Molecular Biology

Contents: Review Content Review Questions

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Review Content

 

DNA provides the set of coded instructions required by every organism for specifying its traits. The DNA molecule also provides for a reliable way for parents to pass their genetic code from one generation to the next. Many scientists have contributed to the knowledge that chromosomes carry hereditary information. It also became known that genes are arranged along the chromosomes. In 1869, Friedrich Miescher who was a biochemist, isolated fish sperm. The material was called nuclein which was made of carbon, hydrogen, oxygen, and nitrogen. It was rich in phosphorus and was also shown to be acidic and the name was changed to nucleic acid. Later researchers found two kinds of nucleic acid, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA can only nucleus and RNA can be found mainly in the cytoplasm. Scientists later discovered that chromosomes contained DNA and protein. 3 scientists, Avery, Colin, MacLeod, and McCarty transferred materials in Griffith�s experiment to prove that DNA produced new inherited traits in genetic substance. The conclusive evidence that proved that DNA contained genetic information was obtained by Alfred Hershey and Martha Chase.

1. Composition of DNA � In the 1920s it was discovered that DNA was made of a polymer (a chain of repeating subunits) of nucleotides which are composed of a phosphate group, 5-carbon sugars, and four nitrogenous bases.

2. Structure of DNA � By 1953 Watson and Crick developed their models of the structure of DNA. The DNA was shaped as a helix which is the shape of a coiled spring. In the double helix model adenine (A) joins with thymine (T), and guanine (G) connects with cytosine (C).

Ex: A� ���� T

����� �G� ���� C

������ G������ C

����� �T������� A

������ T������� A

������ A������� T

����� �C������� G

3. DNA vs. RNA

	DNA	RNA
Structure:	Double Helix	Single stranded
Nitrogenous Bases:	Adenine Thymine����� > ATGC Guanine Cytosine	Adenine Uracil������������ > AUGC Guanine Cytosine
5-Carbon Sugars:	Deoxyribose	Ribose
Function:	Reproduction, controls cell�s activities	Protein synthesis, (mRNA, tRNA (rRNA), messenger, transporter, translator

4. Replication of DNA � The double helix model also explains how an exact copy of each chromosome is made during cell division. The base pairs that form the DNA are held together by weak hydrogen bonds. Before the replication begins the bonds break and the two strands of the DNA molecule come apart. The bases are exposed along each strand. The bases if the free nucleotides in the nucleus of the cell can then fasten onto complementary bases on each exposed strand. When the nucleotides join together they make a complete complementary strand like the one that originally divided. This way, two double stranded molecules of DNA exactly like the original molecule are made. Each double-stranded molecule contains one old strand and one new strand of DNA. In all organisms, the coded instructions for specifying the characteristics of the organism are carried in DNA. The genetic code is contained in the four nitrogenous bases of DNA; adenine, guanine, cytosine, and thymine. These bases are often indicated only by using their beginning letters A, G, C, and T. Each individual DNA strand serves as a template or model for the formation of other DNA molecules by replication
5. RNA � DNA codes for the formation of RNA in the nucleus of the cell. RNA is short for another kind of nucleic acid called ribonucleic acid. RNA is very similar in structure to DNA except for three small differences. These differences include the fact that RNA is a single stranded molecule, lacks the base thymine (T) as it is replaced by the base uracil (U), and its five carbon sugar ribose has one more oxygen atom than the sugar in DNA. Three different types of RNA exist, mRNA or messenger RNA, tRNA or transfer RNA, and rRNA or ribosomal RNA.
1. mRNA � the complementary nucleotides attach to the exposed DNA strands. When the assembled RNA sequence reaches the DNA �stop� code, it leaves the DNA strand. The RNA strand is now a separate molecule that carries the complete message for a single polypeptide. Now each A in the DNA is represented by a U in the RNA, each T by an A, each G by a C, and each C by a G. A strand of RNA that copies a genetic message from DNA in this way is called messenger RNA. The copying of a genetic message into a molecule of mRNA is called transcription. Each group of three bases in the mRNA that specifies an amino acid is called a codon.
2. Transfer RNA � another kind of RNA. At one end of the tRNA there is a short tail where an amino acid can become attached. Each tRNA molecule will pick up one kind of amino acid. At the other end of the tRNA molecule there is a loop of exposed nucleotides. In this loop there is a sequence of 3 bases called an anticodon that are complements of an mRNA codon. The tRNA is a device for bringing a certain amino acid to a certain place specified by mRNA.

 

3. Ribosomal RNA � is formed in the nucleoli of the cell. A ribosome consists of protein and rRNA. The ribosomal protein made in the cytoplasm then travels into the nucleus. In the nucleoli, the protein and the rRNA join together to form complete ribosomes. The ribosome is where a polypeptide is assembled during protein synthesis.
5. Cells store and use coded information. The genetic information stored in DNA is used to direct the synthesis of the thousands of proteins that each cell requires. The chemical and structural properties of DNA are the basis for how the genetic information that underlies heredity. DNA is encoded in the sequence of nitrogenous bases which directs the formation of proteins in the cell. First, the DNA code is copied on to the mRNA (messenger RNA) codon. A codon is a sequence of three nitrogenous bases. This process is called transcription. This mRNA codon is then carried from the nucleus out to the ribosome. Messenger RNA attaches to another kind of RNA called tRNA (transfer RNA). Transfer RNA attaches to amino acids and carries them to the ribosome. This assembly of amino acids due to the code provided to RNA by the original DNA molecule is what produces proteins for the cell. Remember a protein is a long molecule formed from amino acid subunits.
6. The code of DNA directs the synthesis of RNA, which in turn directs the making of proteins on the ribosomes. This is sometimes referred to as being the central dogma or idea of biology. There are 64 possible combinations of triplets (sequences of 3 nitrogenous bases) which code for the 20 different possible amino acids. As the DNA of different organisms and most individuals (except for identical twins) is different, this means the proteins produced by different humans and other organisms exhibit differences. It is these differences which make us unique individuals. The work of the cell is carried out by the many different types of molecules it assembles, mostly proteins. Protein molecules are long, usually folded chains made from 20 different kinds of amino acids in a specific sequence. This sequence influences the shape of the protein. The shape of the protein, in turn, determines its function. Offspring resemble their parents because they inherit similar genes (DNA sequences) that code for the production of proteins that form similar structures and perform similar functions.
7. Cell Regulation � Cell functions are regulated. Regulation occurs both through changes in the activity of proteins and through the selective expression of individual genes, as humans and other organisms have genes which direct the expression of other genes. This regulation allows cells to respond to their environment and to control and coordinate cell growth and division.

8. Assembly of a Polypeptide � The synthesis of 3 types of RNA as well as the assembly of ribosomes, occurs in the cell nucleus. The RNA and complete ribosomes migrate separately through the nuclear pores to the cytoplasm. Within the cytoplasm, there is contained a supply of all the amino acids that are needed to make the cell�s proteins. Within the cytoplasm polypeptides are assembled according to the instruction carried by mRNA. In the cytoplasm, amino acid molecules become attached to their specific varieties of tRNA. Ribosomes become attached to different places along each strand of mRNA. Where a ribosome is attached to mRNA, a molecule of tRNA with the right anticodon temporarily joins with the corresponding codon on the mRNA. The amino acid then joins with the last amino acid in the chain and separates from tRNA. A new tRNA takes its place on the mRNA strands and its amino acid joins the polypeptide chain. Amino acids are added onto the polypeptide chain until the ribosome reaches a �stop� codon. The polypeptide is then let go and it forms into a complete protein molecule.

9. Translation � the process by which the information coded in RNA is used for the assembly of a particular amino acid sequence that is known as translation. All the cell�s proteins are synthesized in the cytoplasm, while the chromosomes carrying the hereditary instruction for this synthesis remain in the nucleus.

 

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Review Questions

 

1. For which organic compounds must information be encoded in DNA for green plants to synthesize the other three compounds?
1. sugars
2. starches
3. fats
4. proteins

 

2. Strand II of this DNA molecule will include

 

 

a.       AGC

b.      TCG

c.       TAC

d.      GAT

 

3. The letters in the diagram below represent genes on a particular chromosome.

Gene B contains the code for an enzyme that cannot be synthesized unless gene A is also active. Which statement explains why this can occur?

a.       A hereditary trait can be determined by more than one gene

b.      Genes are made up of double-stranded segments of DNA

c.       All the genes on a chromosome act to produce a single trait

d.      The first gene on each chromosome controls all the other genes on the chromosome

 

4. When DNA separates into two strands, the DNA would most likely be directly involved in
1. replication
2. fertilization
3. differentiation
4. evolution

 

5. The instructions for the traits of an organism are coded in the arrangement of
1. glucose units in carbohydrate molecules
2. bases in DNA in the nucleus
3. fat molecules in the cell membrane
4. energy-rich bonds in starch molecules

 

6. Which statement is true regarding an alteration or change in DNA?
1. it is always known as a mutation
2. it is always advantageous to an individual
3. it is always passed in to offspring
4. it is always detected by the process of chromatography

 

7. What determines the kind of genes an organism possesses?
1. type of amino acids in the cells of the organism
2. sequence of the subunits A, T, C, and G in the DNA of the organism
3. size of simple sugar molecules in the organs of the organism
4. shape of the protein molecules in the organelles of the organism

 

8. Which statement best describe the relationship between the terms chromosomes, genes, and nuclei?
1. Chromosomes are found on genes. Genes are found in nuclei.
2. Chromosomes are found in nuclei. Nuclei are found in genes.
3. Genes are found on chromosomes. Chromosomes are found in nuclei.
4. Genes are found in nuclei. Nuclei are found in chromosomes.

 

�      Answers:

1.      d Proteins. Proteins are the organic compounds for which information must be encoded in DNA for green plants to synthesize to other compounds. DNA codes provide templates for the synthesis of protein chains consisting of specific sequences of amino acids. Some of these proteins, known as enzymes, are specialized to catalyze cellular reactions such as photosynthesis and dehydration synthesis.

2.      d GAT.� These letters represent the nitrogenous bases of DNA and match up with those in strand I.

3.      a A hereditary trait can be determined by more than one gene. This is the statement that best explains why gene B cannot synthesize its enzymes unless gene A is also active. In this hypothetical model, a product of gene A is most likely necessary to activate gene B. Without with product, gene B cannot carry out the synthesis activities necessary to allow gene A to produce its enzymes.

4.      a Replication. When DNA separates into two strands, the DNA is replicated. During this process the double-stranded DNA molecule �unzips� as the nitrogenous base pairs separate. Free nucleotides then attach to the exposed bases on the separated strands, forming two new DNA molecules that are the exact duplicate of the original DNA molecule.

5.      b bases in DNA in the nucleus. The arrangement of the bases in the DNA molecule contains the instructions for the traits of an organism. Collectively, these instructions are known as the �genetic code�. The characteristics that make an organism both similar to and unique from other members of its species are held in the specific sequences of the bases A, T, G, and C.

6.      a it is always known as a mutation. Such alterations may involve addition, deletion, or rearrangement of genetic information contained in the DNA molecule.

7.      b sequence of the subunits A, T, C, and G in the DNA of the organism. This sequence codes for the production of specific proteins in the cell, each of which has a unique function to perform. As these cell functions are carried out, the results are the specific genetic physical or chemical features that make up the characteristics of an organism.

8.      c Genes are found on chromosomes. Chromosomes are found in nuclei.