Chapter 4-Nucleic Acids and the Origin of Life(1)

4 Nucleic Acids and the Origin of Life

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

Nucleic acids are macromolecules, polymers specialized for the storage and transmission of genetic information.

The two types of nucleic acids are:

DNA = deoxyribonucleic acid (the structure of DNA was first described by James Watson, Francis Crick and Maurice Wilkins).

RNA = ribonucleic acid

The Deoxyribonucleic Acid or DNA

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

The monomeric units of nucleic acids are called nucleotides.

Nucleotides consist of :

1. a pentose sugar (ribose or deoxyribose)

2. a phosphate group

3. a nitrogen-containing base (purines and pyrimidines).

Question 1.A nucleotide contains a pentose sugar, a phosphate group, and

A. a lipid.

B. an acid.

C. a nitrogen-containing base.

D. an amino acid.

E. a glycerol

Name: __________________________ Date: _____________

Q. 2. The difference between nucleosides and nucleotides is

A. only nucleotides have nitrogenous bases.

B.a phosphate group is found in nucleotides but not in nucleosides.

C.nucleotides have the pentose sugar ribose while nucleosides have the pentose sugar deoxyribose.

D.in nucleosides the monomers are joined by phosphodiester bonds; in nucleotides the monomers are joined by hydrogen bonds.

E.nucleosides are the monomers of DNA; nucleotides are the monomers of RNA.

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

RNA has ribose sugar

DNA has deoxyribose sugar

Figure 4.1 Nucleotides Have Three Components

• Pyrimidines are one-ringed structures• Cytosine and thymine are components of DNA. Uracil is a base in RNA which replaces thymine.

•Purines have two rings.• Adenine and Guanine.

Q. 3 The bases of nucleic acids are purines or pyrimidines. Purines and pyrimidines are distinguished by the fact that

A.purines include the bases of cytosine and thymine; pyrimidines include the bases of adenine and guanine.

B.pyrimidines are found in RNA; purines are found in DNA.

C.purines consist of hydrogen, carbon, oxygen, and nitrogen, whereas pyrimidines have phosphorus, hydrogen, carbon, oxygen, and nitrogen.

D.purines only have single bonds in their structure, whereas pyrimidines have both single and double bonds in their structure.

E.purines are double-ring structures, whereas pyrimidines are single-ring structures

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

The “backbone” of DNA and RNA is a chain of sugars and phosphate groups, bonded by phosphodiester linkages.

The phosphate groups link carbon 3′ in one sugar to carbon 5′ in another sugar.

The two strands of DNA run in opposite directions (antiparallel).

Figure 4.2 Distinguishing Characteristics of DNA and RNA Polymers (Part 1)

Figure 4.2 Distinguishing Characteristics of DNA and RNA Polymers (Part 2)

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

DNA bases: adenine (A), cytosine (C), guanine (G), and thymine (T)

Complementary base pairing:

A–T

C–G

Purines pair with pyrimidines by hydrogen bonding.

Q. 5.According to the principle of complementary base pairing, purines always pair with

A. deoxyribose sugars.

B. uracil.

C. pyrimidines.

D. adenine.

E. guanine.

Q. 6.What is the nucleotide sequence of the complementary strand of the DNA molecule: A A T G C G A?

A. T T A C G C T

B. A A T G C G A

C. G G C A T A G

D. C C G T T A T

E. A G C G T A A

DNA Video

http://www.youtube.com/watch?v=qy8dk5iS1f0

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

Instead of thymine, RNA uses the base uracil (U).

RNA is single-stranded, but complementary base pairing occurs in the structure of some types of RNA.

Q. 7. All of the following bases are found in DNA except

A. thymine.

B. adenine.

C. uracil.

D. guanine.

E. cytosine

Figure 4.3 Hydrogen Bonding in RNA

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

The two strands of a DNA molecule form a double helix.

All DNA molecules have the same structure; diversity lies in the sequence of base pairs.

DNA is an informational molecule: information is encoded in the sequences of bases.

The central dogma of molecular biology

The Central Dogma of Molecular Biology is: DNA is transcribed to RNA which is translated to protein. Protein is never back-translated to RNA or DNA; and except for retroviruses, DNA is never created from RNA. Furthermore, DNA is never directly translated to protein.

Information flows from DNA to RNA to protein

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

The two functions of DNA comprise the central dogma of molecular biology:

• DNA can reproduce itself (replication).

• DNA can copy its information into RNA (transcription). RNA can specify a sequence of amino acids in a polypeptide (translation).

Figure 4.5 DNA Stores Information

The central dogma

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

DNA replication and transcription depend on base pairing.

DNA replication involves the entire molecule (i.e. replication of the genome), but only relatively small sections of the DNA are transcribed into RNA (transcription of genes).

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

The complete set of DNA in a living organism is called its genome.

Not all the information is needed at all times; sequences of DNA that encode specific proteins are called genes.

Sequences of DNA encoding specific proteins known as genes

Figure 4.6 DNA Replication and Transcription

Q. 8. The central dogma of molecular biology states that

A. the genetic code is ambiguous.

B.the information flow between DNA, RNA, and a protein is reversible.

C.the information flow in a cell is from DNA to RNA to protein.

D.the information flow in a cell is from protein to RNA to DNA.

E.the information flow in a cell is from DNA to a protein to RNA.

4.1 What Are the Chemical Structures and Functions of Nucleic Acids?

DNA carries hereditary information between generations.

Determining the sequence of bases helps reveal evolutionary relationships.

The closest living relative of humans is the chimpanzee.

4.2 How and Where Did the Small Molecules of Life Originate?

The Origins of Life

Many cultures believed in the spontaneous generation of life: i.e life arose from non-living matter.

The first experiment to disprove spontaneous generation was done in 1668.

Experiments by Louis Pasteur showed that microorganisms can arise only from other microorganisms.

Figure 4.7 Disproving the Spontaneous Generation of Life (Part 1)

Figure 4.7 Disproving the Spontaneous Generation of Life (Part 2)

In 1668, Francesco Redi did some experiments to disprove spontaneous generation. He began by putting pieces of meat into identical jars. Some jars were left open to the air, and some were sealed. He then did the same experiment with one variation: instead of sealing the jars completely, he covered them with gauze (which kept out flies while allowing the meat to be exposed to the air). In both experiments, he checked if fly maggots appeared in the meat or not. What hypothesis was being tested?

A.Spontaneous generation is more likely to occur during the hot days of summer.

B. Maggots do not arise spontaneously but from eggs laid by adult flies.C. The type of meat affects whether spontaneous generation occurs.D. Spontaneous generation can occur only if air is present.E. Flies will develop from rotting meat if the meat is exposed to air.

4.2 How and Where Did the Small Molecules of Life Originate?

But their experiments did not prove that spontaneous generation never occurred.

Eons ago,conditions on Earth and in the atmosphere were vastly different.

About 4 billion years ago, chemical conditions, including the presence of water, became just right for life.

4.2 How and Where Did the Small Molecules of Life Originate?

Two of the theories on the origin of life:

• Life came from outside of Earth.

• Life arose on Earth through chemical evolution.

4.2 How and Where Did the Small Molecules of Life Originate?

In 1969, fragments of a meteorite were found to contain molecules unique to life, including purines, pyrimidines, sugars, and ten amino acids.

Evidence from other meteorites suggest that living organisms could possibly have reached Earth within a meteorite.

Figure 4.8 The Murchison Meteorite

4.2 How and Where Did the Small Molecules of Life Originate?

Chemical evolution: conditions on primitive Earth led to formation of simple molecules (prebiotic synthesis); these molecules led to formation of life forms.

Scientists have experimented with reconstructing those primitive conditions.

4.2 How and Where Did the Small Molecules of Life Originate?

Miller and Urey (1950s) set up an experiment with gases thought to have been present in Earth’s early atmosphere.

An electric spark simulated lightning as a source of energy to drive chemical reactions.

After several days, amino acids, purines, and pyrimidines were formed.

Figure 4.9 Miller & Urey Synthesized Prebiotic Molecules in an Experimental Atmosphere (Part 1)

Figure 4.9 Miller & Urey Synthesized Prebiotic Molecules in an Experimental Atmosphere (Part 2)

Miller’s experiment gave rise to many small molecules

•All five bases present in the DNA and RNA were formedi.e. A, T, G, C and U

•17 of the 20 amino acids were also produced

•3 to 6 carbon sugars were produced.

•However, a 5 carbon ribose or deoxyribose were not produced.

• The original experiment was redesigned: using other gases in addition to the ones used.

• This time around Ribose was formed