Biology Chapter: DNA Structure and Organization

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Questions and Answers

What is the structural composition of a nucleosome?

A linear arrangement of histones

A complex of DNA and ribosomal RNA

DNA wrapped around eight histone proteins (correct)

DNA wrapped around a single protein

What is the primary function of euchromatin?

To package DNA tightly

To allow transcription and gene expression (correct)

To silence gene expression

To form visible chromosomes during cell division

Which type of heterochromatin is always in a condensed state?

Active heterochromatin

Constitutive heterochromatin (correct)

Transcriptional heterochromatin

Facultative heterochromatin

Who discovered the concept of DNA base pairing?

Erwin Chargaff Signup and view all the answers

What does heterochromatin NOT typically do?

Participate actively in transcription Signup and view all the answers

Which researcher is known for using X-ray diffraction to study DNA?

Rosalind Franklin Signup and view all the answers

What is the role of specific proteins and histone modifications in constitutive heterochromatin?

Maintain its condensed state Signup and view all the answers

What type of chromatin is primarily involved in gene expression and is less condensed?

Euchromatin Signup and view all the answers

What is the fundamental structure of DNA?

A double helix Signup and view all the answers

Which nitrogenous bases pair together in DNA?

Adenine with Thymine Signup and view all the answers

What is the role of helicase in DNA replication?

To unwind the DNA double helix Signup and view all the answers

What type of process is DNA replication classified as?

Semi-conservative Signup and view all the answers

What is the primary function of DNA polymerase during replication?

To synthesize new DNA strands Signup and view all the answers

Which part of the DNA is formed by the covalent bonds between the phosphate group and sugar?

Sugar-phosphate backbone Signup and view all the answers

How do prokaryotic and eukaryotic DNA replication origins differ?

Prokaryotes have a single origin, while eukaryotes have multiple origins Signup and view all the answers

What is the role of single-strand binding proteins (SSBPs) in DNA replication?

To stabilize separated DNA strands Signup and view all the answers

What happens to the lac operon in the absence of lactose?

The lac repressor binds to the operator. Signup and view all the answers

What effect does allolactose have on the lac repressor?

It causes a conformational change in the repressor. Signup and view all the answers

What is the role of cAMP in the positive regulation of the lac operon?

It binds to CAP and facilitates RNA polymerase binding. Signup and view all the answers

Under which condition is the lac operon most active?

Lactose is present and glucose is absent. Signup and view all the answers

What occurs when both glucose and lactose are present?

Transcription is minimal due to catabolite repression. Signup and view all the answers

What is the effect of high glucose levels on cAMP and CAP?

CAP cannot bind to the CAP-binding site. Signup and view all the answers

What is the primary function of the lac operon?

To regulate the production of enzymes based on sugar availability. Signup and view all the answers

In the presence of lactose and absence of glucose, what is true about the lac operon?

RNA polymerase has high binding affinity due to CAP. Signup and view all the answers

What effect does high glucose have on cAMP levels?

High glucose leads to low cAMP. Signup and view all the answers

How does catabolite repression influence E. coli's metabolism?

It ensures glucose is used first before lactose. Signup and view all the answers

What happens to the lac operon when glucose levels are low?

CAP-cAMP binds to DNA enhancing transcription if lactose is present. Signup and view all the answers

What is one of the key roles of the epigenome?

To regulate gene activity and expression without changing the DNA sequence. Signup and view all the answers

Which modification is crucial for processes like X-chromosome inactivation?

DNA methylation Signup and view all the answers

How does DNA methylation affect gene expression?

It represses gene expression by hindering transcription factor access. Signup and view all the answers

What is the relationship between CAP-cAMP and the lac operon?

CAP-cAMP enhances the transcription of the lac operon in absence of glucose. Signup and view all the answers

What happens when glucose is present and lactose is available?

Low cAMP prevents strong activation of the lac operon. Signup and view all the answers

What is the primary role of RNA in relation to DNA?

Transports genetic information for protein synthesis Signup and view all the answers

Which of the following statements correctly describes genomic DNA?

Comprises all genes and non-coding sequences Signup and view all the answers

What distinguishes mRNA from tRNA?

mRNA contains codons, whereas tRNA contains anticodons Signup and view all the answers

Which type of RNA is primarily responsible for forming ribosomes?

rRNA Signup and view all the answers

What type of biological molecule is more susceptible to degradation?

RNA Signup and view all the answers

What characteristic differentiates DNA from RNA regarding their structure?

DNA is more stable due to a double-stranded structure, whereas RNA is single-stranded Signup and view all the answers

How does a harmful mutation typically affect an organism?

It disrupts normal cell growth and regulation. Signup and view all the answers

Which of the following statements about RNA lifespan is true?

mRNA is generally short-lived compared to rRNA. Signup and view all the answers

What is the function of tRNA in protein synthesis?

It carries specific amino acids to the ribosome. Signup and view all the answers

Study Notes

Levels of Organization

Nucleosomes: DNA coiled around a core of eight histone proteins, forming a "beads on a string" structure.
Chromatin Types:
- Euchromatin: Less condensed, actively involved in transcription and gene expression, representing about 90% of the human genome.
- Heterochromatin: Highly condensed, generally transcriptionally inactive. Two types:
  - Constitutive heterochromatin: Always condensed, contains few genes.
  - Facultative heterochromatin: Can transition between condensed and less condensed states based on gene expression needs.
Chromosome Structure: During cell division, chromatin condenses to form visible chromosomes. Each chromosome consists of two sister chromatids joined at the centromere.

Discovery of DNA

Friedrich Miescher (1869): Discovered "nuclein" (DNA).
Oswald Avery (1944): Demonstrated DNA is the substance responsible for heredity, transforming non-virulent bacteria into virulent ones.
Erwin Chargaff (1940s): Formulated Chargaff's rules: Adenine (A) pairs with Thymine (T) and Cytosine (C) pairs with Guanine (G).
Rosalind Franklin (1952): Used X-ray diffraction to capture images of DNA, revealing its helical structure.
James Watson and Francis Crick (1953): Proposed the double-helix model of DNA, incorporating findings from previous researchers.

DNA Structure

Double Helix: Two strands of DNA wind around each other.
Nucleotides: Each strand is composed of alternating sugar (deoxyribose) and phosphate groups, with a nitrogenous base attached to each sugar.
Base Pairs:
- Adenine (A) pairs with Thymine (T) (two hydrogen bonds).
- Cytosine (C) pairs with Guanine (G) (three hydrogen bonds).

DNA Replication

Origin of Replication: Specific sequence where DNA replication begins. Prokaryotes have one origin, while eukaryotes have multiple.
Semi-conservative Replication: Each new DNA molecule has one original strand and one newly synthesized strand.
Key Enzymes:
- Helicase: Unwinds the DNA double helix, separating the strands.
- Single-Strand Binding Proteins (SSBPs): Prevent separated strands from re-annealing.
- Topoisomerase: Relieves tension ahead of the replication fork by cutting and rejoining DNA strands.
- Primase: Synthesizes short RNA primers, providing a starting point for DNA polymerase.
- DNA Polymerase: Synthesizes new DNA strands by adding nucleotides complementary to the template strand.

The Lac Operon

Negative Regulation by Lac Repressor:
- In the absence of lactose, the lac repressor binds to the operator, blocking RNA polymerase from transcribing the lac operon.
- With lactose present, allolactose (an inducer) binds to the repressor, causing it to detach from the operator, allowing transcription to occur.
Positive Regulation by CAP:
- When glucose is scarce, cAMP levels rise. cAMP binds to CAP, which in turn binds to the CAP-binding site, enhancing RNA polymerase binding and increasing transcription.
- When glucose is abundant, cAMP levels decrease, CAP doesn't bind, reducing transcription.
Dual Regulation:
- The lac operon is most active when lactose is present and glucose is absent.
- When glucose is available, it is preferentially utilized, and the lac operon remains minimally expressed until glucose is depleted (catabolite repression).

The Epigenome

Chemical Modifications and Markers: The epigenome regulates gene activity and expression without altering DNA sequence.
Key Components:
- DNA Methylation: Addition of a methyl group (CH₃) to DNA, typically at cytosine bases in CpG dinucleotides. Usually represses gene expression.
  - Plays a role in X-chromosome inactivation, genomic imprinting, and silencing transposable elements.
- Histone Modifications: Modifications to the histone proteins around which DNA is wrapped. Can alter chromatin structure and accessibility for transcription factors.
  - Play a role in regulating gene expression, DNA replication, and repair.

Harmful Mutations

Can disrupt normal cell regulation and growth, potentially leading to genetic disorders or contributing to diseases like cancer.

Conclusion

The study of genes and genomes is crucial for advancements in biotechnology and for understanding biological processes.
Understanding DNA replication, transcription, translation, and gene regulation is essential for developing biotechnological applications and addressing genetic disorders.

DNA vs. RNA

Structure: DNA is double stranded, RNA is single stranded.
Sugar: DNA contains deoxyribose, RNA contains ribose.
Bases: DNA has Adenine (A), Thymine (T), Cytosine (C), and Guanine (G). RNA has Adenine (A), Uracil (U), Cytosine (C), and Guanine (G).
Function: DNA stores genetic information, RNA is involved in protein synthesis and gene regulation.
Location: DNA is primarily found in the nucleus (eukaryotes) or the nucleoid region (prokaryotes). RNA is found in the nucleus, cytoplasm, and ribosomes.
Stability: DNA is more stable due to its double-stranded structure. RNA is less stable and more prone to degradation.
Replication: DNA is self-replicating. RNA is synthesized from DNA during transcription.
Types: DNA has one main type. RNA has three main types: mRNA, rRNA, and tRNA.

mRNA vs. rRNA vs. tRNA

mRNA (Messenger RNA): Carries genetic information from DNA to ribosomes for protein synthesis.
** rRNA (Ribosomal RNA):** Forms the core component of ribosomes and catalyzes protein synthesis.
tRNA (Transfer RNA): Transfers specific amino acids to the ribosome during translation.

Types of DNA

Genomic DNA: The complete set of DNA, including all genes and non-coding sequences.

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Description

Explore the intricate levels of organization within DNA, including nucleosomes and the two types of chromatin: euchromatin and heterochromatin. Learn about the remarkable history of DNA discovery, from Friedrich Miescher's identification of nuclein to Oswald Avery's pivotal experiments demonstrating DNA's role in heredity.