Meiosis in Humans | MCAT 2015
Meiosis in Humans
• Knowing the mechanics of
meiosis, we should now discuss
the specifics of meiosis in human
reproductive cells
• Before meiosis, reproductive
cells in males and females are
called spermatogonia and
oogonia, respectively
o They are diploid cells
with 23 chromosome
pairs
• In S phase of the cell cycle, their
DNA replicates, making them
primary spermatocytes and
primary oocytes
o Remember, they are still
diploid, as DNA
replication does not affect
the cell’s ploidy
• In females, all reproductive cells
will become primary oocytes
before birth
o They will not continue
with meiosis until she hits
puberty
• We will look first at the process
of spermatogenesis, which occurs
in males
Spermatogenesis
• First, our primary spermatocytes
will undergo meiosis I, forming 2
secondary spermatocytes
o Recall that meiosis I is
reductive division,
meaning the secondary
spermatocytes will be
haploid
• The secondary spermatocytes
will then undergo meiosis II,
forming 4 haploid spermatids
• Finally, the spermatids grow
flagella, making them fullfledged spermatozoa (sperm)
Spermatogenesis
Spermatogonia (not shown) have their DNA
replicated during S phase of the cell cycle. That
produces diploid primary spermatocytes which
undergo meiosis I to form 2 haploid secondary
spermatocytes. Meiosis II generates 4 haploid
spermatids. Those spermatids grow flagella,
making them into spermatozoa.
Oogenesis
• When a female hits puberty,
meiosis will begin
• Meiosis I of a primary oocyte
produces:
o One secondary oocyte
o One polar body
Oogenesis: Meiosis I
Meiosis I of a primary oocyte produces a
secondary oocyte and a polar body. The polar
body is essentially a waste product, but since it is
so small, it allows the secondary oocyte to have
most of the cytoplasm.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-17-2017
Meiosis in Humans | MCAT 2015
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Rather than produce two equallysized secondary oocytes, meiosis
I produces a waste product- the
polar body
o The polar body allows the
majority of the cytoplasm
to be conserved for the
secondary oocyte
And, since meiosis I is reductive
division, both the secondary
oocyte and the polar body are
haploid
During ovulation, the secondary
oocyte is released
A sperm from a male can fertilize
the secondary oocyte
o This causes the two
haploid cells to fuse into
a diploid cell known as a
zygote
separate during anaphase
I, or
o The chromatids of a
chromosome don’t
separate during anaphase
II
(a)
(b)
Nondisjunction
Nondisjunction during (a) anaphase I occurs
when homologous chromosomes fail to separate
and move to opposite ends of the cell. In (b)
anaphase II, it can occur when the individual
chromatids of the chromosome fail to separate.
•
In either scenario, nondisjunction
will result in one daughter cell
having excess genetic
information, while the other
lacks genetic information
Fusion of a Sperm and Secondary Oocyte
A haploid spermatozoa and secondary
spermatocytes fuse to form a diploid zygote.
•
Since the secondary oocyte never
underwent meiosis II, that occurs
now
o And on the side, another
polar body is formed
Errors During Meiosis
• Errors during meiosis are costly,
since the reproductive cells
contain the genetic information
to be replicated throughout the
entire body
• One type of error is known as
nondisjunction, which occurs
when either:
o Homologous
chromosomes fail to
(a)
(b)
Result of Nondisjunction
Whether nondisjunction occurs in (a) anaphase I
or in (b) anaphase II, it will result in one
daughter cell having an extra genetic
information, while the other daughter lacks
genetic information.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-17-2017