How would you define a chromosome?
What do you picture in your head?
In eukaryotes, a chromosome consists of a single molecule
of DNA associated with proteins that help maintain
its structure and function.
You've probably seen a chromosome depicted like this.
However, this commonly used image
can be misleading, as it actually
depicts a linked pair of identical sister
chromatids after DNA replication.
A single condensed chromosome actually
should be depicted like this.
This is one of several instances where imperfect terminology
can make biological processes seem
more puzzling than they actually are.
So in this video, we will give an overview
of a few important terms and concepts
that are used throughout biology.
Every human cell has a mass of DNA associated
with other proteins and RNA stored in the nucleus.
We call this complex of macromolecules chromatin.
You may have heard chromatin described as a tangled
mess of DNA and protein, but we're
starting to figure out that it does actually
have its own organization.
Each chromosome contains a single molecule
of DNA associated with proteins to help
it maintain its structure and function.
Chromosomes can be un-condensed or condensed.
The compact structure of a condensed chromosome
allows the cell to more easily move and manipulate chromosomes
independently, which is useful for cellular processes
like mitosis and cytokinesis.
Humans have 23 unique chromosomes,
each including the proteins associated
with a single molecule of DNA containing
many genes, regulatory elements, and non-coding DNA.
Biologists quantify this number of unique chromosomes
in an organism-- also known as the number of chromosomes
in a haploid cell, such as ovum or sperm in humans--
with the variable N. Therefore, we
say that human cells have N equals 23.
n changes between species and does not
indicate how complex the organism is.
For example, the cells of budding yeast--
Saccharomyces cerevisiae-- have n equals 16.
The cells of fruit flies-- Drosophila melanogaster--
have n equals 4, while the cells of domestic dogs-- Canis
familiaris-- have N equals 39.
Even though humans have 23 unique chromosomes
in each haploid cell, our somatic, non-sex cells
are diploid.
So in most of our cells, we have two slightly different sets
of these unique chromosomes.
Therefore, we describe diploid cells
as 2N, or as having 46 chromosomes in total.
You receive one set of 23 chromosomes from your mom
and one set of 23 chromosomes from your dad.
These chromosomes contain various similar sequences
of DNA, but have different versions
of the same gene located in the same spot, or locus on the DNA.
We call these different versions of the same genes alleles.
Also, because these chromosomes are very similar
but don't contain identical DNA sequences,
we call them homologous chromosomes.
Same-- homo, relation-- logos.
In some, there are two types of cells in humans
and many eukaryotes-- haploid and diploid.
Haploid cells have one unique copy
of each chromosome-- for example, N equals 23 in humans.
Diploid cells have two copies of homologous chromosomes--
for example, 2N equals 46 in humans.
So what happens to our DNA and chromosomes
when we have to make more cells through cell division?
The series of biological events that occur during each cell
division is called the cell cycle.
In this video, we will focus on the mitotic cell cycle
with cells that undergo mitosis to create two daughter cells.
When a human cell undergoes replication,
every single one of our 46 homologous chromosomes
is duplicated to form identical pairs.
Each member in an identical pair of chromosomes
is called a sister chromatid.
Each sister chromatid is just a copy of one of 46 homologous
chromosomes, and they are linked together into one unit.
This is where you may have heard some people describe
the cell as having four N chromosomes, which
can be misleading.
These cells are still diploid with 2N, or 46, homologous
chromosomes.
Even though they are currently duplicated
as pairs of sister chromatids, you
didn't receive two more sets of new homologous chromosomes
with different alleles, and the amount of unique chromosomes
has not increased.
Instead, you can describe this cell
as diploid with two N chromosomes and 4C DNA content.
The C value stands for constant-- the genome size
or amount of DNA that is characteristic
of a particular haploid cell.
As such, a haploid cell will have 1C DNA content,
and a diploid cell that hasn't undergone DNA replication
will have 2C DNA content.
This specific terminology allows us
to describe how the diploid cell that
has undergone DNA replication still has
only two sets of homologous chromosomes-- 2N--
but now contains 4C DNA content.
That is, this cell contains twice the DNA content it once
had-- 2C-- because each chromosome has been replicated
to form a pair of sister chromatids.
This DNA replication ensures that at the end of each cell
cycle involving mitosis, the cell
can split these pairs of sister chromatids apart to divide
into two diploid daughter cells with 2N chromosomes and 2C DNA
content.
Hopefully, now you can explain the difference
between chromatin and chromosomes
and can describe how the homologous chromosomes
duplicate to form pairs of sister chromatids
when the cellular DNA is replicated.
These concepts and their vocabulary
are tricky, but are very useful to understand thoroughly
when talking or thinking about biology.
Thanks for watching.