DNA Packaging and Chromosomes Overview

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Questions and Answers

What is the primary role of DNA Polymerase III during strand elongation?

It removes Okazaki fragments.

It catalyzes the elongation of new DNA strands. (correct)

It repairs damaged DNA.

It catalyzes the formation of RNA primers.

Which of the following correctly describes the leading strand during DNA replication?

It is synthesized continuously and grows from 5' to 3'. (correct)

It synthesizes in short, discontinuous segments.

It is synthesized in segments of 100-200 nucleotides.

It requires multiple RNA primers for elongation.

What are Okazaki fragments primarily associated with?

Synthesis of the leading strand.

Reinforcement of DNA ligase activity.

Discontinuous synthesis on the lagging strand. (correct)

Replication in eukaryotic cells only.

Which enzyme is responsible for filling in the gaps between Okazaki fragments?

DNA Ligase (C) Signup and view all the answers

How are the lagging and leading strands oriented in relation to each other?

They are antiparallel to each other. (D) Signup and view all the answers

What distinguishes the synthesis of the lagging strand from the leading strand?

It proceeds in short, discontinuous fragments. (D) Signup and view all the answers

What is a unique feature of prokaryotic Okazaki fragments compared to eukaryotic fragments?

They are larger, consisting of 1000-2000 nucleotides. (B) Signup and view all the answers

What is the function of DNA Polymerase III during DNA replication?

Proofreads and adds DNA nucleotides (B) Signup and view all the answers

In which direction do nucleotides always get added to the growing DNA strand?

5' to 3' (D) Signup and view all the answers

What is a key role of the primer in DNA replication?

It initiates DNA strand synthesis (D) Signup and view all the answers

What enzyme is responsible for joining Okazaki fragments during DNA replication?

Ligase (A) Signup and view all the answers

Which statement about Okazaki fragments is correct?

They are short DNA fragments on the lagging strand. (C) Signup and view all the answers

Which enzyme unwinds the DNA strands during replication?

Helicase (B) Signup and view all the answers

How is energy provided for the elongation of the DNA strand?

By breaking the bonds of triphosphate groups (B) Signup and view all the answers

What is the role of primases in DNA replication?

To synthesize RNA primers (D) Signup and view all the answers

What type of strand does DNA Polymerase I primarily act on?

Lagging strand (D) Signup and view all the answers

What occurs during the elongation phase of DNA replication?

Proteins connect nucleotides into a continuous new strand. (B) Signup and view all the answers

Which enzyme is primarily responsible for synthesizing new DNA during elongation?

DNA polymerase (C) Signup and view all the answers

How do single strand binding proteins contribute to DNA replication during elongation?

They stabilize the single-stranded DNA and prevent rejoining. (A) Signup and view all the answers

What role does primase play in the elongation phase of DNA replication?

It synthesizes RNA primers to initiate DNA synthesis. (D) Signup and view all the answers

What is the primary function of topoisomerases during the process of elongation?

To prevent torsional strain by breaking and rejoining DNA strands. (A) Signup and view all the answers

Study Notes

DNA Packaging and Replication

DNA is very thin (2.0 nm) and fragile, but needs to be protected and accessible within a cell's nucleus (5-10 μm in diameter).
DNA packaging is essential for gene expression and cellular responsiveness.
DNA must be compacted into a 3-D conformation aided by DNA-binding proteins (like histones).

Chromosomes

Chromosomes consist of a single DNA molecule associated with proteins.
A karyotype displays the number and types of chromosomes present in a cell.
Chromosomes are comprised of chromatin, a complex of DNA and proteins.

Normal Human Chromosomes

Normal human cells contain 23 pairs of homologous chromosomes.
22 pairs are autosomes (same in males and females).
1 pair are sex chromosomes (XX in females, XY in males; X is homologous, Y is much smaller with fewer genes).

Three-Dimensional Structure of DNA

DNA must be packaged extensively to fit within a cell.
This compaction is aided by DNA-binding proteins, primarily histones.
DNA is wrapped around histones, forming nucleosomes (11 nm in diameter).
Nucleosomes coil into a 30 nm fiber, and further compact into radial loops (300 nm in diameter).
The final structure forms metaphase chromosomes (700 nm in diameter).

Chromatin

Chromatin is a complex of DNA and proteins forming eukaryotic chromosomes.
Two classes of chromatin proteins:
- Histones (4 core histones: H2A, H2B, H3, H4, and other histones)
- Non-histone proteins
  - Structural proteins associated with chromosomes
  - Involved in gene regulation

Histones

Five classes of histones (H1, H2A, H2B, H3, H4)
Rich in lysine and arginine amino acids.
Histones are basic proteins.

Nucleosomes

Nucleosomes are the fundamental structural units of chromatin.
A nucleosome core particle consists of 146 base pairs (bp) of DNA wrapped around an octamer of histone proteins (2 H2A, 2 H2B, 2 H3, 2 H4).

Importance of DNA Packaging

DNA packaging allows DNA to fit within the cell.
Protecting the DNA from damage.
Efficient DNA transmission during cell division.
Facilitating gene expression.

DNA Replication

Replication is the process of duplicating the entire genome prior to cell division.
Biological significance:
- Extreme accuracy is essential for preserving genome integrity across generations.
- In eukaryotes, replication is restricted to the S phase of the cell cycle.
  - Slower replication rates result in higher fidelity.
Prokaryotic DNA polymerase: 1000 bases per second
Eukaryotic DNA polymerase: 50 bases per second

Basic Rules of Replication

Semi-conservative: Each new DNA molecule is composed of one original strand and a newly synthesized strand.
Starts at the origin: replication initiates at specific sites on the DNA.
Synthesis in the 5' to 3' direction: DNA polymerases add nucleotides only to the 3' end of the growing strand.
Semi-discontinuous: DNA synthesis is continuous on the leading strand and discontinuous on the lagging strand.
RNA primers are needed: DNA polymerase cannot initiate synthesis; RNA primers provide a starting point.

DNA Replication Bubbles and Forks

Replication bubbles eventually fuse to form newly replicated strands.
Replication forks are Y-shaped regions where DNA is unwound and new strands grow.
Linear eukaryotic replication proceeds from multiple origins and produces two linear DNA molecules.

Direction of Replication

DNA polymerases always add nucleotides to the 3' end of a growing strand.
Replication proceeds in the 5' to 3' direction.

Semi-discontinuous Replication

Leading strand synthesizes continuously in the 5' to 3' direction.
Lagging strand synthesizes discontinuously in the 5' to 3' direction via Okazaki fragments.

RNA Primers

RNA primers are short RNA sequences needed to initiate DNA synthesis.
RNA primers are synthesized by primase.
Primers are removed and replaced by DNA polymerase.

DNA Polymerase

DNA polymerase I: removes RNA primers and fills in gaps with DNA, particularly on the lagging strand.
DNA polymerase III: elongates the new DNA strands by adding nucleotides.

Okazaki Fragments

Short, discontinuous DNA fragments synthesized on the lagging strand.
DNA ligase links adjacent Okazaki fragments.

DNA Ligase

Joins DNA fragments, especially Okazaki fragments, together.

Core Proteins at the Replication Fork

Topoisomerases: prevent supercoiling ahead of the replication fork.
Helicases: separate the DNA strands.
Primase: synthesize RNA primers.
Single-strand binding proteins: stabilize the single-stranded DNA.
DNA polymerase: synthesizes new DNA strands.
DNA ligase: joins Okazaki fragments.

Mechanism of DNA Replication

Initiation: Proteins bind to DNA, opening it up for complementary base pairing.
Elongation: Proteins add nucleotides to the growing strand to form new DNA.
Termination: Proteins release the replication complex.

When and Where Replication Occurs

In eukaryotes, replication occurs during the S phase of interphase.
DNA replication takes place within the nucleus of eukaryotic cells.

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Description

Explore the intricate processes of DNA packaging and the structure of chromosomes in this quiz. Learn how DNA is protected and organized within the cell nucleus, and understand the composition and significance of normal human chromosomes. This quiz covers essential concepts related to genetics and cellular biology.