Basics of genetics.

1. Basics of Genetics
DR. SHUMAILA ASHRAF
INSTITUTE OF CHEMISTRY, UNIVERSITY OF SARGODHA

2. Contents:
Chromosomes
➢ Chromosome’s composition
➢ Structure of chromosomes
➢ Types of chromosomes
➢ Number of chromosomes in different organisms,
➢ Chromosome’s size
Gene
➢ What is gene?
➢ Number of genes in different organisms
➢ Chemical structure of genes
➢ DNA Replication
➢ Gene expression

3. Chromosomes
➢ In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called
chromosomes which are carriers of genes.
➢ Each chromosome is made up of DNA tightly coiled many times around proteins called histones
that support its structure.
➢ Chromosomes were first described by Strasburger (1815), and the term ‘chromosome’ was first
used by Waldeyer in 1888 (Gr. Chrom- color, soma- body).
➢ They appear as rod-shaped dark stained bodies during the stages of cell division under light
microscope, otherwise they are not visible in an active nucleus due to high water contents.

4. Chromosome’s Composition
Major components of chromosomes are
1. DNA
2. RNA
3. Basic proteins of low molecular weights known
as “histones”.
4. Acidic proteins (non-histone proteins).
5. Chromosomes are made up of thin chromatic
threads called “chromatin fibers”.

5. • Interphase Chromosomes consists of about 30-40% DNA, 50-60% Proteins and 1-10%
RNA.
• Metaphase Chromosomes contain 15-20% DNA, 65-75% Proteins and 10-15% RNA.
• Chromatin fibers undergo folding, coiling and supercoiling during “prophase” so the
chromosomes become progressively thicker, smaller and visible.
• At the end of cell division these fibers unfold and extend as fine chromatin threads.
Chromosomes during different stages of cell division

6. Histones
Histones are highly basic proteins that provide structural support for chromosomes. Each chromosome contains a
long molecule of DNA, which must fit into the cell nucleus. To do that, the DNA wraps around complexes of histone
proteins, giving the chromosome a more compact shape. They act as coils around which DNA winds to create
structural units called nucleosomes. Nucleosomes in turn are wrapped into 30 nm fibers that form tightly
packed chromatin. Histones prevent DNA from becoming tangled and protect it from damage. In addition, histones
play important roles in gene regulation and DNA replication. Without histones, unwound DNA in chromosome would
be very long. For example, each human cell has about 1.8 meters of DNA if completely stretched out; however, when
wound about histones, this length is reduced to about 90 micrometers (0.09 mm) of 30 nm diameter chromatin fibers.
There are five families of histones which are designated H1/H5 (linker histones), H2, H3, and H4 (core histones).
The nucleosome core is formed of two H2A-H2B dimers and a H3-H4 tetramer. The tight wrapping of DNA around
histones is to a large degree a result of electrostatic attraction between the positively charged histones and negatively
charged phosphate backbone of DNA.

8. Structure of Chromosomes
Chromatid:
A chromosomes is made up of two strands which are identical to each other and these are called chromatids.
They are connected near the center. Chromatids are prepared when new cells are going to be made.
Centromere :
It is the primary constriction at the center to which the
chromatids or spindle fibers are attached. Its function is to enable movement
of the chromosome during the anaphase stage of cell division.

9. Continued
Chromatin: It is a complex of DNA and proteins that forms chromosomes within the nucleus of
eukaryotic cells. The primary function is to package long DNA molecules into more compact, denser
structures. This prevents the strands from becoming tangled and also plays important roles in
reinforcing the DNA during cell division, preventing DNA damage, and regulating gene
expression and DNA replication.
Telomere: Telomere is the terminal region of each side of the chromosome.

11. On the basis of the location of the centromere, chromosomes are classified into four types:
Metacentric Chromosomes: Metacentric chromosomes have the centromere present exactly in the center. Both the sections of
metacentric chromosomes are therefore of equal length. Example: Human chromosome 1 and 3 are metacentric.
Sub-metacentric Chromosomes: In Submetacentric chromosomes, the centromere is not present exactly at the center. The
centromere is slightly offset from the center. Both the sections are therefore not of equal length or are asymmetrical. Example:
Human chromosomes 4 to 12 are submetacentric.
Acrocentric Chromosomes: Acrocentric chromosomes have a centromere which is highly offset from the center. Therefore,
one of the strands is very long and one very short. Example: Human chromosomes 13,15, 21, and 22 are acrocentric.
Telocentric Chromosomes: In telocentric chromosomes, the centromere is present at the very end of the chromosome.
Telocentric chromosomes are present in species such as mice. Humans do not possess telocentric chromosomes.
Types of Chromosomes

12. Among many organisms that have separate sexes, there are two basic types of chromosomes:
❖ Autosomes (body Chromosomes)
❖ Allosomes (sex chromosomes)
Autosomes: Body chromosomes or non sex chromosomes (humans have 44 chromosomes in 22
pairs). Autosomes control the inheritance of all the characteristics except the sex-linked ones, which are
controlled by the sex chromosomes
Allosomes (Sex Chromosomes): XX or XY (23rd pair for humans) determines the sex of the offspring..
Humans have 22 pairs of autosomes and one pair of sex chromosomes.
Types of Chromosomes

13. Number of Chromosomes
Precise number of chromosomes is typical for a given species.
In any given asexually reproducing species, the chromosomes
number is always the same in all the cells of an organism.
While in sexually reproducing organisms, the number of
chromosomes in the body (somatic) cells typically is diploid (2n; a
pair of each chromosome).
While in the sex cells, or gametes the number of chromosomes is
haploid (1n). The haploid number is produced during meiosis.

14. No of Chromosomes in Different Organisms
The number of chromosomes does not correlate with the apparent complexity of an animal or a plant, in humans for
example, the diploid number is 2n = 46 (that is, 23 pairs), compared with 2n = 78, or 39 pairs, in the dog.
Organisms No. Of Chromosomes Organisms No. Of Chromosomes
Humans 46 Chimpanzee 48
Dog 78 Horse 64
Chicken 78 Goldfish 94
Fruit Fly 8 Mosquito 6
Horsetail 216 Round Worm 2

15. Chromosome Size
Unlike other cell organelles, the size of chromosomes vary significantly depending upon stages of
cell division.
Interphase: Chromosomes are longest & thinnest.
Prophase: Progressive decrease in their length accompanied with increase in thickness.
Anaphase: Chromosomes are smallest.
Metaphase: Chromosomes are easily observed and studied during metaphase when they are thick
small and well spread in the cell. The size of chromosomes in mitotic phase generally varies between
0.5 µ to 32 µ in length and between 0.2 µ to 3 µ in diameter.

16. Gene
Word Gene is derived from Greek word genos meaning generation/ birth/ gender. “It is the segment of DNA that has the
information (the code) for specific proteins. These proteins have different functions throughout the body and allow humans to
live, grow, and reproduce. In other words gene is a unit of hereditary information that occupies a fixed position (locus) on a
chromosome”. A single molecule of DNA has thousands of genes.
Genes are a set of instructions passed down from parents to offspring. They contain the information that determines a person’s
specific physical and biological traits, like hair color, eye color, and blood type.
➢In eukaryotes (such as animals, plants, and fungi), genes are contained within the cell nucleus. The mitochondria (in animals)
and the chloroplast (in plants) also contain small subsets of genes distinct from the genes found in the nucleus.
➢ In prokaryotes (organisms lacking a distinct nucleus, such as bacteria), genes are contained in a single chromosome that is
free-floating in the cell cytoplasm.

17. Genome
The genome is the total genetic material of an organism and includes both the genes and non-coding sequences.
Number of Genes in Different Organisms
The number of genes in an organism’s genome (the entire set of chromosomes) varies significantly between species.
For example, the human genome contains an estimated 20,000 to 25,000 genes, the genome of the bacterium
Escherichia coli houses precisely 5,416 genes. The smallest genomes occur in viruses, and viroids* (which act as a
single non-coding RNA gene). Conversely, plants can have extremely large genomes, with rice containing >46,000
protein-coding genes. The total number of protein-coding genes (the Earth's proteome) is estimated to be 5 million.
*infectious agents that consist only of naked RNA without any protective layer such as a protein coat.

18. Chemical Structure of Genes
Genes are composed of deoxyribonucleic acid (DNA), except in some viruses, which have genes consisting of
RNA. The sequence of bases along a strand of DNA determines the genetic code.
A DNA molecule is composed of two chains of nucleotides that wind about each other to resemble a twisted ladder.
The sides of the ladder are made up of sugars and phosphates, and the rungs are formed by bonded pairs of
nitrogenous bases. These bases are adenine (A), guanine (G), cytosine (C), and thymine (T). An A on one chain
bonds to a T on the other (thus forming an A–T ladder rung); similarly, a C on one chain bonds to a G on the other.
.

19. DNA Replication
DNA replicates by separating into two single strands, each of which serves as a template for a new
strand. If the bonds between the bases are broken, the two chains unwind, and free nucleotides within
the cell attach themselves to the exposed bases of the now-separated chains.
The free nucleotides line up along each chain according to the base-pairing rule—A bonds to T, C bonds
to G. This process results in the creation of two identical DNA molecules each containing one of the
original strands and one new strand and is the method by which hereditary information is passed from
one generation of cells to the next. The new strands are copied by the same principle of hydrogen-bond
pairing between bases that exists in the double helix. This replication is the key to the stable inheritance
of genetic traits.

20. Gene Expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene
product that enables it to produce end products, protein or non-coding RNA, and ultimately affect a phenotype
(physical traits and structure of an organism e.g. eye color, height , body structure and its behavioral characteristics
etc.) as the final effect.
In all organisms, two steps are required to read the information encoded in a gene's DNA and produce the protein it
specifies. First, the gene's DNA is transcribed to messenger RNA (mRNA). Second, that mRNA is translated to
protein. RNA-coding genes must still go through the first step, but are not translated into protein. The process of
producing a biologically functional molecule of either RNA or protein is called gene expression, and the
resulting molecule is called a gene product.

21. Transcription and Translation
When the product of a particular gene is needed, the portion of the DNA molecule that contains that gene will split. Through the
process of transcription, a strand of RNA with bases complementary to those of the gene is created from the free nucleotides in the
cell (RNA has the base uracil [U] instead of thymine, so A and U form base pairs during RNA synthesis).
Transcription is performed by an enzyme called an RNA polymerase, which reads the template strand in the 3' to 5' direction and
synthesizes the RNA from 5' to 3'. This single chain of RNA, called messenger RNA (mRNA), then passes to the organelles called
ribosomes, where the process of translation, or protein synthesis, takes place. In prokaryotes, transcription occurs in the cytoplasm
while in In eukaryotes, transcription occurs in the nucleus, where the cell's DNA is stored.
During translation, a second type of RNA, transfer RNA (tRNA), matches up the nucleotides on mRNA with specific amino acids.
Each set of three nucleotides codes for one amino acid. The series of amino acids built according to the sequence of nucleotides
forms a polypeptide chain; all proteins are made from one or more linked polypeptide chains.

22. Gene Transcription and Translation