Download The Molecular Basis of Heredity

The Molecular Basis of Heredity
Introduction:
The inherited set of instructions that are passed from parent to offspring exist in
the form of a code contained in DNA molecules. The code carried by the DNA
molecules form the molecular basis of heredity and must be accurately replicated or
duplicated before being passed on to the next generation. Once the encoded information
is passed on, it is used by a cell to make proteins. The proteins that are made become
cell parts and carry out most of the functions of the cell.
DNA Molecules:
It wasn’t until modern times that scientists began to understand the chemical
makeup of genes and chromosomes. In the 1940s and 1950s experiments showed genes,
the basic units of heredity, are made up of the chemical compound DNA, or
deoxyribonucleic acid. DNA is a large, complex molecule found in high concentrations
in the nucleus of the cell. It is responsible for passing genetic information from
generation to generation. DNA also controls the manufacture of enzymes that control
cellular activity.
The DNA molecule is made up of a long chain of repeating nucleotide subunits.
Nucleotides contain nitrogenous bases, a sugar, and a phosphate group. The four
different nitrogenous bases in DNA nucleotides are adenine, guanine, cytosine, and
thymine. The sugar in DNA is deoxyribose, a 5-carbon sugar. The phosphate group
contains phosphorus and oxygen. A model of a typical DNA nucleotide is shown.
1. What does DNA stand for and what is its job in the cell?
2. What are the building blocks of DNA?
3. What do nucleotides consist of?
DNA Double Helix:
The structure of DNA was discovered in the 1950s by James Watson and Francis
Crick. According to Watson and Crick, DNA molecules are shaped like a twisted ladder.
The twisted ladder structure is called a double helix. The DNA double helix has two
strands or sides. The strands are connected at the rungs. The sides of the ladder consist
of alternating sugar and phosphate molecules. The rungs are pairs of nitrogenous bases.
The nitrogen bases are attached to each other by weak hydrogen bonds.
The four nitrogenous molecular bases in DNA bond (join) together in a certain
way known as base pairing. According to the DNA base pairing rule, adenine (A) and
thymine(T) bond together and guanine (G) and cytosine (C) bond together. No other
combinations are possible. Whenever there is a C on one strand, it is bonded to a G on
the other strand. Whenever there is an A on one strand, there is a T on the other strand.
The base-pairing rule for DNA is: A-T and C-G as shown in the figure.
1. Who discovered the structure of DNA?
2. What is the structure of DNA like?
3. What is the base pairing rule for DNA?
DNA Replication:
During reproduction, DNA makes exact copies of itself called replication. These
exact copies are passes from the parent cell to daughter cells during cell division.
Replication of DNA occurs both in mitosis and meiosis. The process begins with the
untwisting of the DNA helix as shown in the figure. The two strands that make up the
helix then “unzip.” The bonds holding the nitrogenous bases together break, leaving the
molecule in the form of two strands of nucleotides. Each strand is a pattern or template
for the new nucleotide strand. Free nucleotides present in the cytoplasm pair with free
nitrogenous bases of both strands according to the DNA base pairing rule. Because the
nitrogenous bases can pair in only one way, two identical DNA molecules that are
identical to the original molecule are produced.
1. Describe the process of DNA replication step by step.
The Genetic Code:
Genetic information is present in the structure and organization of the DNA
molecule. This hereditary information, known as the genetic code, depends upon the
order of the different nucleotides in the DNA molecule. The genetic code is a message to
the cell to make certain proteins. The kind of protein to be made is coded in the
arrangement of groups of three nucleotide bases called triplet codes. The production of
one type of protein is controlled by a sequence (order) of nucleotide triplets. This
sequence of nucleotides is called a gene.
1. What is a triplet code and why is it important?
RNA:
Control of cell activities involves two kinds of nucleic acids: DNA and RNA
(ribonucleic acid). RNA is involved in the synthesis of proteins by the cell. Offspring
resemble their parents because they inherit similar genes that code for the production of
proteins that form similar structures and perform similar functions. Both DNA and RNA
are composed of nucleotides, but the two types of molecules differ in several ways; for
example, RNA contains the nitrogenous base uracil (U) instead of thymine (T). The
base pairing rule of RNA is A-U and C-G. The table below compares DNA and RNA.
DNA
RNA
Contains the sugar deoxyribose
Contains the sugar ribose
Contains the nitrogen bases adenine,
Contains the nitrogen bases adenine, uracil
thymine, guanine, and cytosine
(instead of thymine), guanine, and cytosine
Double-stranded
Single-stranded
Only one kind
Three kinds: messenger RNA (mRNA),
transfer RNA (tRNA), and ribosomal RNA,
(rRNA)
1. What is the role of RNA in the cell?
2. What do DNA and RNA have in common? What are their differences?
Protein Formation:
Proteins are made at the cell’s ribosomes. They are long, usually folded chains
made of 20 different amino acids in a specific sequence. This sequence influences
the shape of the protein. The shape of the protein, in turn, determines its function.
When proteins are formed, special chemical molecules known as messenger
RNA (mRNA) carry the genetic code information of DNA from the nucleus to the
ribosomes in the cytoplasm. The information in the genetic code of DNA is copied
into molecules of messenger RNA in a process that is similar to DNA replication.
RNA however, is single-stranded. The base pairing rule for RNA is the same as for
DNA, except, adenine pairs with uracil instead of thymine. The newly formed
molecule is now called messenger RNA or mRNA. The mRNA separates from the
DNA and passes out of the nucleus through the cytoplasm to the ribosome. Transfer
RNA (tRNA) molecules located in the cytoplasm carry specific amino acids.
Transfer RNA reads the message for protein formation carried by mRNA and tRNA
then transfers the appropriate amino acids along the mRNA at the ribosome forming
protein chains. Bonds form between the amino acids and then the proteins leave the
ribosome and travel to wherever they are needed in the cell. Protein synthesis is
shown in the figure.
At the ribosomes, the messenger RNA, because of the specific information it
carries, controls the making of certain proteins. Some of these proteins are enzymes;
others include the cell membrane, chromosomes, and organelles. Certain cellular
processes and chemical reactions are enzyme-controlled. Because they control
certain reactions, enzymes control specific traits. It is through this mechanism of
enzyme production that genes control genetic traits.
1. Where are proteins made in the cell?
2. What are the building blocks of proteins?
3. Why is the sequence or order of amino acids in a protein chain so important?
4. What is the job of mRNA in protein synthesis? Of tRNA in protein synthesis?
5. Describe the process of protein formation step by step.