Cell Growth and Division

Cell Growth When living things grow what happens to its cells? Living things grow by producing more cells. On average, the cell size of an adult animal = the cell size of a young animal. There are just more! Does an animal get larger because each cell increases in size or because it produces more of them?

Limits to Cell Growth 2 main reasons why cells divide rather than continue to grow indefinitely The larger the cell becomes, the more demands the cell places on its DNA The cell has more trouble moving enough nutrients and wastes across the cell membrane

Limit to Cell Growth: DNA overload As cell increases in size, it does not make extra copies of DNA…causes information crisis A larger cell would have to make greater demands on its available genetic library = DNA Analogy: if a small town’s library had only a few thousand books, and the population grew, there would be more people borrowing books and greater demand on the popular books.

Limits to Cell Growth: Exchange Cell size is limited because the rate at which food and oxygen are used up and waste products produced depends on the cell’s volume If a cell got too large, it would be more difficult to get sufficient amounts of oxygen and nutrients in and waste products out.

Cell Size Small cells function more efficiently Small cells can exchange substances more readily than larger cells because small objects have a higher surface area-to-volume ratio than larger objects As a result, substances do not need to travel as far to reach the center of a smaller cell Analogy: a main street in the town. As the population grew, more and more people would walk on the street. The size of the street is not getting bigger, so people would encounter more and more traffic.  

Higher SA:V, the more efficient Relative Cell Size

Higher SA:V, the more efficient

Division of the Cell Before it becomes too large, a growing cell divides to form two daughter cells Each daughter cell gets one complete set of genetic information By dividing the cell, surface area to volume ratio will be high and allow for efficient exchange

Cell Division

Cell Division Cell division is at the heart of the reproduction of cells and organisms Organisms can reproduce sexually or asexually

Asexual Reproduction: Some organisms make exact copies of themselves Review Cell Theory

Cells arise only from pre-existing cells All cells come from other cells Cellular reproduction is called cell division Allows an embryo to develop into an adult It also ensures the continuity of life from one generation to the next

Prokaryotes reproduce by binary fission Prokaryotic cells divide asexually These cells contain a single chromosome, containing genes The chromosome is replicated The cell then divides into two cells, a process called binary fission

Binary Fission of Prokaryote Plasma membrane Prokaryotic chromosome Cell wall Duplication of chromosome and separation of copies Continued growth of the cell and movement of copies Division into two cells

Multicellular Organisms Grow and Develop by Mitosis

Cell Division : Mitosis In multicellular organisms, mitosis is the source of new cells during growth and development Mitosis occurs in humans shortly after the egg is fertilized- produces the vast number of cells needed for the embryo to take form

Cells divide at different rates. The rate of cell division varies with the need for those types of cells. Liver cells go through mitosis about once a year.   Skin and intestinal cells go through mitosis every few hours.

Large, complex chromosomes of eukaryotes duplicate with each cell division A eukaryotic cell has many more genes than a prokaryotic cell. The genes are grouped into multiple chromosomes

Chromosomes Humans have 46 chromosomes Chromatin condense to form chromosomes only during cell division 23 pairs

Chromosomes Before cell division, each chromosome is replicated. Same genetic information Before cell division, each chromosome is replicated. One half of a duplicated chromosome is a chromatid

Chromosome distribution to daughter cells When the cell divides, the sister chromatids separate Chromosome duplication Sister chromatids Centromere Two daughter cells are produced Each has a complete and identical set of chromosomes Chromosome distribution to daughter cells Figure 8.4C

The cell cycle multiplies cells The cell cycle consists of two major phases: Interphase, where DNA duplicates and cell parts are made The mitotic phase, when cell division occurs Figure 8.5

The Cell Cycle a cell grows, prepares for division, and divides to form 2 daughter cells, each of which begin the cycle again. 2 phases Interphase G1 phase (gap) S phase (DNA replication or synthesis) G2 phase (gap) M phase mitosis and cytokinesis

The Cell Cycle

Interphase Made up of G1, S, G2 phase Gap 1 (G1): cells increase in size Synthesis (S) phase: DNA is replicated

Interphase S-Phase DNA must be duplicated… cell nucleus cell nucleus chromosomes in cell 4 single-stranded chromosomes nucleus cell DNA in chromosomes nucleus cell duplicated chromosomes duplicated chromosomes 4 double-stranded chromosomes

Interphase Made up of G1, S, G2 phase Gap1 (G1): cells increase in size, organelles duplicate Synthesis phase: DNA is replicated Gap2 (G2): shortest phase, cells check for mistakes, make needed repairs