2.5: The Nucleus

Found within eukaryotic cells, the nucleus contains the genetic material that determines the entire structure and function of that cell.

Master this section and you'll be able to

• Explain the purpose of the nucleus in eukaryotic cells (include storing DNA and directing the synthesis of ribosomes and proteins)
• Identify the nucleolus as a condensed region within the nucleoplasm where ribosome synthesis takes place
• Explain the structure and role of chromatin (including how DNA is wrapped around histone proteins and stored within the nucleoplasm).
• Describe briefly the composition and function of ribosomes. 

The Nucleus

One of the biggest differences between prokaryotic and eukaryotic cells is the nucleus — the “command center” of the cell. As you learned earlier, prokaryotic cells (such as bacteria) do not have a true nucleus. Their DNA (deoxyribonucleic acid) just floats around in the cytoplasm. But eukaryotic cells (like our own) do have a membrane-bound nucleus that stores and protects DNA, and also helps direct important activities like making ribosomes and proteins. In fact the nucleus actually has a double-membrane

Inside this nucleus is a gel-like substance called the nucleoplasm, which holds the cell’s chromatin and other substances. What is chromatin? It is the material that makes up chromosomes, the structures made of DNA (your genetic blueprint) and proteins.

In prokaryotes, DNA comes in the form of a single circular chromosome. In eukaryotes, chromosomes are linear, and each species has a specific number. For example, humans have 46 chromosomes, while cats have 38 and dogs 78.  Chromosomes are arranged in pairs.  Therefore, humans have 

• 18 pairs of autosomes. Any chromosome that is not a sex chromosome and carries genes that determine most of an organism’s traits is called an autosome.

• 1 pair of sex chromosomes. The sex chromosomes determine an organism’s biological sex — typically XX for females and XY for males in most mammals.

To keep the long, fragile DNA organized, the DNA is wrapped around special proteins called histones (see image below). This protein-associated DNA is what we call chromatin.  The long DNA strands are wound around special proteins called histones, like thread wrapped around spools.

To keep the long, fragile DNA organized, it is wrapped around special proteins called histones (see image below). This combination of DNA and proteins is called chromatin. You can think of it like thread wrapped around spools — the histones help neatly package the DNA so it fits inside the cell’s nucleus.

Figure \(\PageIndex{1}\): Most of the time, your DNA is kept as chromatin in the nucleus. This relaxed structure makes it easy for the cell to read the DNA and make proteins. But when a cell is about to divide, it needs to make sure each new cell gets the right amount of DNA. To do that, the chromatin coils up tightly into thick, visible structures called chromosomes. 

Think of it like this:

• Chromatin is like a loose pile of yarn when the cell is working.

• Chromosomes are like neatly wrapped balls of yarn when the cell is getting ready to divide and pack things up.

Once the cell finishes dividing, the chromosomes unwind back into chromatin so the new cells can get back to work.

 

Humans have 23 different chromosomes, but we have two copies of each — one from each parent. That means each cell in your body has 46 chromosomes total. Because we have two sets, humans are called diploid.

Some organisms only have one copy of each chromosome — they are called haploid. Others may have three or even four copies of each chromosome, and these are called triploid and tetraploid, respectively.

 

The nucleoplasm is kind of like the cytoplasm, but just for the nucleus.  It is the gel-like fluid inside the nucleus that surrounds and supports the DNA and other structures inside the nucleus. 

The nucleolus is a dense region inside the nucleoplasm, and it is where ribosomes are made. Specifically, it is a concentrated area of chromatin involved in building the components of ribosomes.

Ribosomes are large structures made of proteins and RNA (ribonucleic acid). They are the organelles responsible for making proteins. Ribosomes get their instructions from the DNA in the nucleus, where the DNA is first transcribed into messenger RNA (mRNA).

The mRNA then travels out of the nucleus to the ribosomes. There, the ribosomes read the mRNA code — a sequence of nitrogenous bases — and translate it into a specific chain of amino acids, forming a protein.

Figure \(\PageIndex{2}\): Ribosomes are responsible for protein synthesis. Ribosomes are made up of a large subunit (top) and a small subunit (bottom). During protein synthesis, ribosomes assemble amino acids into proteins.

 

Lastly, the boundary of the nucleus is called the nuclear envelope. It consists of two phospholipid bilayers: an outer membrane and an inner membrane. The nuclear membrane is continuous with the endoplasmic reticulum. Nuclear pores allow substances to enter and exit the nucleus

Figure \(\PageIndex{3}\): Nuclear envelope with nuclear pores. The nuclear envelope is a double membrane that encloses the nucleus, separating it from the cytoplasm. Each layer is made of a phospholipid bilayer. This is in contrast to the cell membrane, which has just one bilayer. The envelope is studded with nuclear pores, which act like selective gateways, allowing materials like RNA and proteins to move in and out of the nucleus while protecting the DNA inside.