HAN 200 Cell Division

Interphase

DNA replication

Majority of cell’s life spent in Interphase

• Nuclear material called chromatin

• Four subphases:

• G1 (gap 1)—vigorous growth and metabolism

• G0—gap phase in cells that permanently cease dividing, nondividing state

• S (synthetic)—DNA replication (duplicates chromosomes)

• G2 (gap 2)—preparation for division, makes more organelles, repairs any errors

DNA Replication

(Occurs during Interphase)

Helicase untwists the double helix and exposes complementary chains

• The Y-shaped site of replication is the replication fork

• Each nucleotide strand serves as a template for building a new complementary strand

Mitotic (M) phase

Moves DNA from mother cell to 2 daughter cells

• Essential for body growth and tissue repair

• Does not occur in most mature cells of nervous tissue, skeletal muscle, and cardiac muscle, repairs are made with scar tissue.

Prophase early phase

Chromosomes become visible, each with two chromatids joined at a centromere

Centrosomes separate and migrate toward opposite poles

Mitotic spindles and asters form

Prophase late phase

Nuclear envelope fragments

Kinetochore microtubules attach to centromeres and draw them toward the equator of the cell.

Metaphase

Centromeres of chromosomes are aligned at the equator

This plane midway between the poles is called the metaphase plate.

Anaphase

Shortest phase

Centromeres of chromosomes splits simultaneously – each chromatid now becomes a chromosome

Chromosome (V shaped) are pulled toward poles by motor proteins of kinetochore

Telophase

Begins when chromosome movement stops

The two sets of chromosomes uncoil to form chromatin

New nuclear membrane forms around each chromatin mass

Nucleoli reappear, spindle disappears

Cytokinesis

Ring of actin microfilaments contracts to form a cleavage – “draw string”

Go signals

Critical volume of cell when area of membrane is inadequate for exchange – gets too big

• Chemicals (e.g., growth factors, hormones, cyclins, and cyclin-dependent kinases)

Stop signals

Contact inhibition – contact their neighbor cell

• An absence of contact inhibition, leading to uncontrolled cell division , is one of the characteristics of cancer

Protein Synthesis

DNA is the master blueprint for protein synthesis

• Gene: Segment of DNA with blueprint for one polypeptide (protein)

• Four nucleotide bases (A, G, T and C) are the “letters” in the genetic

alphabet

• Triplets of nucleotide bases form genetic library (triplets = words)

• Each triplet specifies coding for one amino acid

• GCA = alanine

mRNA

Carriers instructions for building a polypeptide, from gene in DNA to ribosomes in cytoplasm

rRNA

Helps translate messages from mRNA

tRNA

Bring amino acids to the ribosomes

Transcription

occurs in the nucleus

Information is transferred from one format to another

• Transfers DNA gene base sequence to a complementary base sequence of an mRNA

• RNA polymerase – enzyme that oversees the synthesis of mRNA

• Formation of mRNA

Translation

From one language to another; language of nucleic acids to language of amino acids

• Converts base sequence of nucleic acids into the amino acid sequence of proteins

• Involves mRNAs, tRNAs, and rRNAs

Genetic Code

Each three-base sequence on DNA is represented by a codon

• Codon—complementary three-base sequence on mRNA

• Certain codon start (initiation) or stop (termination) production

• Each codon will specify a different amino acid