AP Bio Unit 5: Cell Communication & Cell Cycle

DNA Function

Stores genetic info & is the blueprint for building proteins

DNA Structure

• Double-stranded helix (2 sugar phosphate backbones)

• Nucleic acids (polymer) are made up of nucleotide monomers

• Nucleotides have three parts:

  • Phosphate Groups (- charged)

  • Deoxyribose (5-carbon sugars)

  • Nitrogen Bases

    • Adenine

    • Thymine

    • Guanine

    • Cytosine

RNA Structure

• Single-stranded (1 sugar phosphate backbone)

• Ribose Sugar

• Nitrogen Bases:

  • Adenine

  • Uracil

  • Cytosine

  • Guanine

Nucleotides are connected by a ______ bond between the sugar of one and the phosphate group of the other

Phosphodiester

In DNA, the complementary nitrogen bases are connected via ______ bonds.

Hydrogen

Nucleotides are attached together through ______ ______.

Dehydration Synthesis

Pyrimidines

Nitrogen bases that are single ringed - C, T, U

Purines

Nitrogen bases that are double ringed - A, G

Pairing of Nucleotides

• Pyrimidines always bond with purines

• A & T = 2 Hydrogen Bonds

• G & C = 3 Hydrogen Bonds

Deoxyribose has __ Carbon atoms, which are numbered clockwise.

5

DNA Directionality

DNA is antiparallel because complementary strands run in opposite directions (5’ → 3’ & 3’ → 5’)

DNA Synthesis

Occurs in the S phase of the cell cycle, when the DNA is in chromatin form

1. Cell reproduction (mitosis)

2. Gamete production (meiosis)

Semiconservative Process (in DNA synthesis)

The two DNA strands are complementary, which means base pairing allows each strand to serve as a template for a new strand

Helicase in DNA Replication

Unwinds part of the DNA double helix

Topoisomerase in DNA Replication

Helps relieve the strain of unwinding by breaking, swiveling, and rejoining DNA strands

DNA Polymerases in DNA Replication

Connects nucleotides together to make a strand

RNA Polymerase (a.k.a Primase, DNA Primase, etc) in DNA Replication

Adds a few nucleotides of RNA (RNA primer) to get the process started

Ligase in DNA Replication

Connects DNA fragments together

DNA Synthesis: Step 1

DNA Helicase unwinds the DNA strands. Topoisomerase relaxes supercoiling in front of the replication fork.

DNA Synthesis: Step 2

Complementary nucleotides are matched with the ones of the original DNA parent strand to create a new strand:

1. RNA polymerase (primase) adds a few nucleotides so DNA polymerase can get started; the RNA nucleotides are later replaced

2. DNA polymerase connects the nucleotides but can only add nucleotides to the 3’ end of a nucleotide

Leading Strand

Once an RNA primer is added, DNA polymerase can continuously add nucleotides in the 5’ to 3’ direction - the DNA strand is copied in a continuous way

Lagging Strand

Made in Okazaki fragments that are later joined together by ligase - the DNA strand is copied discontinuously

→ RNA primers are later removed and replaced by DNA nucleotides

Telomeres

The ends of chromosomes in eukaryotes have repeating, non-coding sequences called telomeres that serve as protective caps

→ Loss of bases at 5′ ends in every replication: Chromosomes get shorter with each replication (limits # of cell divisions to about 50) → aging process

Telomerase

Can add DNA bases at 5’ end; high activity in stem cells and cancers but not in most somatic cells

The cell cycle consists of two main stages:

Interphase (G1, S, G2) & Cell Division (Mitosis & Cytokinesis)

How do unicellular organisms use the cell cycle?

Binary Fission

How do multicellular organisms use the cell cycle?

Growth & repair

Nucleus in the Cell Cycle

Protects the DNA

Cytoskeleton in the Cell Cycle

• Organizes structures in the cell

• Includes the centrosomes, made up of centrioles, which are responsible for the spindle fibers that guide the chromosomes during mitosis

Spindle Fibers

• Made of microtubules

• Extend from the centrosome

• Separates duplicated chromosomes during Anaphase by attaching to the chromesomes’ centromeres

Cells spend 90% of their time in ______.

Interphase

G1

• 1st gap

• Everyday tasks, such as making proteins

• Cell grows

G₀

• Resting stage

• Cells could enter this stage during G1

• Cell continues doing its job until it receives a signal to reenter G1 to get ready to divide

S

• DNA Synthesis

• Copies genetic material so each cell gets a copy

G2

• 2nd Gap

• Prepares for cell division

• Cell grows more

• Produces proteins, organelles, & membranes

The four stages of mitosis are:

Prophase → Metaphase → Anaphase → Telophase

Prophase

• Chromatin condenses into chromosomes, and chromosomes become visible

• Centrioles (in an animal cell) move to opposite ends of the cell

• Protein fibers form across the cell

• The nucleolus disappears

• The nuclear membrane breaks down

Metaphase

• Chromosomes line up in the middle of the cell

• Spindle fibers (attached to kinetochores) coordinate movement

Anaphase

• Sister chromatids separate at kinetochores

  • Proteins holding the sister chromatids together are inactivated

  • Pulled by motor proteins “walking” along microtubules

• Poles move farther apart

Telophase

• Chromosomes arrive at opposite poles

• Daughter nuclei form

• Chromosomes disperse

• Spindle fibers disperse

• Cytokinesis begins

Cytokinesis

Organelles & cytoplasm are divided

Cytokinesis in Animal Cells

• Microfilaments contract, forming a cleavage furrow

Cytokinesis in Plant Cells

• Cell plate forms

• Vesicles from the Golgi fuse to form two cell membranes

• A new cell wall is laid down between the cell membranes

G1 Checkpoint

• Do I need a new cell?

• Is this cell healthy?

• Are there enough nutrients to divide?

Pass - Cell enters S phase

Fail - Goes to G₀

G2 Checkpoint

Did the DNA copy correctly in the S phase?

Pass - Cell enters mitosis

Fail - Apoptosis

M Checkpoint

When the DNA lines up in the middle (metaphase), will each cell get the same amount of DNA?

Pass - Cell divides

Fail - Apoptosis

Kinases

Proteins that activate or inactivate other proteins by phosphorylating them

Cyclin-Dependent Kinases (Cdks)

Kinases that are only active when attached to a cyclin

Cyclin

A protein that fluctuates in concentration in the cell

• Made at certain times during the cell cycle

• Also destroyed after they are no longer needed by the cell

Benign Cancer Cells

Stays in the same place

Malignant Cancer Cells

Spreads to other parts of the body

Cancer cells bypass ______.

Cell cycle controls

Proto-Oncogenes - Normal Function

When activated, they signal for cell division to start (G1 checkpoint)

Proto-Oncogenes - Mutation Function (called an oncogene)

• The gene is always activated, so it continues to divide (ignores the G1 checkpoint)

• Dominant - only one copy of the defective gene is needed to impact the cell

Tumor Suppressor Genes - Normal Function

Slow cell division, repairs mistakes, or triggers apoptosis

Tumor Suppressor Genes - Mutated Function

• Cell does not stop division if mistakes are found

• Recessive - both copies of the gene must be mutated to impact the cell

• Ex: p53