Cell Size and the Cell Cycle

Why do cells divide? The larger a cell becomes: the more demands are placed on its DNA. the more trouble a cell has moving materials across the cell membrane.

1. DNA Demand as cells grow larger, extra copies of DNA are not typically made Therefore, as a cell grows, there is greater demand placed on the information contained in its DNA. “harder to get to the instructions” as the cell gets bigger In time the cell would not be able to keep up with the demands for information from its DNA.

2. Material Transport Issues As cells grow larger it becomes more difficult to efficiently move nutrients into the cell and wastes out of the cell by diffusion. the smaller a cell, the larger the surface area to volume ratio large surface area to volume ratios make cells efficient Cell Size 1x1x1 cm 2x2x2 3x3x3 Surface Area 6cm2 24cm2 54cm2 Volume 1cm3 8cm3 27cm3 Surface area to volume ratio 6:1 3:1 2:1

Cell Division when a cell becomes too large it divides to form two new “daughter” cells through cell division before this happens the DNA is replicated each new daughter cell gets a complete set of the genetic information cell size is reduced

Why is having “new” cells helpful? “new” cells helps us: grow heal replace dead cells

Cell Cycle repeating sequence of cell growth and division during an organism’s lifetime During the cell cycle a cell grows, prepares for division, and divides to form two daughter cells, each of which then begins the cell cycle again. the cell cycle is regulated by proteins that receive and send signals (checkpoints in the cell cycle)

Cell Cycle Phases G1 Gap 1 S Synthesis   G2 Gap 2 M Cell Division

Cell Cycle Phases G1 Gap 1 S Synthesis G2 Gap 2 Interphase   G2 Gap 2 M Cell Division Interphase

Interphase every phase except ‘M’ Phase (cell division) cells spend most of their lives in these phases (G1, S, and G2)

Interphase G1 First Gap Phase S Synthesis Phase grow and make new proteins and organelles S Synthesis Phase chromosomes (DNA) are replicated

DNA Replication making copies of DNA DNA strands are held together by WEAK hydrogen bonds these WEAK hydrogen bonds are easily broken by an enzyme

-ase suffix often used to identify enzymes

Helicase enzyme that breaks the double helix of DNA into two single strands “old strands” are called “parent strands”

Semiconservative Replication each single strand then serves as a template (guide) for a new complementary strand

Semi-conservative Replication each parent strand remains intact Glencoe Biology, Fig. 13-7, p. 212

Semiconservative Replication replicated DNA molecules are half “old” and half “new” “new strands” are called “complementary strands”

Replication Fork areas where the double helix is split

Replication Fork areas where the double helix is split can occur in more than one place speeds up replication

DNA Polymerases a major group of enzymes involved in DNA replication some polymerases add complementary nucleotides to each DNA strand after the strands have been “unzipped” by helicase A always paired with T (and vice versa) C always paired with G (and vice versa)

DNA Polymerases other polymerases will proofread the newly produced strand and correct any mistakes in base pairing

Base Pairing during Replication

Base Pairing and Semiconservative Replication Below is a group of letters that represents the nitrogen-containing bases in a double stranded piece of DNA. What would the new pieces of DNA look like after replication? A T C G G C T A

Base Pairing and Semiconservative Replication Bases from the parent strands are in white text while bases on the newly produced complementary strands are in black text. A T C G G C T A A T A T C G C G G C G C T A T A

Cell Cycle Phases G1 Gap 1 S Synthesis   G2 Gap 2 M Cell Division

Why does DNA need to be replicated for a cell to divide? DNA contains “instructions” for building a cell and for completing the cell’s function the “instructions” come in the form of genes the genes provide instructions for how to make proteins the cell needs each new cell will need a full copy of the instructions to produce proteins need for the cells’ survival

Cell Cycle Phases G1 Gap 1 S Synthesis   G2 Gap 2 M Cell Division

Interphase G1 First Gap Phase S Synthesis Phase G2 Second Gap Phase grow and make new proteins and organelles S Synthesis Phase chromosomes (DNA) are replicated G2 Second Gap Phase shortest phase when the is preparing for mitosis organelles and molecules required for cell division are produced

Cell Cycle Phases G1 Gap 1 S Synthesis   G2 Gap 2 M Cell Division

M Phase Cell Division divided into two parts Mitosis Cytokinesis division of the cell’s nucleus Cytokinesis division of the cell’s cytoplasm includes division of organelles like mitochondria and chloroplasts