Chromosomes | MCAT 2015
Chromosomes
• A chromosome is a continuous
piece of DNA
• Human somatic cells have 23
pairs of chromosomes
Nucleosome
Eight histone proteins come together to form a
nucleosome.
•
Many nucleosomes coil together,
forming solenoids
Human Chromosomes
Human somatic cells have 23 pairs of
homologous chromosomes.
•
Each chromosomes contains an
extraordinarily large sum of
genetic information
o So, chromosomes must be
packed very efficiently in
order to fit in the nucleus
Packing Chromosomes
• Chromosomes are wound around
small proteins called histones
o Kind of like thread being
wound around a spool
Solenoid
Many nucleosomes (red rectangles) coil together
to form a solenoid.
•
And finally, several solenoids
together form supercoils
Chromosomes Wrapped Around Histones
Chromosomes are wrapped around histone
proteins. Segments of DNA that are needed less
frequently are wound more tightly, thereby
making them less accessible
•
Eight histones bind together to
form a nucleosome
Supercoil
Several solenoids combine to form a supercoil.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-17-2017
Chromosomes | MCAT 2015
•
•
Chromosomes, along with their
wrapping proteins (histones)
make up chromatin
Chromatin comes in two forms:
o Heterochromatin is
tightly-wrapped
chromatin, which is
transcribed less
frequently
o Euchromatin is looselywrapped chromatin,
which is transcribed more
often
Homologous Chromosomes
• We’ve routinely said that human
somatic cells have 23
chromosome pairs instead of
saying there are 46 chromosomes
• This is because each
chromosome has a homologue
o Homologous
chromosomes code for
the same traits
So, it is useful to
pair them together
• A cell whose chromosomes are
paired in homologues are said to
be diploid
o Alternatively, we can say
they have a ploidy of 2n
The 2 indicates
each chromosome
is paired in 2s
Diploid Cell
A human somatic cell is diploid, because the
chromosomes all have homologous pairs.
•
By contrast, a cell whose
chromosomes are unpaired is
said to be haploid
o As we may expect, the
ploidy of these cells is
just n
Haploid Cell
A human gamete cell is haploid, because there
are only 23 chromosomes- not 23 pairs of
chromosomes. None of the chromosomes have
homologues.
•
•
We said that homologous
chromosomes encode the same
traits
o But this doesn’t mean
they have the same genes
i.e. 2 homologous chromosomes
might code for eye color
o But, one may code for
blue eyes, while the other
codes for brown eyes
o The trait- eye color- is the
same, but the genes- blue
and brown- are not
Homologous Chromosomes
These homologous chromosomes will code for
the same traits (i.e. eye color), but their specific
genes may vary. Evidently, one encodes brown
eyes, while the other encodes blue eyes.
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© 2017 J Co Review, Inc., Accessed by Guest on 06-17-2017