The nucleus is where the DNA is kept and RNA is transcribed.
RNA is moved out of the nucleus through the nuclear pores. Proteins
needed inside the nucleus are transported in through the nuclear pores.
The nucleolus is usually visible as a dark spot in the nucleus, and
is the location of ribosome formation.
Ribosomes are where RNA is translated into protein. This process
is called protein synthesis. Protein synthesis is very important to
cells, therefore large numbers of ribosomes are found in cells. Ribosomes
float freely in the cytoplasm, and are also bound to the endoplasmic
reticulum (ER). ER bound to ribosomes is called rough ER because the
ribosomes on the ER give it a rough sandpaper like look.. These organelles
are very small, made up of 50 proteins and several long RNAs bound
together. Ribosomes do not have a membrane. Ribosomes fall into two
seperate units while not synthesizing protein.
The endoplasmic reticulum is the transport system for molecules needed
for certain changes and specific destinations, instead of molecules
that float freely in the cytoplasm. There are two types of ER, rough
and smooth. Rough ER has ribosomes attached to it, as mentioned before,
and smooth ER does not.
The golgi changes molecules and divides them into small membrane
contained sacs called vesicles. These sacs can be sent to various
locations in the cell.
The lysosome is the digestive system in the cell. It breaks down
molecules into their base components digestive enzymes. This demonstrates
one of the reasons for having all parts of a cell compartmentalized,
the cell couldnt use the destructive enzymes if they werent sealed
off from the rest of the cell.
The instructions for making an organism is called a
genome. It is the plan for all structures and activities for the exsitance
of the cell or organism. In the nucleus in every single cell, the
genome within is made up of coiled threads of deoxyribonucleic acid
(DNA) and protein molecules, combined in structures called chromosomes.
DNA is only 50 trillionths of an inch wide. Every organism has DNA
that is the pattern of its life.. Understanding how DNA does this
requires some knowledge of its structure and organization.
In humans and other higher organisms, DNA molecules consists of two
strands that wrap around each other to resemble a twisted ladder whose
sides, which are made of sugar and phosphate molecules, are connected
by rungs of nitrogen- containing chemicals called bases. Each strand
is a arrangement of repeating similar units called nucleotides, which
are each composed of one sugar, one phosphate, and a nitrogenous base.
Four different bases are present in DNA adenine (A), thymine (T),
cytosine (C), and guanine (G).
The order of the bases arranged along the sugar- phosphate backbone
is called the DNA sequence; the sequence specifies the exact genetic
instructions required to create a particular organism with its own
The two DNA strands are held together by weak bonds between the bases
on each strand, forming base pairs (bp). Genome size is usually stated
as the total number of base pairs; the human genome contains about
3 billion bp.
Each time a cell divides into two new cells, its full genome is duplicated;
for humans and other complex organisms, this duplication occurs in
the nucleus. During the cell division, the DNA molecule unwinds and
the weak bonds between the base pairs break, allowing the strands
to separate. Each strand directs the synthesis of a new strand, with
free nucleotides matching up with their bases on each of the separated
strands. Base pairing rules are adhered to, adenine will pair only
with thymine (an A - T pair) and cytosine only with guanine (a C -
G pair). Each new cell receives one old and one new DNA strand. Since
the cells follow the pairing rules, it ensures that the new strand
is an exact copy of the old one.
Each DNA molecule contains many genes, the basic physical and functional
units of heredity. A gene is a sequence of nucleotide bases, whose
sequences carry the information required for constructing proteins,
which provide the structural components of cells and tissues as well
as enzymes for essential biochemical reactions, such as the lysosomes
digestive enzymes. The human genome is has about one hundred thousand
Human genes vary in length, often extending over thousands of bases,
but only about 10% of the genome is known to include the protein-
coding sequences (exons) of genes. All living organisms are composed
largely of proteins; humans can synthesize at least 100,000 different
kinds. Proteins are large, complex molecules made up of long chains
of subunits called amino acids. Twenty different kinds of amino acids
are usually found in proteins. Within the gene, each specific sequence
of three DNA bases (codons) directs the cells protein synthesizing
machinery to add specific amino acids. For example, the base sequence
ATG codes for the amino acid methionine. Since 3 bases code for 1
amino acid, the protein coded by an average- sized gene (3000 bp)
will contain 1000 amino acids. The genetic code is thus a series of
codons that specify which amino acids are required to make up specific
The protein coding instructions from the genes are transmitted indirectly
through messenger ribonucleic acid (mRNA), a intermediary molecule
similar to a single strand of DNA. For the information within a gene
to be expressed, a complementary RNA strand is produced (a process
called transcription) from the DNA template in the nucleus. This mRNA
is moved from the nucleus to the cellular cytoplasm, where it serves
as the template for protein synthesis. We have created a program that allows you to convert a CGAT sequence into a protein
that shows this.The cells protein synthesizing machinery then translates
the codons into a string of amino acids that will constitute the protein
molecule for which it codes. In the laboratory, the mRNA molecule
can be isolated and used as a template to synthesize a complementary
DNA (cDNA) strand, which can then be used to locate the corresponding
genes on a chromosome map.
The 3 billion bp in the human genome are organized into 24 units
called chromosomes. All genes are arranged linearly along the chromosomes.
The nucleus of most human cells contains 2 sets of chromosomes, 1
set given by each parent. (The expection to this is the sex cells,
they have half the normal, to combine and add up to the normal amount).
Each set has 23 single chromosomes, 22 autosomes and an X or Y sex
chromosome. (A normal female will have a pair of X chromosomes; a
male will have an X and Y pair.) Chromosomes contain roughly equal
parts of protein and DNA; chromosomal DNA contains an average of 150
Chromosomes can be seen under a light microscope and, when stained
with certain dyes, reveal a pattern of light and dark bands reflecting
regional variations in the amounts of A and T vs G and C. Differences
in size and banding pattern allow the 24 chromosomes to be distinguished
from each other, an analysis called a karyotype. A few abnormalities,
including missing or extra copies of a chromosome or breaks and rejoinings,
can be detected by microscopic examination; Downs syndrome, in which
an individual's cells contain a third copy of chromosome 21, is diagnosed
by karyotype analysis. Most changes in DNA, however, are too subtle
to be detected by this technique and require molecular analysis. These
subtle DNA abnormalities (mutations) are responsible for many inherited
diseases such as cystic fibrosis and sickle cell anemia or may predispose
an individual to cancer, major psychiatric illnesses, and other complex
Mitochondria (singular: mitochondrion) are the sites
of aerobic respiration, and generally are the major energy production
center in eukaryotes. Mitochondria have two membranes, an inner and
an outer, visible in this electron microscope photo of a mitochondrion. Note the reticulations,
or many infoldings, of the inner membrane, This serves to increase
the surface area of membrane on which membrane-bound reactions can
take place. The existence of this double membrane has led many biologists
to theorize that mitochondria are the descendants of some bacteria
that was endocytosed by a larger cell billions of years ago, but not
digested. This fascinating theory of symbiosis, which might lend an
explanation to the development of eukaryotic cells, has additional
supporting evidence. Mitochondria have their own DNA and their own
ribosomes; and those ribosomes are more similar to bacterial ribosomes
than to eukaryotic ribosomes.
Chloroplasts are the site of photosynthesis in eukaryotic cells.
They are disk-like structures composed of a single membrane surrounding
a fluidcontaining stacks of membranous disks. Because of their geen
color chloroplast are the only organelles that can be easily seen
with a light microscope.
There is a single membrane surrounding the chloroplast. This membrane
surrounds a fluid called thestroma. Floating in the stroma are stacks
of disks made up of membranes, these are the grana. The grana resemble
a stack of coins, however instead of coins this stack is made up of
individual hollow disks called thylakoids.
That's it! You have made it through lesson 2, the parts of cells.
If you want to proceed to the next lesson, first take the lesson 2 quiz.