This document discusses the history and science of genetics. It explains that early humans recognized inheritance of traits but it was Gregor Mendel who solved the mystery of heredity by discovering genes are passed from parents to offspring. The document also details the composition of DNA and RNA, including the nucleotides, sugars, phosphates, and nitrogenous bases that make up these molecules. It describes how DNA is packaged into chromosomes in the nucleus and outlines the double helix structure of DNA discovered by Watson and Crick.

1. • The historical background of the science of genetics
• Even in prehistoric times, humans recognized that certain
physical characteristics of plants, livestock, and people were
passed on from one generation to the next.
• This rudimentary knowledge of genetics was important for
improving the cultivation of corn and wheat, as well as the
domestication of cattle, horses, and dogs.
• Gregor Mendel solved the mystery of heredity and laid
foundation for modern genetics.
• Passing of parental characteristics to their offspring to
heritable ‘factors’. These ‘factors’ are now known as genes.

2. • Genetics, the science of heredity, is at its core the study of
biological information.
• All living organisms—from single-celled bacteria and protozoa
to multicellular plants and animals—must store, replicate,
transmit to the next generation, and use vast quantities of
information to develop, reproduce, and survive in their
environments.
• Genetics is concerned primarily with understanding biological
properties that are transmitted from parent to offspring.
• The subject matter of genetics includes;
heredity,
the molecular nature of the genetic material,
the ways in which genes control life functions, and
the distribution and behavior of genes in populations.

3. What is gene?
• Gene is the basic physical and functional unit of heredity.
• Genes, which are made up of DNA, act as instructions to make
molecules called proteins.
• Alleles are forms of the same gene with small differences in their
sequence of DNA bases.
What is a chromosome?
• In the nucleus of each cell, the DNA molecule is packaged into
thread-like structures called chromosomes.
• Each chromosome is made up of DNA tightly coiled many times
around proteins called histones that support its structure.

4. • The unique structure of chromosomes keeps DNA tightly wrapped
around spool-like histones proteins.
• Without such packaging, DNA molecules would be too long to fit inside
cells.
• For example, if all of the DNA molecules in a single human cell were
unwound from their histones and placed end-to-end, they would stretch
nearly 2m.

5. • The Composition and Structure of DNA and RNA
• DNA and RNA are polymers—large molecules that consist of many
similar smaller molecules, called monomers, linked together.
• The monomers that make up DNA and RNA are nucleotides.
• Each nucleotide consists of:
. Pentose (five-carbon) sugar, . Phosphate group and,
. Nitrogenous base (usually just called a base).
• In DNA, the pentose sugar is deoxyribose, and in RNA it is ribose.
• The two sugars differ by the chemical groups attached to the 2’
carbon: a hydrogen atom (H) in deoxyribose and a hydroxyl group
(OH) in ribose.
• The carbon atoms in the pentose sugar are numbered 1’ to 5’ to
distinguish them from the numbered carbon and nitrogen atoms in
the rings of the bases.)

6. • There are two classes of nitrogenous bases:
• Purines; which are double-ringed structures, —adenine (A) and guanine (G)
• Pyrimidines; which are single-ringed structures. — thymine (T), cytosine (C),
and uracil (U) in DNA and RNA
• Both DNA and RNA contain adenine, guanine, and cytosine; however,
thymine is found only in DNA, and uracil is found only in RNA.

7. • To form polynucleotides of either DNA or RNA, nucleotides are linked together by a
covalent bond between the phosphate group of one nucleotide and the 3’ carbon of
the sugar of another nucleotide.
• These 5’ to 3’ phosphate linkages are called phosphodiester bonds.
• The phosphodiester bonds are relatively strong, so the repeated sugar–phosphate–
sugar–phosphate backbone of DNA and RNA is a stable structure
• Polynucleotide chains have polarity, meaning that the two ends are different: there is
a 5’ carbon (with a phosphate group on it) at one end, and a 3’ carbon (with a
hydroxyl group on it) at the other end.
• The ends of a polynucleotide are routinely referred to as the 5’ end and the 3’ end.

8. Discovering the structure of DNA
• DNA = Deoxyribose nucleic acid
• Made out of sugars (deoxyribose), phosphates
and nitrogen bases

9. Discovering the structure of DNA
“Chargoff’s rule”
A = T & C = G