Category: Biology Second Paper

During DNA replication, some enzymes and proteins join near the replication fork to form complex molecular structures. It is called replisome or replication complex. The main components of replisome are-
1. DNA Helicase: The DNA helicase enzyme unwinds the DNA double helix at the replication fork.
2. DNA Polymerase: DNA polymerase enzyme joins nucleotides to form a complementary strand directed 5→3. DNA proof reading.
3. Topo-Isomerase: Topo-isomerase enzyme frees DNA from super coiled state.
4. DNA Clumps: DNA clumps inhibit maternal elongation of DNA.
5. DNA gyrase: The enzyme DNA gyrase creates supercoils (coils) at the end of DNA replication.
6. DNA ligase: The enzyme DNA ligase joins the Okazaki fragment to the complementary strand. Stops chain DNA sequence elongation.
7. SSBP: Single Strand Binding Proteins (SSBP) prevent single strand DNA molecules from refolding back into the double helix state.
8. Primase: Primase enzyme joins the RNA primer to the ends of the chain.
9. Telomerase: The telomerase enzyme adds nucleotides to telomeric DNA-molecules in eukaryotic chromosomes.
10. Exonuclease: Exonuclease enzyme removes the primers from the chain.
11. Nuclease: Nuclease enzymes remove incorrect nucleotides from complementary primers or strands.

DNA replication is the most beneficial process in the living world. True cell replication is a complex process. The process can be discussed under three headings. Double helix separation, complementary strand formation and new DNA formation.
1. Separation of the double helix
(i) At the beginning of replication, base pairs are released at specific sites in the DNA and bubble-like ori points or replicon points or initiation points are formed. Ori spots are usually formed at sites where adenine and thymine are abundant in DNA. Because adenine and thymine have two hydrogen (A=T) bonds. Adikosha has one and Prakharkosha has multiple ori points.
(ii) First the helicase enzyme binds to the Ori point and starts unwinding the double helix. The helicase enzyme then uses energy from ATP to break the hydrogen bond. A Y-shaped structure is formed at the point where the two strands diverge. This is called replication fork.
(iii) After opening the patch, the two strands separated due to tension or attraction tend to gather again by re-adjusting the patch. The enzyme topoisomerase cuts the strand near the replication fork. As a result, the attraction of forming patches and gathering of the formula is lost. In primary cells, gyrase enzyme breaks down the attractive pull of formation and aggregation of sutra patches. The severed formula is then reattached.
(iv) The two formulas are complementary to each other. So it wants to re-attach by forming a hydrogen bond. Single Strand Binding Protein (SSBP) does not allow hydrogen bonds to form. As a result the two formulas cannot be rejoined.
(v) Replication forks move in opposite directions and balloon or eye-like structures are formed in the intervening space. It is called Replication eye or Replication bubble. Many replication bubbles are formed simultaneously. The bubbles elongate and coalesce, separating the two sutras. The separation of the two strands of DNA is called denaturation.
2. Creating complementary chains
(i) RNA primase enzyme uses the formula two as a template and creates smaller primers. The R3 end of the primer has a free -OH group.
(ii) DNA polymerase enzyme adds new nucleotides to the 3 -OH end of the primer. Nucleotides tend to join 5-3 carbons. As a result, two new sources started to emerge. One is the leading formula and the other is the lagging formula.
(iii) The strand that grows towards the replication fork is called the leading strand. Leading formulas continuously create counterpoints. The strain that grows in the opposite direction of the replication fork is called the lagging strain. The lagging formula produces unpaired segmental counterparts.
(iv) Each term of the lagging formula is called Okazaki. Okazaki of protocells is composed of 1000-2000 nucleotides and Okazaki of true cells is 100-200 nucleotides. Japanese scientist Reiji Okazaki and his wife Tsuneko Okazaki discovered it.
3. Creation of new DNA
(i) Exonuclease enzyme removes the primers complementary to the new sequence and the empty sites are filled with complementary nucleotides.
(ii) Incorrect nucleotides are removed by nuclease enzymes and correct nucleotides are added by DNA polymerase enzymes. This mismatch repair (MMR) is called DNA proof reading. Only one in 1,000 genes in humans can be a mismatch.
(iii) Ligase enzyme joins Okazaki. Purine and pyrimidine bases are joined by hydrogen bonds. As a result, two new DNAs are created.

Replication occurs in the interphase or S phase of the cell cycle and in the leptin subphase. Primitive cells contain circular DNA and have no ends or centers. Replication starts anywhere in the progenitor cell and replication forks move in both directions and meet at the midpoint. Replication of circular DNA is rapid and can add up to a million base pairs per minute. A cell’s circular DNA has two ends, and replication begins at multiple sites between the strands. Replication of circular DNA is slow and can add up to 500-5000 base pairs per minute. Replication of E. coli takes 20-30 minutes. Replication in animal cells takes 1.4-24 hours. Replication can start at 50,000 sites in Dosophila.

Meselson and Stahl (1958) grew E. coli bacteria in a culture medium enriched in heavy isotopes of 14N and 15N. After several generations, the DNA of E. coli bacteria is labeled with 15N. The 15N enriched bacteria were then transferred back to the 14N medium. The next generation showed that the newly formed bacterial DNA double helix had one strand of 14N and the other strand of 15N. That is, one old formula and the other new formula of the double helix. This proves that DNA replication occurs in a semi-conservative manner.

1. Conservative hypothesis: In the conservative process, DNA sequences are separated into two. Each sutra acts as a mold and creates new sutras. Then the old sutras are placed together and the new sutras are placed together. As a result, two strands of DNA are formed. That is, two strands of one DNA are old and two strands of other DNA are new.
2. Dispersive hypothesis: In the dispersive process, DNA strands are broken into two. Then different amounts of old and new fragments combine to form two DNA molecules. This process is not acceptable.
3. Semiconservative hypothesis: The process in which two new DNAs are formed from one maternal DNA and each of the two new DNAs has a maternal source and a new source is called a semiconservative process. In 1957, scientist Stent first used the term semiconservative. In 1958 Mathew Meselson & Franklin Stahl proved the semiconservative hypothesis in E. coli bacteria. In 1960, Herbert Taylor demonstrated semiconservative replication by experimenting with bean plant root cells. In 1960, scientist Sueka demonstrated semiconservative processes in human HeLa cells. In 1961, scientist Symon demonstrated the semiconservative process in the algae Chlamydomonas.

The process in which two new DNAs of exactly the same nature are made from a double helix DNA is called DNA replication. Bacterial circular DNA molecules can be replicated at 100,000 base pairs per minute. Replication of long DNA molecules in real cells can add 500-5000 base pairs per minute. Replication does not start at either end of a real cell’s DNA.

A dogma is a core belief or set of ideas. Cell replication, transcription and translation together are called the central dogma. In 1958, scientist Francis Crick first proposed the idea of ​​the central dogma. Barry Commoner gave it the cyclic form in 1968. It is now considered a fundamental principle of molecular genetics. Francis Crick considers three different paths to be the basic tenets of the Central Dogma.
1. Replication: Through replication, another DNA is created from DNA. DNA replication occurs in the S phase of the cell cycle.
2. Transcription: mRNA is produced from DNA through transcription. Transcription occurs inside the cell nucleus.
3. Translation: Protein is produced from mRNA through translation. Translation occurs in the cytoplasm of the cell.

Ribonucleic acid is abbreviated as RNA. The monomeric units of nucleotides of nucleic acid are made up of ribose sugar, adenine, guanine, cytosine, uracil and phosphoric acid is called RNA. In 1963 P.J. Gomatos & I. Tamm discovered the existence of double-stranded RNA molecules in Reovirus. Rice dwarf virus contains double-stranded RNA.
Amplification of RNA
All cells contain RNA. It is located in the cell cytoplasm, nucleus, ribosomes, chromosomes, mitochondria, chloroplasts etc. 90% of the RNA of the cell is in the cytoplasm and 10% of the RNA is in the nucleus. Some bacteria E. coli, Xanthomonas, Cyanobacteria, Francisella etc. contain RNA. Most plant viruses contain RNA. RNA viruses such as TMV, HIV, dengue, polio, mumps, measles, measles, rabies, influenza, arbovirus, coronavirus, reovirus, rice dwarf virus, retrovirus etc.
Chemical structure of RNA
RNA molecules are made up of three chemical components. These are-
1. Nitrogen Alkali: This alkali is formed by carbon, hydrogen, oxygen and nitrogen. Alkaline compounds form rings. Based on the number of rings, nitrogenous bases can be divided into two groups.
(i) Purines: Dicyclic nitrogenous bases are called purines. Its common symbol is C5H4N4. It is composed of adenine and guanine.
(ii) Pyrimidine: A cyclic nitrogenous base is called pyrimidine. Its common symbol is C4H4N2. It is composed of cytosine and uracil.
2. Pentose sugars: Five carbon sugars are called pentose sugars. RNA molecules contain the ribose sugar. It is a type of monosaccharide. Ribose sugar is so named because it has a hydroxyl group at the 2nd carbon.
3. Phosphoric acid: One of the chemical components of RNA molecules is phosphoric acid. Its molecular symbol is H3PO4. It contains one divalent oxygen atom and three monovalent hydroxyl groups. Oxygen and hydroxyl groups combine with a pentavalent phosphorus atom to form phosphoric acid.
A molecule of pentose sugar and a molecule of nitrogen base combine to form a molecule of nucleoside. Again, one molecule of phosphate joins with one molecule of nucleoside to form one molecule of nucleotide. The smallest RNA molecule can contain 22 and the largest RNA molecule can contain 10,000 nucleotides.
Physical structure of RNA
The structure of RNA molecules is constant. Its helix is grooved in places to stabilize the structure. The grooves are formed in a special way. These are called secondary coils. In the secondary coil, the nitrogenous bases are joined by hydrogen bonds. RNA molecules can be mainly divided into two parts. Genetic RNA and non-genetic RNA.

Genetic RNA
RNAs that act as carriers of heredity are called genetic RNAs. Some viruses contain RNA instead of DNA. All these RNAs carry hereditary characteristics. Genetic RNA can cause enzyme synthesis and protein coating. Viruses like Dengue, Polio, Mumps, TMV, HIV, Rabies, Reo virus, Dwarf virus, Retro virus etc. contain genetic RNA.
Non-genetic RNA
RNAs that do not function as carriers of heredity are called non-genetic RNAs. Different types of non-genetic RNA are-
1. tRNA
RNA that carries amino acids to the ribosome where proteins are made is called tRNA or transfer RNA. It is the smallest RNA and has a molecular weight of 25,000 daltons. About 15% of the cell is tRNA. Each tRNA molecule is made up of 90 nucleotides. It is produced in the nucleolus of the cell. tRNA molecules contain some unusual bases. Such as inosinic acid, thiamine etc. They are highly stable and make proteins for cells. There are about 100 types of tRNA in living organisms. A cell contains 31-42 types of tRNA.
According to scientist R. Holley’s clover leaf model in 1965, tRNA molecule consists of 5 arms.
(i) Subscriber arm: 3 and 5 ends of tRNA lie side by side to form the subscriber arm. It contains 7 base pairs. In addition, there are 4 additional unpaired nucleotides at the R3 end. 3 ends contain the -CCA sequence and the fourth A or G. The CCA sequence is called the amino acid attachment site. Amino acids are attached at this point. 5 ends with G or C.
(ii) D arm: The second arm of tRNA is D arm. It consists of 15-18 nucleotides. The D arm consists of 3-4 base pairs and its loop consists of 7 unpaired nucleotides. D arm loop is called loop-1 or Dihydrouridine or D loop.
(iii) Anticodon arm: The third arm of tRNA is called anticodon arm. The anticodon arm consists of 5 base pairs and its loop consists of 7 unpaired nucleotides. Its leak is called leak-2. The 3 nucleotides between the loops form the anticodon. It acts as a complementary codon to mRNA during protein synthesis.
(iv) Variable arm: Variable arm is of two types. Stemless and stemmed arms. Stemless arms have 4-5 bases and a loop. Stem arms consist of 13-21 bases and a loop.
(v) T arm: T arm has both stem and loop. Its stem is made up of 5 base pairs and the loop is made up of 7 nucleotides. The T arm contains a ribosome site. Its loop contains ribothymine, pseudouracil and cytosine bases.
tRNA carries amino acids to mRNA during protein synthesis.

2. mRNA
RNAs that carry genetic signals from the DNA of cells are called mRNAs or messenger RNAs. This is called template RNA or monocistronic mRNA. It is a beginning and very temporary. Their molecular weight is 5-20 lakh daltons. About 5-10% of the cell is mRNA.

Structure of mRNA
(i) Guanine cap: Guanine is added to the 5 end of mRNA to form a cap. This is called a guanine cap. The guanine cap consists of 7 guanosine nucleotides. This cap ensures ribosome attachment during translation.
(ii) Non-coding region-1: The region which does not synthesize protein is called non-coding region. This region consists of 10-100 nucleotides after the cap. It is rich in adenine and uracil. Translation does not occur here.
(iii) Initiation codon: The codon which initiates protein synthesis is called initiation codon. The start codon is AUG. It binds to the amino acid methionine.
(iv) Coding region: The region that synthesizes the protein is called coding region. This region consists of 1500 nucleotides. Translation occurs in this region.
(v) Termination codon: The codon which terminates protein synthesis is called termination codon. The termination codons are UAA, UAG and UGA. If any one of these codons is present, protein synthesis stops.
(vi) Non-coding region-2 : This region consists of 50-150 nucleotides. No translation here.
(vii) Poly A tail: Adenine is added to the 3 end of mRNA to form poly A tail. It is composed of 200-250 adenines.
Function: mRNA carries the protein-making signal from the nucleus to the cytoplasm and forms the chain of amino acids.
3. rRNA
The RNA that acts as the main structural component of ribosomes is called ribosomal RNA or rRNA. They are extremely permanent and insoluble. 80-90% of cells are rRNA. That is, it is the main part of the ribosome. Its molecular weight is 5-20 lakh daltons. Each rRNA molecule is made up of about 3,000 nucleotides. It is unbranched and unbranched. However, if the ionic level increases, a two-factor condition may occur in some places. Adenine and uracil and guanine and cytosine pair at the double-stranded position. rRNA combines with proteins to form ribonucleoprotein particles. There are three types of rRNA in progenitor cells (23S, 16S, 5S) and four types (28S, 18S, 5.8S, 5S) in protocells.
Function: rRNA forms ribosomes and helps in protein synthesis.
4. Minor RNA
RNAs that combine with proteins to form enzyme structures are called minor RNAs. They are located in cytoplasm and nucleus. They are also called ribozymes because they have the properties of enzymes. eg ribonucleoprotein.

Importance of RNA
1. Genetic RNA carries hereditary characteristics.
2. tRNA carries amino acids to mRNA during protein synthesis.
3. mRNA carries the protein-making signal from the nucleus to the cytoplasm and builds the chain of amino acids.
4. rRNA forms ribosomes and helps in protein synthesis.
5. Minor RNA combines with proteins to form enzyme structures.

1. Genetic RNA carries hereditary characteristics.
2. tRNA carries amino acids to mRNA during protein synthesis.
3. mRNA carries the protein-making signal from the nucleus to the cytoplasm and builds the chain of amino acids.
4. rRNA forms ribosomes and helps in protein synthesis.
5. Minor RNA combines with proteins to form enzyme structures.

RNAs that combine with proteins to form enzyme structures are called minor RNAs. They are located in cytoplasm and nucleus. They are also called ribozymes because they have the properties of enzymes. eg ribonucleoprotein.