CIS Biology Team
Intro.
In order to understand DNA, Chromosomes and Genes (and how they are related),one must
understand a little bit about a cell and its nucleus. When the cell is not dividing, its nucleus contains
chromatin, a combination of protein and DNA. When the cell is dividing into two new cells, the DNA
contained within the nucleus is duplicated via mitosis, which ensures that each new cell contains a
complete copy of all genetic material found within the parent cell. Mitosis is a process by which a
eukaryotic cell (a cell containing complex structures) separates the chromosomes within its nucleus
into two identical sets of nuclei.
During the division of a cell, the chromatin organizes itself into chromosomes. Each chromosome
contains a DNA molecule, and each molecule is comprised of many genes, each containing
instructions needed to direct the synthesis of a protein for a specific function.
DNA
DNA, or Deoxyribonucleic acid, is a nucleic acid that contains the genetic instructions that are used
in the development and functioning of all living organisms, with the exception of an RNA virus. The
role of DNA molecules is the storage of information for the long-term. The instructions it contains
are needed to construct other components of cells, such as RNA molecules and proteins, much like a
blueprint. The segments in a DNA strand that contain this information are called genes, but other
sequences of DNA have structural purposes, or are for regulating the use of the information.
DNA consists of two long polymers made from
repeating units called nucleotides, with sugars and
phosphate groups joined by ester bonds. These strands
run in opposite directions, and are therefore antiparallel. Attached to each sugar is one of four bases
known as nucleobases. These four bases are Adenine,
Guanine, Thymine and Cytosine. These four bases
appear in the DNA, so they are known as DNA-bases.
There is a fifth base called Uracil, which appears in RNA.
The sequences of these nucleobases encode
information that is read using the genetic code, which
specifies the sequence of amino acids within proteins.
The code is read by copying a stretch of DNA into the
related nucleic acid RNA in a process called
transcription.
A nucleobase linked to a sugar is called a nucleoside, and a base linked to a sugar and one or more
phosphate groups is called a nucleotide. The table below shows the Nucleobases, and when they are
linked with a sugar.
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Nucleobase
Adenine
Guanine
Thymine
Cytosine
Uracil
Nucleoside
Adenosine
A
Guanosine
G
Thymidine
T
Cytidine
C
Uridine
U
Chromosomes
So what exactly are chromosomes? Chromosomes are organized structures of DNA and protein
found within a cell. It is a single strand of DNA containing many genes, regulatory elements and
other nucleotide sequences. A regulatory sequence is a segment of DNA where regulatory proteins
(DNA-binding protein) bind in a preferential manner. These proteins bind to short stretches of DNA
called regulatory regions, which are positioned a short distance from the gene being regulated.
Chromosomes vary widely between different organisms. The DNA molecule may be circular or linear,
and can be composed of a varying number of nucleotides. Nucleotides are molecules that make up
the structural units of RNA and DNA when joined together. A typical eukaryotic cell (cells with nuclei)
has linear chromosomes, while prokaryotic cells (cells without defined nuclei) have circular
chromosomes, though there are exceptions to both rules. In addition to this, cells may contain more
than one type of chromosomes. For example, the mitochondria present within eukaryotes have their
own small chromosomes.
In eukaryotes, chromosomes are packaged by proteins into a condensed structure known as
chromatin. This allows the strand of DNA to fit within the nucleus. Chromosomes are also essential
for cellular division, and must be replicated, divided and passed successfully to their daughter cells
to ensure genetic diversity and the survival of their offspring. Compaction of the duplicated
chromosomes during mitosis and meiosis generally result in the classic four armed structure.
Chromosomes can be divided into two types – autosomes and sex chromosomes. Certain generic
traits linked to a person’s sex are passed on through the sex chromosomes. Put simply, the
autosomes contain everything else (the rest of the genetic hereditary information). All act in the
same way during cell division.
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The images above represents the 23 paired chromosomes in an average human. For those who are
struggling with the math, that’s 46 total chromosomes in a human. The image on the left shows the
karyogram of a human male, as noted by the sex chromosomes (pair #23), which contains one X and
one Y chromosome. This is known as heterogametic sex. The image on the right shows the
karyogram of a human female, which is shown through the presence of two X chromosomes. This is
known as homogametic sex. Note that the presence of solely two Y chromosomes is impossible.
Females and males alike both retain one of their mother’s X chromosomes, and females receive the
second X chromosome from their fathers (or rather from their paternal grandmother on her father’s
side, as the father retains his X chromosome from his mother). Males therefore receive their Y
chromosome from their fathers as well. While the X chromosome does contain a few thousand
genes, few of those genes actually play a part in sex determination; instead they are autosomal
genes, genes that code for several proteins related to non-sexual traits. In mammals, the Y
chromosome contains a gene known as SRY (Sex-determining region Y), which triggers development
as a male. The Y chromosomes also contain genes necessary for the production of sperm.
During the embryonic development in females, one of the X chromosomes is randomly inactivated,
permanently. This happens in nearly all somatic cells (a biological cell forming the body of an
organism). This phenomenon is known as X-inactivation or Lyonization and results in the creation of
a barr body. Since women have two X chromosomes, one of them is deactivated to ensure that the
female does not have twice as many X chromosome gene products as the male, which possesses
only a single copy of the X chromosome. Once the chromosome is deactivated, it will forever be
deactivated. When seen through a microscope, the inactive X chromosome is seen as a granule on
the nucleus. This granule is known as a Barr body. A Barr body is the inactive X chromosome in a
female somatic cell. In men and women with more than one X chromosome, the number of Barr
bodies visible at interphase is always one less than the total number of X chromosomes present.
Note that X-inactivation isn’t generally confined to women alone, but to anyone with two X
chromosomes present. In order to better understand Barr bodies, here is an example: A man with a
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47, XXY karyotype1 have a single Barr body, whilst a woman with a 47, XXX karyotype have two Barr
bodies. The image below points out the location of a Barr body.
Besides the common XX/XY system for sex determination, there are other sex determination
systems. The XX/XO system is used for many insects. The females are XX, signifying two X
chromosomes, and the males are XO, signifying only one X chromosome. The O represents the lack
of another X chromosome. There is also the ZW sex determination system, which is used for birds
and certain fish and insects. Males are the homogametic sex in this system, using ZZ, while females
are heterogametic, using ZW. The Z chromosome is larger and contains more genes, much like the X
chromosome in the XX/XY system.
Genes
The image above is an illustration of the placement of genes within a chromosome.
A gene can be defined as a region of DNA that controls one, and only one hereditary characteristic. It
usually corresponds to a sequence used during the production of a specific protein or of RNA. Living
organisms depend on genes to function, as they hold the information needed to build and maintain
1
Karyotype is the number and appearance of chromosomes in the nucleus of a eukaryotic cell. It can also refer
to the complete set of chromosomes in a species, or an individual organism.
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the cells within an organism and to pass on the cells traits to offspring. Genes can code for various
traits, such as eye color, blood type, number of eyes etc. When a gene is copied and transmitted
from the cell to its offspring, everything is copied. This includes the coding DNA sequences, noncoding regulatory DNA sequences and introns. An intron is a nucleotide sequence within a gene that
is removed via RNA splicing to generate the final product of a gene.
The total complement of genes in an organism or cell is known as its genome. In prokaryotes, the
majority of genes are located on a chromosome of circular DNA, while eurkaryotes usually possess
multiple linear DNA helices packed into complexes called chromosomes. Many species can carry
more than one copy of their genome within each somatic cell. Cells with only one copy are called
haploid, while those with two are called diploid. Those with three or more are called polypoids.
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Bibliography
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