Cell Structure and Function Lecture 26

The Two Main Divisions

The cycle is split into M Phase (physical division) and Interphase (growth and prep).

G1 Phase

Gap Phase 1

Growth: The cell gets bigger and prepares for DNA replication.

S Phase

Synthesis Phase

DNA Replication: The genetic code is copied (2N → 4N).

G2 Phase

Gap Phase 2

Checkpoint: The cell double-checks that DNA was copied correctly.

M Phase

Mitosis

Division: The cell physically splits into two.

What are the longest and shortest parts of the cell cycle

The longest part of the cell cycle is interphase, which includes G1, S, and G2 phases, while the shortest part is mitosis (M phase) during which the cell divides.

What is the most variable phase of the cell cycle

G1 Phase. Its length changes the most between different types of cells (typically 8–10 hours).

S Phase Duration

Typically takes 6–8 hours to copy all the DNA.

G2 Phase Duration

Typically takes 4–6 hours for final checks and prep

G0 Phase

A "resting" state where the cell exits the cycle and stops dividing.

Permanent Go vs Temporary G0

Permanent: Terminally differentiated cells (like neurons) that stay in G0 forever unless they become cancerous.

Temporary: Cells (like stem cells) that wait in G0 until they are needed for things like wound healing.

G1 to G0 Transition

Triggered by Differentiation Factors that tell a cell to become specialized (e.g., becoming a liver cell).

G0 to G1 Transition

Triggered by Growth Factors (like EGF) that tell the cell it is time to multiply again.

Why is G1 needed?

To increase cell volume. Without it, daughter cells would get smaller and smaller every time they divided

Fast-Dividing Cells

These cells have a very short or even no G1 phase because they don't stop to grow

Embryonic Cells

They skip G1 because the initial egg is already huge; they can divide many times without needing to grow first.

What determines S Phase length?

The number of active DNA Replicons (starting points for copying).

Speeding up S Phase

To finish faster, the cell activates all replicons at once rather than doing them in small groups.

The Result of S Phase

The total amount of DNA in the cell doubles.

What is G2 Phase?

A final growth phase that serves as the "pre-game" for Mitosis

What di Meselson-Stahl prove?

DNA replication is Semi-Conservative (each new DNA molecule is 1/2 "old" and 1/2 "new")

What are the two labels of the Meselson-Stahl Experiment

15N (heavy) and 14N(light). These are isotopes of nitrogen used to weigh the DNA.

Centrifugation role in the Meselson-Stahl Experiment

Generation 1 result of the Meselson-Stahl Experiment

A single "Hybrid" band in the middle. This proved the DNA was half-heavy and half-light

Generation 2 result of the Meselson-Stahl Experiment

Two bands: One Hybrid and one Light. This confirmed the semi-conservative model.

Meselson-Stahl

Used Nitrogen to prove HOW DNA replicates (Semi-conservative).

Hershey-Chase

Used Phosphorus & Sulfur to prove THAT DNA is the genetic material (not protein).

Pulse-Chase

A technique to track a molecule's path over time by "tagging" it (the pulse) and then following it through the cell (the chase)

Meselson-Stahl Steps

1. The Start

• Setup: All 15N

• Result: One Low Band

• Logic: 100% heavy DNA

2. Gen 1

• Setup: 15N moved to N14 for one round

• Result: One middle Band

• Logic: Disproved conservative theory

3. Gen 2

• Setup: Stayed in 14N for 2nd round

• Result: One middle, one high

• Logic: Proved that the hybrid strands split to create pure light copies.

What is a Replicon?

An individual unit of DNA that has its own Origin of Replication (starting point).

Replication Bubbles

The open areas of DNA where copying is happening. They grow in both directions until they meet and fuse

Why many origins?

Eukaryotic chromosomes are too long to start at one end. Multiple origins allow the cell to finish the job much faster.

Location Matters

Early replication happens in the nucleus interior; Late replication happens at the periphery (edges).

What is "Licensing"?

A "tagging" system that ensures every origin of replication fires exactly once per cell cycle.

G1 Phase (The Green Light) of licensing

Licensing is Active. Proteins like ORC and MCM "license" the origins to get them ready.

S Phase (The Red Light) of licensing

Licensing is Inhibited. Proteins like Geminin stop new licenses from forming so you don't copy the same DNA twice.

Why is licensing vital?

It prevents "re-replication," which could lead to extra copies of genes and cancer.

DNA Polymerase Rule #1

It can only add new nucleotides to the 3' end of an existing strand

Leading Strand

The new strand that grows continuously in the same direction as the replication fork moves.

Lagging Strand

The new strand that grows discontinuously in the opposite direction of the fork.

Okazaki Fragments

Short "chunks" of DNA created on the lagging strand that are later glued together.

DNA Ligase

The "molecular glue" that joins Okazaki fragments into one solid strand.

DNA Helicase

Enzyme that unwinds the DNA double helix during replication.

Topoisomerase

Enzyme that relieves the tension and supercoiling ahead of the replication fork by making temporary cuts in the DNA strands.

ssDNA Binding Proteins

The Bodyguards: Coat the single strands to keep them from "re-zipping" or being destroyed by enzymes

Where does the energy for the Phosphodiester bond come from?

nucleotides

DNA Polymerase Rule #2

It cannot start a new chain from scratch; it can only add nucleotides to an existing chain.

What is an RNA Primer?

A short stretch of RNA (3–10 nucleotides) that provides the initial "hook" for DNA polymerase

Primase

The enzyme that performs "de novo" synthesis—it builds the RNA primer from scratch using the DNA template

What happens to Primers?

They are temporary. They are later removed by an enzyme with 5'–3' exonuclease activity

De Novo vs. Primed

De Novo: Building from nothing (Primase).

Primed: Adding to an existing chain (DNA Polymerase).

DNA Polymerase Dual Roles

It has two main jobs: Building (5'–3' Polymerase) and Editing (3'–5' Exonuclease).

3'–5' Exonuclease Activity

The "Delete Key." If the wrong base is added, the enzyme moves backward to "snip" it out.

Why is proofreading vital

It ensures "high fidelity," meaning the genetic code is copied with extremely few errors.

Direction of Proofreading

It happens in the 3' to 5' direction (the opposite direction of synthesis).

DNA Synthesis: What Happens and in what direction?

Building the new strand.

5' → 3'

Proofreading: What Happens and in what direction?

Removing a mismatched base at the end.

3' → 5'

Primer Removal: What Happens and in what direction?

Chewing up the RNA "starter" to replace it with DNA.

5' → 3'