DNA Structure & Function, DNAReplication,

DNA: STRUCTURE

DNA, or deoxyribonucleic acid, is the hereditary material in humans and almost all other organisms. Nearly every cell in a person’s body has the same DNA. Most DNA is located in the cell nucleus (where it is called nuclear DNA), but a small amount of DNA can also be found in the mitochondria (where it is called mitochondrial DNA or mtDNA).

DNA is made up of Molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar group and a nitrogen base. The four types of nitrogen bases are adenine (A), thymine (T), guanine (G) and cytosine (C). The order of these bases is what determines DNA’s instructions, or genetic code. Similar to the way the order of letters in the alphabet can be used to form a word, the order of nitrogen bases in a DNA sequence forms genes, which in the language of the cell, tells cells how to make proteins. Another type of nucleic acid, ribonucleic acid, or RNA, translates genetic information from DNA into proteins. The entire human genome contains about3 billion bases and about 20,000 genes.

Nucleotides are attached together to form two long strands that spiral to create a structure called a double helix. If you think of the double helix structure as a ladder, the phosphate and sugar molecules would be the sides, while the bases would be the rungs. The bases on one strand pair with the bases on another strand: adenine pairs with thymine, and guanine pairs with cytosine. DNA molecules are long — so long, in fact, that they can’t fit into cells without the right packaging. To fit inside cells, DNA is coiled tightly to form structures we call Chromosomes. Each chromosome contains a single DNA molecule. Humans have 23 pairs of chromosomes, which are found inside the cell’s nucleus.

Functions of DNA

DNA usually occurs as linear chromosomes in eukaryotes, and circular chromosomes in prokaryotes. The set of chromosomes in a cell makes up its genome; the human genome has approximately 3 billion base pairs of DNA arranged into 46 chromosomes. The information carried by DNA is held in the sequence of pieces of DNA called genes. Transmission of genetic information in genes is achieved via complementary base pairing. For example, in transcription, when a cell uses the information in a gene, the DNA sequence is copied into a complementary RNA sequence through the attraction between the DNA and the correct RNA nucleotides. Usually, this RNA copy is then used to make a matching protein sequence in a process called translation, which depends on the same interaction between RNA nucleotides. In alternative fashion, a cell may simply copy its genetic information in a process called DNA replication. The details of these functions are covered in other articles; here the focus is on the interactions between DNA and other molecules that mediate the function of the genome.

DNA REPLICATION

Process of DNA replication takes place in following steps:

Fork Formation

Before DNA can be replicated, the double stranded molecule must be “unzipped” into two single strands. DNA has four bases called adenine (A), thymine (T), cytosine (C) and guanine (G) that form pairs between the two strands. Adenine only pairs with thymine and cytosine only binds with guanine. In order to unwind DNA, these interactions between base pairs must be broken. This is performed by an enzyme known as DNA helicase. DNA helicase disrupts the hydrogen bonding between base pairs to separate the strands into a Y shape known as the replication fork. This area will be the template for replication to begin. DNA is directional in both strands, signified by a 5′ and 3′ end. This notation signifies which side group is attached the DNA backbone. The 5′ end has a phosphate (P) group attached, while the 3′ end has a hydroxyl (OH) group attached. This directionality is important for replication as it only progresses in the 5′ to 3′ direction. However, the replication fork is bi-directional; one strand is oriented in the 3′ to 5′ direction (leading strand) while the other is oriented 5′ to 3′ (lagging strand). The two sides are therefore replicated with two different processes to accommodate the directional difference.

Primer Binding

The leading strand is the simplest to replicate. Once the DNA strands have been separated, a short piece of RNA called a primer binds to the 3′ end of the strand. The primer always binds as the starting point for replication. Primers are generated by the enzyme DNA primase.

Elongation

ENZYMES known as DNA polymerases are responsible creating the new strand by a process called elongation. There are five different known types of DNA polymerases in bacteria and human cells. In bacteria such as E. coli, polymerase III is the main replication enzyme, while polymerase I, II, IV and V are responsible for error checking and repair. DNA polymerase III binds to the strand at the site of the primer and begins adding new base pairs complementary to the strand during replication. In eukaryotic cells, polymerases alpha, delta, and epsilon are the primary polymerases involved in DNA replication. Because replication proceeds in the 5′ to 3′ direction on the leading strand, the newly formed strand is continuous. The lagging strand begins replication by binding with multiple primers. Each primer is only several bases apart. DNA polymerase then adds pieces of DNA, called Okazaki fragments, to the strand between primers. This process of replication is discontinuous as the newly created fragments are disjointed.

Termination

Once both the continuous and discontinuous strands are formed, an enzyme called exonuclease removes all RNA primers from the original strands. These primers are then replaced with appropriate bases. Another exonuclease “proofreads” the newly formed DNA to check, remove and replace any errors. Another enzyme called DNA ligase joins Okazaki fragments together forming a single unified strand. The ends of the linear DNA present a problem as DNA polymerase can only add nucleotides in the 5? to 3? direction. The ends of the parent strands consist of repeated DNA sequences called telomeres. Telomeres act as protective caps at the end of chromosomes to prevent nearby chromosomes from fusing. A special type of DNA polymerase enzyme called telomerase catalyzes the synthesis of telomere sequences at the ends of the DNA. Once completed, the parent strand and its complementary DNA strand coils into the familiar double helix shape. In the end, replication produces two DNA molecules, each with one strand from the parent molecule and one new strand.

Enzymes involved in the process of DNA replication

DNA replication would not occur without enzymes that catalyze various steps in the process. Enzymes that participate in the eukaryotic DNA replication process include:

DNA helicase: unwinds and separates double stranded DNA as it moves along the DNA. It forms the replication fork by breaking hydrogen Bonds between nucleotide pairs in DNA.

DNA primase: a type of RNA polymerase that generates RNA primers. Primers are short RNA molecules that act as templates for the starting point of DNA replication.

DNA polymerases: synthesize new DNA molecules by adding nucleotides to leading and lagging DNA strands.

Topoisomerase or DNA Gyrase: unwinds and rewinds DNA strands to prevent the DNA from becoming tangled or supercoiled.

Exonucleases: group of enzymes that remove nucleotide bases from the end of a DNA chain.

DNA ligase: joins DNA fragments together by forming phosphodiester bonds between nucleotides.,

DNA is the molecule that contains the genetic instructions for all living things. It is a long, double-stranded molecule made up of nucleotides. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base. The four nitrogenous bases are adenine (A), thymine (T), guanine (G), and cytosine (C).

The two strands of DNA are complementary to each other, meaning that the bases on one strand always pair with the bases on the other strand in a specific way: A always pairs with T, and C always pairs with G. This complementary base pairing is what allows the two strands of DNA to be separated and then re-joined during DNA replication.

DNA is packaged into chromosomes, which are found in the nucleus of every cell. Chromosomes are made up of DNA and proteins. The proteins help to organize the DNA and protect it from damage.

Genes are segments of DNA that code for proteins. Proteins are the building blocks of cells and are responsible for carrying out the functions of the cell.

Gene expression is the process by which the information encoded in genes is used to produce proteins. This process involves two steps: transcription and translation.

Transcription is the process by which the DNA sequence is copied into a molecule of RNA. RNA is similar to DNA, but it has a slightly different structure. The RNA molecule is then transported out of the nucleus to the cytoplasm, where it is used in translation.

Translation is the process by which the RNA sequence is used to produce a protein. This process involves the ribosome, which is a protein complex that reads the RNA sequence and assembles the amino acids into a protein.

DNA replication is the process by which DNA is copied so that each daughter cell has a complete copy of the genetic information. This process is essential for cell division and for the Growth and repair of Tissues.

DNA replication is a semiconservative process, which means that each daughter cell receives one strand of DNA from the parent cell and one strand of newly synthesized DNA. The replication process begins with the separation of the two strands of DNA. This is done by helicase, an enzyme that breaks the hydrogen bonds between the base pairs.

Once the two strands of DNA are separated, each strand serves as a template for the synthesis of a new strand. The new strand is synthesized by DNA polymerase, an enzyme that adds nucleotides to the growing strand in the 5′ to 3′ direction.

The leading strand is synthesized continuously, in the 5′ to 3′ direction. The lagging strand is synthesized discontinuously, in the 5′ to 3′ direction. The lagging strand is synthesized in short segments called Okazaki fragments. Okazaki fragments are joined together by DNA ligase.

After replication is complete, each daughter cell has a complete copy of the genetic information.

DNA replication is a highly accurate process, but errors can occur. These errors can be caused by environmental factors, such as radiation or chemicals. They can also be caused by mistakes made by DNA polymerase.

When errors occur in DNA replication, they can lead to mutations. Mutations can be harmful, beneficial, or neutral. Harmful mutations can cause genetic diseases. Beneficial mutations can give an organism an advantage in its Environment. Neutral mutations have no effect on the organism.

DNA repair mechanisms are in place to correct errors that occur in DNA replication. These mechanisms include proofreading and mismatch repair. Proofreading is the process by which DNA polymerase checks the newly synthesized strand for errors. If an error is detected, DNA polymerase will correct it. Mismatch repair is the process by which errors that are not detected by proofreading are corrected.

DNA repair mechanisms are essential for maintaining the Integrity of the genome. Without these mechanisms, mutations would accumulate and could lead to genetic diseases.

DNA Repair

DNA repair is the process by which cells identify and correct damage to the DNA molecule. This damage can be caused by a variety of factors, including environmental toxins, radiation, and errors during DNA replication. DNA repair is essential for maintaining the integrity of the genome and preventing the development of genetic diseases.

Types of DNA Repair

There are two main types of DNA repair: direct repair and excision repair. Direct repair involves the enzymatic removal of damaged nucleotides and their replacement with undamaged nucleotides. Excision repair involves the removal of larger segments of damaged DNA and their replacement with newly synthesized DNA.

Mechanisms of DNA Repair

There are a number of different mechanisms that cells use to repair damaged DNA. These mechanisms can be divided into two main categories: error-free repair and error-prone repair. Error-free repair mechanisms are able to correct damage without introducing any errors into the DNA sequence. Error-prone repair mechanisms are not able to correct damage perfectly and may introduce mutations into the DNA sequence.

Importance of DNA Repair

DNA repair is essential for maintaining the integrity of the genome and preventing the development of genetic diseases. Without DNA repair, damage to the DNA molecule would accumulate over time, leading to the development of cancer and other genetic diseases.

Frequently Asked Questions

1. What is DNA repair?

2. What are the different types of DNA repair?

3. What are the mechanisms of DNA repair?

4. Why is DNA repair important?

5. What are some of the challenges of DNA repair?

One of the challenges of DNA repair is that the DNA molecule is very complex. It is made up of billions of nucleotides, and each nucleotide can be damaged in a variety of ways. This makes it difficult for cells to identify and correct all of the damage that occurs to the DNA molecule.

Another challenge of DNA repair is that the DNA molecule is constantly being replicated. This means that new DNA is being synthesized all the time, and any damage that occurs to the DNA molecule must be repaired before the new DNA is synthesized. This can be difficult, as the DNA molecule is constantly changing.

6. What are some of the future directions of research in DNA repair?

One of the future directions of research in DNA repair is to develop new methods for repairing damage to the DNA molecule. This could involve developing new enzymes that are able to repair damage that is not currently repairable.

Another future direction of research in DNA repair is to understand how DNA repair is regulated. This could lead to the development of new drugs that can be used to improve the efficiency of DNA repair.

Here are some multiple choice questions about DNA without mentioning the topic DNA Structure & Function, DNAReplication:

DNA is made up of:
(A) Proteins
(B) RNA
(C) Nucleic acids
(D) Carbohydrates
DNA is found in:
(A) The nucleus of cells
(B) The cytoplasm of cells
(C) The mitochondria of cells
(D) All of the above
DNA is made up of four different types of nucleotides:
(A) Adenine, thymine, guanine, and cytosine
(B) Adenine, guanine, cytosine, and uracil
(C) Thymine, cytosine, guanine, and adenine
(D) Uracil, cytosine, guanine, and adenine
The order of the nucleotides in DNA is called the:
(A) Genetic code
(B) Genome
(C) Codon
(D) All of the above
DNA is responsible for:
(A) Determining an organism’s traits
(B) Controlling the production of proteins
(C) Reproducing cells
(D) All of the above
DNA is replicated before a cell divides. This process ensures that each new cell has a complete copy of the DNA.
(A) True
(B) False
Mutations are changes in the DNA sequence. Some mutations are harmful, while others are beneficial.
(A) True
(B) False
Gene therapy is a technique that uses DNA to treat or prevent disease.
(A) True
(B) False
Genetic engineering is the process of modifying an organism’s DNA. This can be done to improve the organism’s traits or to make it resistant to disease.
(A) True
(B) False
DNA is a molecule that is essential for life. It is found in the nucleus of cells and contains the genetic code that determines an organism’s traits. DNA is replicated before a cell divides, and mutations can occur in the DNA sequence. Gene therapy and genetic engineering are techniques that use DNA to treat or prevent disease or to improve an organism’s traits.