A concise, bullet-pointed summary on Nucleic Acids. Covers strictly what you need to know for all tests and assessments (as dictated by the SAGS guidelines). Includes annotated diagrams and images to illustrate points mentioned, as well as additional research, helpful when understanding the work
• When a cell is dividing, its chromosomes become visible
• Analysis of the chromatin reveals that it is made of two major components:
• Deoxyribonucleic acid (DNA)
• The DNA is the molecule of heredity within whose structure is coded the information
of inheritance and the information for controlling the cell
• Proteins, called histones, that stabilise the DNA molecules
Chromosomes
• Each chromosome contains one very long DNA molecule that, when unravelled, would
measure about 4.8 cm in length
• The total length of DNA in the nucleus of one single human cell has been estimated to
be about 2.2 m
• The DNA is intricately folded and tightly bound around histones
• For most of the cell cycle, DNA molecules are in very long strands called chromatin, but
prior to cell division, the chromatin condenses
• This replicated DNA molecule, in its now condensed form, is now referred to as a
chromosome
• But, remember, there are two copies attached to each other until nuclear division occurs
• Each of the two copies of a chromosome are designated as a chromatid and they are
physically linked together at a point on the chromosome called the centromere
Nucleic Acids
• Nucleic acids are molecules that were originally thought to occur only in the nucleus
Two types of nucleic acids found in cells:
1. Deoxyribonucleic acid (DNA)
2. Ribonucleic acid (RNA)
• Both of these molecules are long, chain like structures built up from repeating similar
building blocks
• Both are thus, polymers
• Polymer: long chain of similar units joined end-to-end with each other
• Each unit of a polymer is called a monomer (building block)
• In a nucleic acid, the monomers are called nucleotides
Nucleotide consists of three units bonded together:
1. A sugar molecule (ribose or deoxyribose)
- Deoxyribonucleic acid had nucleotides in which the sugar is deoxyribose, while
ribonucleic acid contains the sugar ribose
2. A phosphate group
3. A nitrogenous base
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, Chloë van Beukering Life Sciences Notes 2020
Deoxyribonucleic Acid (DNA)
• In 1953, the scientists James Watson and Frances Crick published the first description of
the structure of DNA based on a model they constructed mainly from experimental
work done by other scientists, including Rosalind Franklin
• DNA is a macromolecule / polymer that provides the template for making the cells
proteins, called the genetic code
• Most of the DNA in a eukaryotic cell is found in the nucleus but there is also DNA in
mitochondria (mtDNA) and chloroplasts
DNA has 2 characteristics:
1. It is a store of genetic information
2. It can copy itself exactly, time after time (called DNA replication)
Each nucleotide in DNA can contain any of four nitrogenous bases:
• A = adenine
• G = guanine
• C = cytosine
• T = thymine
• Within each nucleotide, the nitrogenous base is joined to the deoxyribose sugar and
the phosphate group is attached to the deoxyribose sugar
• Humans have 23 pairs or 46 chromosomes
• These chromosomes are further broken
down into smaller pieces of DNA called
genes, which make up coding DNA for
providing the template for making proteins
• Each DNA molecule consists of two
strands of nucleotides, bonded together
in the middle by weak hydrogen bonds
between pairs of nitrogenous bases
• Adenine can only bond with thymine
• Guanine can only bond with cytosine
• This is the complementary base pairing
rule
• The sides of the double stranded, ladder-like structure are formed by alternating sugar-
phosphate units, while the base pairs form the cross bridges like the rungs on a ladder
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