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Part a: In this illustration, DNA tightly coiled into two thick cylinders is shown in the upper right. A close-up shows how the DNA is coiled around proteins called histones. Part b: This image shows paired chromosomes.
(a) This image shows various levels of the organization of chromatin (DNA and protein). (b) This image shows paired chromosomes. (credit b: modification of work by NIH; scale-bar data from Matt Russell)

The nucleolus

We already know that the nucleus directs the synthesis of ribosomes, but how does it do this? Some chromosomes have sections of DNA that encode ribosomal RNA. A darkly staining area within the nucleus called the nucleolus    (plural = nucleoli) aggregates the ribosomal RNA with associated proteins to assemble the ribosomal subunits that are then transported out through the pores in the nuclear envelope to the cytoplasm.


Ribosomes are the cellular structures responsible for protein synthesis. When viewed through an electron microscope, ribosomes appear either as clusters (polyribosomes) or single, tiny dots that float freely in the cytoplasm. They may be attached to the cytoplasmic side of the plasma membrane or the cytoplasmic side of the endoplasmic reticulum and the outer membrane of the nuclear envelope ( [link] ). Electron microscopy has shown us that ribosomes, which are large complexes of protein and RNA, consist of two subunits, aptly called large and small ( [link] ). Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA is transcribed into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.

The ribosome consists of a small subunit and a large subunit, which is about three times as big as the small one. The large subunit sits on top of the small one. A chain of mRNA threads between the large and small subunits. A protein chain extends from the top of the large subunit.
Ribosomes are made up of a large subunit (top) and a small subunit (bottom). During protein synthesis, ribosomes assemble amino acids into proteins.

Because proteins synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), ribosomes are found in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.


Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don’t get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid.

Questions & Answers

hetreothalism in fungi
Lekhram Reply
there are 3 trimester in human pregnancy
I don't know answer of this question can u help me
what is a cell
Fatima Reply
A cell is functional and structural unit of life.
what is genetic
Janet Reply
I join
what are the branchas of biology
Prisca Reply
zoology, ecology
genetics, microbiology,botany and embryology
what is a cell
Kulunbawi Reply
cell is smallest unit of life. cells are often cell the building blocks of life...
the first twenty element
Orapinega Reply
what are the characteristics of living things?
growth,respiration,nutrition,sensitivity, movement,irritability, excretion,death.
What is the difference between adaptation and competition in animals
Adeyemi Reply
What is biology
it is a natural science stadey about living things
Biology is the bronch of science which deals with the study of life is called biology
what is the x in 300 stands for?
Ogbudu Reply
the properties of life
Clarinda Reply
response to the environment, reproduction, homeostasis, growth,energy processing etc.....
what is reproduction
Reproduction is a fundamental feature of all known life,each individual organism exist as a result of re production.....or else Multiplying...
a complete virus particle known as
Darlington Reply
These are formed from identical protein subunitscalled capsomeres.
fabace family plant name
Pushpam Reply
in eukaryotes ...protein channel name which transport protein ...
Pushpam Reply
in bacteria ...chromosomal dna duplicate structure called
what is a prokaryotic cell and a eukaryotic cell
Matilda Reply
There are two types of cells. Eukaryotic and Prokaryotic cells. Prokaryotic cells don't have a nucleus or membrane enclosed organelles (little organs within that cell). They do however carry genetic material but it's not maintained in the nucleus. Prokaryotic cells are also one celled.
Prokaryotic cells are one celled (single celled).
Prokaryotic cells are Bacteria and Archea
Prokaryotic cells are smaller than Eukaryotic cells.
Eukaryotic cells are more complex. They are much bigger than Prokaryotic cells.
Eukaryotic cells have a nucleus and membrane bound organelles.
Eukaryotic cells are animals cells which also includes us.
Eukaryotic cells are also multicellular.
nice explaination
eukaryotic cells are individual cells .. but eukaryotes are multicellular organisms which consist of many different types of eukaryotic cells
also eukaryotic cells have mitochondria. prokaryotic cells do not
in prokaryotes only ribosomes are present... in eukaryotes mitochondria ...glogi bodies ..epidermis .....prokaryotes one envelop but eukaryotes compartment envelop....envelop mean membrane bound organelles......
prokaryotic cell are cells dat have no true nuclei i.e no cell membrane while eukaryotic cell are cell dat have true nuclei i.e have cell membrane
we have 46 pair of somatic cell and 23 pair of chromosomes in our body, pls can someone explain it to me. pls
Matilda Reply
we have 22 pairs of somatic chromosomes and one pair of sex chromosome
we have 23 pairs of chromosomes,22 pairs of somatic and one pair of sex chromosomes
23 chromosomes from dad & 23 chromosomes from mom 23 +23=46 total chromosomes
X & Y chromosomes are called sex cells, the very presence of a Y chromosome means the person is Male.
XX Female XY Male
If a Karyotype has more than 46 Chromosomes then nondisjunction occured. For example, having an extra chromosome 21 will cause Down Syndrome.
am caira I want to join
caira,whrere are u from
I'm a Ghanaian

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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