Download DNA, RNA, and Central Dogma

DNA and RNA
I. DNA Structure

Double Helix

In the early 1950s, American James
Watson and Britain Francis Crick
determined that DNA is in the shape of
a double helix.

Rosalind Franklin’s and Maurice Wilkins’s
photographs and crystals led to Watson
and Cricks DNA models.
A. DNA Nucleotides

A nucleotide is two long chains or
strands of repeating subunits.

Is made of three parts:
A five-carbon sugar (deoxyribose)
 A phosphate group (P bonded to 4 O)
 A nitrogenous base (made up of N and C)

B. The Bonds of DNA

Covalent Bonds hold the sugar of one
nucleotide to the phosphate group of
the next nucleotide to form the chains.

Hydrogen bonds join the bases on one
strand of DNA to the bases on the
other strand. Usually 2 or 3 bonds.
C. The Nitrogenous Bases

Purines
(double ring)
 Adenine (A)
 Guanine (G)

Pyrimidines
(single ring)
 Cytosine (C)
 Thymine (T)
D. Complementary Bases

Base-pairing rules
Each pairing contains one purine and one
pyrimidine
 Guanine with Cytosine (G-C)
 Adenine with Thymine (A-T)


The order of nitrogenous bases on a
chain of DNA is its base sequence.
II. DNA Replication

The process by which DNA is copied in a cell
before a cell divides by mitosis, meiosis, or
binary fission.

Mutations occur at a very low frequency
(about one in every billion paired
nucleotides). Some mutations cause disease
such as cancer.

Figure 10-10 in Modern Biology © 2006

Notice the direction of the synthesis
Steps of DNA Replication
(Semi-Conservative Replication)
1.
Helicase separates the DNA strands
resulting in a replication fork. DNA is
unzipped.
2.
Complementary nucleotides are added to
each of the original DNA strands. Gaps are
joined together by DNA ligase.
3.
Two DNA molecules, each made up of one
new strand and one old strand, identical to
the original DNA molecule result.
III. Protein Synthesis

Ribonucleic acid, RNA, plays a role in
protein synthesis.

Central Concept
DNA
RNA
protein
A. RNA Structure & Function

Differences in RNA than in DNA
Sugar ribose instead of deoxyribose
 Nitrogenous base uracil instead thymine
 Single stranded instead of double stranded
 Shorter than DNA

Types of RNA

Messenger RNA (mRNA) –
carries instructions from a
gene to make a protein

Ribosomal RNA (rRNA) –
part of the structure of
ribosomes

Transfer RNA (tRNA) –
transfer amino acids to the
ribosomes to make a
protein
B. Transcription

The process by which the genetic instructions
in a specific gene are transcribed into an RNA
molecule.

Takes place in the nucleus of eukaryotic cells
and in the DNA-containing region in the
cytoplasm of prokaryotic cells.

Figure 12-14 of Prentice Hall Biology
RNA Editing

After the RNA is produced, it must be
edited before it can be used.

Introns are removed and exons are
spliced together before the RNA leaves
the nucleus.
C. The Genetic Code

The term for the rules that relate how a
sequence of nitrogenous bases in nucleotides
corresponds to a particular amino acid.

Three adjacent nucleotides (“letters”) in
mRNA specify an amino acid (“word”)

Three adjacent nucleotides are called a
codon and encodes for an amino acid or
signifies a start or stop signal.

No codon encodes more than one amino acid.
The are 64 possible codons and amino acids are
can be specified by more than one codon.

A start codon is a specific sequence of
nucleotides in mRNA that indicates where
translation should begin. AUG.

Three stop codons are specific sequences of
nucleotides in mRNA that indicates where
translation should end.

Figure 12-17
D. Translation

Decoding of the genetic instructions to form a
polypeptide

Takes place on the surface of the ribosome

Protein structure




Made of one or more polypeptides
Polypeptides are chains of amino acids linked by peptide
bonds.
Only 20 different amino acids
The amino acids sequence determines how the polypeptides
will twist and fold into the protein. The shape of the
protein is critical to its function.
Translation

Figure 12-18

Beginning at the start codon, tRNA
carrying an amino acid pairs its
anticodons pair up with the
complementary codon on the mRNA.
This continues as an assembly line linking
the amino acids and breaking bonds
between the tRNA and the amino acids.
The linked amino acids form an
polypeptide until a stop codon is reached.


E. The Human Genome

A genome is the complete genetic content.

Biologists have now decoded the order of the
3.2 billion base pairs in the 23 human
chromosomes.

Bioinformatics compares different DNA
sequences to try to determine what
information the DNA codon encodes.
DNA
Nitrogen Base – the
Gene –
Short Segment
of double
helix
“rung on the ladder”
Deoxyribose – “sugar
side of the ladder”
Nucleotide – composed
of 3 parts floating free in the
cytoplasm
DNA – the double helix
End of Chapter 10 Notes