Ch 3.3 The Nucleus & DNA Replication

What is the Nucleus Structure?

• The nucleus is enclosed by the nuclear envelope, This envelope is continuous with the ER and is perforated by nuclear pores.

What is the nuclear envelope?

The nuclear envelope is a double membrane structure composed of two lipid bilayers separated by a narrow space.

What are Nuclear Pores?

Nuclear pores are large protein complexes that cross the nuclear envelope, regulating the transport of molecules between the nucleus and the cytoplasm.

What does DNA Organization Mean?

• Within the nucleus, DNA does not exist as free strands but is intricately organized with proteins into chromatin.

What is Chromatin?

Chromatin is a complex of DNA and proteins found in the nucleus, which condenses to form chromosomes during cell division.

• Has two main forms : euchromatin and heterochromatin.

What is Euchromatin?

Euchromatin is a less condensed form of chromatin that is associated with active gene expression, allowing easier access for transcription machinery.

What is Heterochromatin?

Heterochromatin is a densely packed form of chromatin that is generally transcriptionally inactive, playing a role in maintaining the structural integrity of chromosomes and regulating gene expression.

What are Chromosomes?

• chromatin condenses dramatically to form discrete, rod-shaped chromosomes

What is a Nucleolus

• A dense, non-membrane-bound structure within the nucleus. Its primary role is the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits (which are then exported to the cytoplasm to form functional ribosomes).

What is Semi-Conservative Replication?

Semi-conservative replication is the mechanism by which DNA is replicated, where each new DNA molecule consists of one original strand and one newly synthesized strand, ensuring genetic continuity.

What are the Stages Of Semi-Conservative Replication?

The stages of semi-conservative replication include initiation, elongation, and termination. during elongation, new nucleotides are added to each template strand; and in termination, the replication process concludes as the newly synthesized DNA molecules separate.

What is the Initiation Stage of Semi-Conservative Replication?

In initiation, the DNA strands unwind and separate;this process is facilitated by helicase enzymes that break hydrogen bonds between the base pairs.

What does Helicase Do?

• unwinds the double helix, separating the two parental DNA strands. Single-strand binding proteins stabilize the separated strands to prevent them from re-annealing.

What is Topoisomerase?

Topoisomerase is an enzyme that alleviates the torsional strain generated ahead of the replication fork by introducing temporary breaks in the DNA strands, allowing them to unwind and preventing supercoiling.

What is the Elongation Stage of Semi-Conservative Replication?

• Primase synthesizes short RNA primers, providing a starting point for DNA synthesis. DNA polymerase III then adds new complementary DNA nucleotides to the free 3' end of the growing strand, moving in a 5' to 3' direction. Replication proceeds simultaneously on both strands but differently due to their antiparallel nature.

What is a Leading Strand?

The leading strand is the continuously synthesized DNA strand during replication, which is built in the same direction as the replication fork movement, allowing for smooth and uninterrupted elongation.

What is the Lagging Strand?

The lagging strand is the DNA strand synthesized discontinuously during replication, formed in short segments called Okazaki fragments, which are later joined together. This occurs opposite to the direction of the replication fork movement.

What is the Termination Phase?

The termination phase is the last stage of DNA replication, where the newly synthesized DNA strands are completed, and the replication machinery is dismantled. Enzymes such as DNA ligase play a critical role in sealing gaps between Okazaki fragments on the lagging strand.

What are Okazaki Fragments?

Okazaki fragments are short segments of DNA synthesized discontinuously on the lagging strand during DNA replication, which are later joined together by DNA ligase to form a complete strand.

What is Primase?

Primase is an enzyme that synthesizes short RNA primers during DNA replication, providing a starting point for DNA polymerase to extend and synthesize new DNA strands.

What is DNA Polymerase I & III ?

• DNA polymerase III is the primary enzyme for synthesizing new DNA strands; DNA polymerase I removes RNA primers and fills in the gaps.

What is the Proofreading and Repair Function?

The proofreading and repair function of DNA polymerases ensures the accuracy of DNA replication by detecting and correcting errors, such as mismatched bases. This mechanism helps maintain genetic stability.