All language subtitles for 17281x_cell_cycle_terms-en

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.