DNA Structure and Function

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Questions and Answers

What occurs to the absorption value at 260nm during heat denaturation of DNA?

It remains constant.

It decreases gradually.

It increases sharply. (correct)

It fluctuates unpredictably.

Which factor has the most significant impact on the melting temperature (Tm) of DNA?

G-C content in DNA. (correct)

Length of the DNA strand.

Presence of single-stranded RNA.

A-T content in DNA.

What is the result of cooling a denatured DNA solution slowly?

The strands form hybrid molecules with proteins.

The DNA strands remain separate.

The solution becomes more viscous.

The DNA can re-form double-stranded helices. (correct)

During molecular hybridization, what occurs between complementary bases of different DNA or RNA?

They repair to form double strands. Signup and view all the answers

What is typically observed when the biological functions of DNA are lost due to denaturation?

An increase in absorption value at 260nm. Signup and view all the answers

What is the impact of supercoiling on DNA molecules?

It allows DNA to fit more easily inside cells. Signup and view all the answers

What is the basic unit of chromatin in eukaryotic cells?

Nucleosome Signup and view all the answers

During which phase does DNA appear in the highly ordered form called chromosomes?

Metaphase Signup and view all the answers

What type of RNA is responsible for carrying genetic information from DNA to ribosomes for protein synthesis?

mRNA Signup and view all the answers

What characteristic differentiates RNA from DNA structurally?

RNA molecules are typically single-stranded Signup and view all the answers

Which of the following describes the main role of ribosomal RNA (rRNA)?

To form part of the ribosome structure Signup and view all the answers

What physical property is associated with nucleic acids due to their phosphate groups?

Acidity Signup and view all the answers

At what specific wavelength do nucleic acids exhibit strong UV absorption?

260 nm Signup and view all the answers

Which of the following statements describes the organization of DNA?

DNA is organized into informational units called genes. Signup and view all the answers

What significant contribution did Friedrich Miescher make to molecular biology?

He isolated nuclein, later known as nucleic acid. Signup and view all the answers

Where is the majority of DNA found in a eukaryotic cell?

In the nucleus. Signup and view all the answers

Which of the following correctly describes the types of nucleic acids?

DNA consists of deoxyribonucleotides, while RNA consists of ribonucleotides. Signup and view all the answers

What feature of DNA suggests it as the genetic material?

It is present in all cells and primarily located in the nucleus. Signup and view all the answers

Which nucleotide is a pyrimidine found in DNA?

Cytosine Signup and view all the answers

What structural component is NOT part of a nucleotide?

Amino acid Signup and view all the answers

How do ribose and deoxyribose sugars differ chemically?

Ribose contains one more oxygen atom than deoxyribose. Signup and view all the answers

What is the role of DNA primase during DNA replication?

It makes a 10 nucleotide RNA primer to initiate replication. Signup and view all the answers

How does DNA ligase function during replication?

It seals the nicks between Okazaki fragments. Signup and view all the answers

What is the primary function of single-strand binding proteins (SSBs) during DNA replication?

To bind and stabilize the unwound DNA strands. Signup and view all the answers

Which of the following statements is true regarding the differences between prokaryotic and eukaryotic DNA replication?

Eukaryotes possess a larger number of DNA polymerases compared to prokaryotes. Signup and view all the answers

What is the function of topoisomerase II (DNA gyrase) in DNA replication?

To relieve torsional stress caused by DNA unwinding. Signup and view all the answers

Which conditions must be met for DNA ligase to seal nicks between Okazaki fragments?

Close and free 3'-OH and 5'-P ends. Signup and view all the answers

During termination of DNA replication, how are the two replication forks synchronized?

Through binding of Tus proteins to Ter sequences. Signup and view all the answers

In which cellular process is RNA-dependent DNA polymerase utilized?

Reverse transcription in retroviruses. Signup and view all the answers

Which of the following structures represents the secondary structure of DNA?

DNA double helix Signup and view all the answers

What is the role of ligase in DNA replication?

Joins Okazaki fragments Signup and view all the answers

What modification occurs when DNA is transcribed into RNA?

Thymine is replaced by Uracil Signup and view all the answers

Which of the following describes the semiconservative nature of DNA replication?

Each daughter molecule has one original and one new strand Signup and view all the answers

What is the primary function of mRNA during translation?

Serve as a template for protein synthesis Signup and view all the answers

How is the leading strand synthesized during DNA replication?

Continuously in the same direction as the fork opens Signup and view all the answers

What dictates the sequence of amino acids in a protein?

The codons of mRNA Signup and view all the answers

What is the role of DNA polymerase I?

Remove RNA primers and replace them with DNA Signup and view all the answers

Which of the following is NOT a property of nucleic acids?

Formation of peptide bonds Signup and view all the answers

What is the significance of the start codon AUG?

It is where protein synthesis begins Signup and view all the answers

How does DNA replication differ in prokaryotes compared to eukaryotes?

Eukaryotic replication is slower than in prokaryotes Signup and view all the answers

Which enzyme is mainly responsible for synthesizing new DNA strands?

DNA polymerase Signup and view all the answers

Study Notes

DNA

DNA is organized into informational units called genes.
Genes are found on chromosomes, which contain hundreds or thousands of genes.
DNA was first discovered in 1869 by Friedrich Miescher, who isolated "nuclein" from pus cells.
DNA is found in the nucleus of eukaryotic cells, with smaller amounts in mitochondria and chloroplasts.

DNA Structure

DNA is a polymer of nucleotides, with each nucleotide comprising a base, a pentose sugar and a phosphate group.
The sugar in DNA is deoxyribose, while RNA uses ribose.
DNA exists in a double helix structure where two strands run antiparallel to each other and are bonded together by hydrogen bonds between nitrogenous bases.
The bases in DNA are adenine (A), guanine (G), cytosine (C), and thymine (T).
Adenine binds with thymine and guanine binds with cytosine
DNA can be overwound or underwound. This leads to positively or negatively supercoiled DNA respectively
In prokaryotes, DNA is supercoiled in order to fit inside the cell.

Eukaryotic DNA

Eukaryotic DNA is highly packed and organized into chromosomes.
The basic unit of chromatin is the nucleosome.
Nucleosomes consist of DNA wrapped around histone proteins.
DNA packing facilitates chromosome function, providing:
- Efficient packaging within the cell.
- Protection against damage.
- Efficient transmission to daughter cells during cell division.
- Gene expression and recombination.

RNA

RNA is primarily single-stranded, but can have double-stranded regions.
RNA is more diverse in its function in the cell than DNA.
Different types of RNA exist, including:
- mRNA (messenger RNA): Transcribes genetic information from DNA for protein synthesis
- tRNA (transfer RNA): Carries amino acids to ribosomes during protein synthesis
- rRNA (ribosomal RNA): Forms part of ribosomes that are involved in protein synthesis
- hnRNA (heterogeneous nuclear RNA)
- snRNA (small nuclear RNA)

Properties of Nucleic Acids

Nucleic acids are acidic due to the phosphate group.
They are amphiphilic, meaning they have both hydrophobic and hydrophilic properties.
Solid DNA is a white fiber, while RNA is a white powder.
They are insoluble in organic solvents and can be precipitated by ethanol.
They absorb ultraviolet (UV) light, particularly at a wavelength of 260nm.
Double-stranded DNA (dsDNA) can be denatured or separated into single strands by heat.

Denaturation of DNA

Denaturation occurs when hydrogen bonds between base pairs are broken, leading to a loss of the 3-D structure.
Denaturation results in two single strands of DNA that are unstructured.
UV absorption increases at 260nm during denaturation as the single strands absorb more UV than double strands.
The melting temperature (Tm) of DNA is the temperature at which half of the DNA molecules are denatured.

Renaturation of DNA

When slowly cooling down a denatured DNA solution, single strands of DNA can reanneal to form a double helix.
This process is known as hybridization.
In hybridization, different source DNAs or single-stranded DNA and RNA can reanneal if they have complementary bases.

Central Dogma

The central dogma outlines the flow of genetic information from DNA to RNA to protein:
- Replication: The process of copying DNA.
- Transcription: The process of creating an RNA copy from DNA.
- Translation: The process of synthesizing protein using the genetic code in mRNA.

Replication

Replication is semiconservative, meaning that each new DNA molecule contains one original strand and one newly synthesized strand.
Enzymes involved in replication include:
- Topoisomerases: Relieve torsional stress in DNA during replication.
- Helicase: Unwinds the DNA double helix.
- DNA polymerases: Synthesize new DNA strands.
- Ligase: Joins Okazaki fragments.

Replication Process

Replication starts at specific sites on the DNA called origins of replication.
DNA polymerase synthesizes new DNA strands in the 5’ => 3’ direction.
The leading strand is synthesized continuously, while the lagging strand is synthesized discontinuously in short fragments called Okazaki fragments.
Primers are used to initiate DNA synthesis.
Okazaki fragments on the lagging strand are joined together by DNA ligase.
Proofreading mechanisms ensure high fidelity in DNA replication, with an error rate of approximately 1 error per billion nucleotides.

Transcription

Transcription uses a DNA template to create an RNA copy.
Uracil replaces thymine in RNA.
One strand of DNA serves as the template strand (antisense), while the other strand is the coding strand (sense).
Transcription occurs in the nucleus, specifically in the nucleoli.

Translation

Translation is the process of converting the genetic code in mRNA into protein.
Translation occurs on ribosomes.
During translation, tRNAs carrying specific amino acids bind to codons on mRNA, adding amino acids in a specific sequence to form a protein.
The initiation codon AUG signals the start of translation.
Stop codons signal the end of translation.
The genetic code is degenerate, meaning that multiple codons can code for the same amino acid.

Prokaryotic DNA Replication

Prokaryotic DNA replication is performed by a multienzyme complex called the replisome.
Important components of the replisome include:
- DNA polymerases
- Helicase
- Primase
- Single-stranded binding proteins (SSBs)
- DNA ligase
- Clamps
- Topoisomerases
Prokaryotic DNA replication is semiconservative, accurate, and fast.
There is one origin of replication in the prokaryotic chromosome.

DNA Polymerase III

DNA polymerase III is a complex protein responsible for DNA synthesis in E. coli.
It is a multi-subunit enzyme with distinct functions for each subunit.

DNA Polymerase I

DNA polymerase I was discovered by Arthur Kornberg in the 1950s.
It has three enzymatic activities: polymerase activity, 3’-5’ exonuclease activity, and 5’-3’ exonuclease activity.
The Klenow enzyme lacks the 5’-3’ exonuclease activity.

Requirements for DNA Polymerase Activity

A free 3’-OH group from an RNA primer is needed to start polymerization.
Mg2+ ions are essential for the enzyme's activity.
A template DNA strand is required to copy.

Replication Origin (OriC)

In E.coli, replication originates at a specific site called OriC, which is around 245 base pairs long and AT-rich.
Initiation proteins bind to specific consensus sequences within OriC.

Replication Fork

Replication proceeds bidirectionally, with two replication forks moving in opposite directions.
DNA is synthesized in the 5’ => 3’ direction at the replication fork.

Okazaki Fragments

The lagging strand is synthesized discontinuously in fragments called Okazaki fragments.
The Okazaki fragments are later joined together by DNA ligase.
Okazaki fragments are typically 1000-2000 nucleotides long in prokaryotes, but 100-200 nucleotides long in eukaryotes.

Proofreading

DNA polymerase III has a 3’-5' exonuclease activity that acts as a proofreading mechanism to ensure accuracy during replication.

Topoisomerases

Topoisomerases relieve the torsional stress caused by unwinding DNA during replication.
There are two types of topoisomerases:
- Topoisomerase I: Breaks one strand of the DNA double helix.
- Topoisomerase II (DNA gyrase): Breaks both strands of the DNA double helix.

Termination of Replication

Replication terminates at specific sequences called Ter sequences, which bind Tus proteins.
Tus proteins block replisome movement, stopping replication.

Reverse Transcription

Reverse transcription is a process used by retroviruses to create DNA from an RNA template.
The enzyme responsible for this process is reverse transcriptase, or RNA-dependent DNA polymerase.

Comparison of Prokaryotic and Eukaryotic Replication

Feature	Prokaryotic	Eukaryotic
# Origins of Replication	1	1000-10000
Speed	500 nt/s	50 nt/s
Time for Replication	40 min	8 hours
Okazaki Fragment Length	1000-2000 nt	100-200 nt
DNA Polymerases	3 (5)	5 (10)
Chromosomes	1, circular	46, linear
Other		Telomeres, histones

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Description

Explore the fascinating world of DNA, the molecular basis of heredity. This quiz covers its discovery, structure, and the role of genes organized on chromosomes. Test your understanding of how DNA's unique structure contributes to its function in living organisms.