindle.
• These chromosomes
line up randomly
behind other pairs.
This is called
independent
assortment.
(Metaphase in
the 2nd round
of Meiosis)
Anaphase I
• Homologous chromosomes
separate, and move to
opposite ends of the cell.
Note that they still have
centromeres.
Telophase I
• Just like in telophase
of mitosis, the
nucleus reforms
around the
chromosomes at each
end of the cell, and
the cells begin to
pinch in and separate
by cytokinesis.
• The chromosomes
unwind to chromatin
Prophase II
• Chromatin condenses
again, to reform
chromosomes.
• These chromosomes
only have 2
chromatids, just like
prophase in mitosis.
• The nuclear
membrane
disappears and
spindle forms.
Metaphase II
• The chromosomes
line up in the middle.
• The way that they line
up is completely
random- once again,
this is called
independent
assortment.
Anaphase II
• The centromeres break, and chromatids move away
from each other to opposite ends of the cell.
• How they separate is also random, and is called
segregation.
Telophase II
• The nuclear membrane
reforms, and
chromosomes unwind.
• The cells pinch inward,
and cytokinesis occurs.
Daughter Cells: Gametes
• The end result of
meiosis is 4 haploid
daughter cells.
• Each daughter cell
has half the number
of chromosomes a
normal diploid cell.
Sperm vs. Egg Production
• The process just
described is exactly how
sperm are produced.
• However, eggs are
produced slightly
different. Instead of the
cytoplasm being evenly
split between daughter
cells, most of it is donated
to one of the cells. The
smaller egg cells die.
Why sex? Variation!
• The advantage of sexually reproducing is
the variation in offspring that it causes.
• Variation comes from 3 things in meiosis:
– Crossing over- when homologous
chromosomes exchange legs
– Independent Assortment- how
chromosomes line up behind others randomly
– Segregation- how chromosomes separate
into the daughter cells randomly
Animated Meiosis
http://highered.mcgraw-hill.com/olc/dl/120074/bio17.swf