Assessment 4 BIO 1107

Why do cells divide?

Growth, replacement, repair

4 major phases of the cell cycle

G1, S, G2, M

What is the G1 phase in the cell cycle?

Cell growth, receiving signal

What is the S phase in the cell cycle?

DNA replication

What is the G2 phase in the cell cycle?

Checking for completion and damage, repairs damage

What is the M phase in the cell cycle?

Mitosis and cytokinesis

What is G0 phase in the cell cycle?

Resting phase

How does flow cytometry work?

Cells are stained with fluorescent dye that binds to DNA and depending on level of florescence, distinguishes cells in different phases

What does a survival factor do in a cell?

Promote cell survival by preventing apoptosis

Apoptosis

Programmed cell death

What does a growth factor do in a cell?

Stimulates cells to grow and increase in size

What do mitogens do in a cell?

Stimulate cells to enter and progress through the cell cycle

What do checkpoints do in the cell cycle?

Monitor internal conditions before allowing cell cycle progression

What does the G1 to S phase checkpoint look for?

If environment is favorable

What does the G2 to M phase checkpoint look for?

If DNA is undamaged and fully replicated

What does the M phase checkpoint look for?

If chromosomes are attached to mitotic spindle

Cyclins

Regulatory proteins whose concentrations rise and fall throughout the cell cycle

CDKs

Cyclin-dependent kinases. Protein kinases that become active when bound to cyclins

Cyclin-CDK complexes

Phosphorylate target proteins, promoting progression through the cell cycle

CDK is always present, but what does it need to become active?

Cyclin

What happens if cyclin is at a high concentration?

CDK is activated and pushes cells through the cell cycle

What happens if cyclin is ALWAYS at a high concentration?

CDKs remain permanently activated, resulting in uncontrolled cell division and proliferation

What happens if cyclin is at a low concentration?

CDK is inactive, the cell cycle is not active

What happens if cyclin is ALWAYS at low concentration

CDK remains permanently inactive, resulting in the cessation of the cell cycle. No more DNA replication or division, cells cannot be repaired nor replaced

What is Rb?

A protein that binds to E2F to stop cell division. Known as the bodys most important tumor suppressor

What happens if the Rb protein is broken / inactive?

Cells divide uncontrollably

What protein controls entry in the S phase?

Rb

What do cyclin-CDK complexes do to Rb proteins?

Phosphorylate it, causing it to release E2F

What does Free E2F do?

Activates genes required for entry into the S phase

What does the phosphorylation of Rb do?

Releases E2F and allows cells to progress through the cell cycle to DNA replication

In which phase of the cell cycle does DNA replication occur?

S phase

What is the replication fork?

The Y-shaped region where 2 strands of the parental DNA helix are unwound and separated into single strands so new complementary strands can be formed

When does DNA replication begin?

When the two parental strands in DNA separate and form the replication fork

What does helicase do in the replication fork?

Unwinds DNA by breaking hydrogen bonds between complementary bases

After helicase unwinds the DNA into 2 separate strands, what stops them from rejoining?

Single-strand binding proteins (SSBPs)

What is topoisomerase?

A enzyme that relieves stress and tension ahead of replication fork during unwinding process

Why is topoisomerase important?

Prevents DNA from tangling or straining, which prevents breakage or stalling of replication

Why can’t you inject more Rb into cells to stop cancer?

Overexpressed Rb would lock cells in their given state

What happens if single-strand binding proteins stopped working?

The parental strands would snap back together and DNA polymerase and primase can no longer access the template strands to read them or build new strands

Why do separated DNA strands tend to snap back together after helicase unwinds them?

Complementary nitrogen bases of DNA are naturally attracted to one another via hydrogen bonds

What is DNA polymerase?

An enzyme that synthesizes a complementary strand from a single parental strand in DNA replication

What does DNA polymerase need to begin synthesis?

Primer

What end of a DNA strand can DNA polymerase add nucleotides to?

3’

What direction is DNA synthesized in?

5’ to ‘3

What helps DNA replications error from happening?

DNA polymerase proofreads and corrects any errors found

What happens if DNA polymerase stops working entirely?

DNA replication stops and triggers internal cell checkpoints to start apoptosis because DNA is half replicated or damaged as a result. Cells can no longer divide to replace dead / dying cells

What happens if DNA polymerase’ ability to proofread or create the correct nucleotide base is impaired?

Cells have another repair enzyme as backup. Otherwise, the error becomes a permanent mutation the next time the cell divides

What can happen with mutations in DNA?

It can do nothing, alter a protein function, or cause cancer

What is a leading strand?

Synthesized continuously TOWARD the replication fork

What is a lagging strand?

Synthesized in short, discontinuous chunks AWAY from the replication fork

What are okazaki fragments?

Short, complementary segments of DNA synthesized discontinuously on the lagging strand in the correct 5’ to 3’ direction. Are sealed into a continuous strand by DNA ligase

Direction of DNA replication

Bidirectional towards 5’ and away from 3’ to 2 different replication forks

What is DNA ligase?

Enzyme that binds the okazaki fragments together. Catalyzes phosphodiester bond to join the 5′ phosphate group and 3′ hydroxyl group

What is primase?

Enzyme that creates short RNA primer (3’ hydroxyl group) to attach to a point on parental strand that allows polymerase to begin synthesizing a new DNA strand from that point

RNA Primer

Produced by primase, a short segment of ribonucleotides that act as a start point for DNA polymerase to begin replication. After new DNA strand is built, its replaced by DNA nucleotides

After a new DNA strand is formed, what seals the remaining breaks in the sugar-phosphate backbone?

DNA ligase

What would happen if DNA ligase stopped working?

The leading strands would be mostly fine, but the lagging strands would remain segmented. Phosphodiester bonds would not form between okazacki fragments and leaves the lagging strand extremely fragile, and DNA would not be able to hold its double helix shape

What is mitosis?

The process of a single cell dividing to make 2 identical daughter cells

What are the 4 stages of mitosis?

G1, S, G2, M

In mitosis, what are the 5 stages in the M phase?

Prophase, prometaphase, metaphase, anaphase, telophase and cytokinesis

What does DNA replication form?

Sister chromatids

What are sister chromatids?

2 identical copies of a single chromosome that are connected to each other at the centromere, creating an X shape

What is the prophase in mitosis?

First phase of mitosis where chromosomes are condensed to save space and make them easier to manipulate

What stages are in the interphase?

G1, S, G2

What are 2 important events of the S phase?

DNA replication forms sister chomatids, centrosomes are duplicated

What is a centrosome?

Microtubule organizing center