Cell Reproduction/Mitosis

Chapter 12

What you need to know!

• Mitotic Cell division results in genetically identical daughter cells

• The mitotic phase alternates with interphase in the cell cycle

Overview

I. Cell Division Overview

II. The Cell Cycle

III. Mitosis – nuclear division/cytokinesis

IV. Stages of Mitosis

Cell Division

• Life depends on the ability of cells to reproduce/copy

• Cell Division = 1 cell divides into 2

• Unicellular organisms cell division = reproduction– Prokaryotes, Protists– Bacteria = binary fission

Cell Division

• Multicellular organisms cell division = growth and development, repair and replace.

– Protists, fungi, plants and animals

• 2 types of multicellular organism cells:

1. Somatic Cells: Body cells of an organism that do all the daily function of the organism

2. Germ Cells: reproductive cells (eggs and sperm)

Cell Cycle

• G0: the cell spends its life working and growing (i.e. breaking down sugars, making ATP, enzymes)

• Sometimes a cell will prepare to divide– 1 mother cell 2 daughter cells

• Interphase– G1 (Gap1): Growth, development, organelle

production, etc. Cell must enter reproduction (S phase) or not (G0)

– S (Synthesis): DNA replication

– G2 (Gap2): continued growth and preparation for

division (centrosomes/centrioles duplicate)

Cell Cycle

• Mitosis (M): nuclear division

• Cytokinesis: division of cytoplasm and cell membrane/cell wall

DNA

• Chromatin: spread out DNA-protein complex. In Eukaryotes the proteins are histones. Human cells have 3 billion base pairs.

• Chromosomes: condensed strands of DNA distinct in number and length for each organism. One chromosome is made up of 30 to 150 million base pairs.

DNA

• Humans have 46 chromosomes (23 pairs).

• Each chromosome contains a few thousand genes that code for cellular proteins

DNA

• Diploid indicates that our somatic (body) cells have 23 pairs of chromosomes (2n)

• Haploid indicates that our gametes (sex cells) have 23 chromosomes (1n or n)

Mitosis: Division of the Nucleus

• Problem: distribution of one exact copy of each of their chromosomes to each new cell.

• Each chromosome contains thousands of genes, each necessary to the proper functioning of the organism.

• Humans have ~120,000 genes spread over 46 chromosomes in each somatic cell.

Mitosis

• During Mitosis all access to the DNA stops as chromatin coils up and condenses into visible chromosomes

• Each replicated chromosome is composed of 2 identical parts (sister chromatids) held together by a centromere.

Mitosis

• Sister chromatids are pulled apart during mitosis, and partitioned into 2 daughter cells

• Result of mitosis & cytokenesis: 2 daughter cells genetically identical to the parent cell.

IV. Stages of Mitosis

• Prophase: Chromosomes condense, centrosomes are pushed apart by growing spindle, nuclear envelope disintegrates

Stages of Mitosis

• Prometaphase: the 2 centromeres of each chromosome attach to one kinetochore spindle fiber; centrosomes move to opposite poles

Kinetochore spindle fiber

Non Kinetochore spindle fiber

Stages of Mitosis

• Metaphase: Chromosomes line up at the metaphase plate (equator); centrosomes are at opposite poles

Stages of Mitosis

• Anaphase: Chromosomes separate due to shortening of kinetochore spindle fibers; cell elongates due to lengthening of non-kinetochore spindle fibers

Kinetochore spindle fiber

Non Kinetochore spindle fiber

Stages of Mitosis

• Telophase: Chromosomes immediately uncoil and resume transcription activities; spindle proteins disassemble; nuclear envelope reassembles

Mitosis Animation

• http://vcell.ndsu.nodak.edu/animations/mitosis/movie-flash.htm

Cytoskeleton Creates Chromosome Migration• Spindle fibers are microtubules

made of tubulin proteins that are always present in the cytoplasm

• Enzymes assemble and disassemble spindle fibers

Centromere region contains

1. Centromere protein clamp, holding the 2 sister chromatids together; deactivated during anaphase

2. Kinetochore motor protein; 1 on each sister chromatid

Kinetochore microtubule

• Several microtubules attach to each kinetochore motor protein

• Motor protein starts moving (walks along the spindle fibers); kinetochore microtubules shorten, pulling chromsomes (at the metaphase plate) apart– ATP hydrolysis powers the motion

Non Kinetochore Microtubules

• Microtubules lengthen, pushing the two poles apart (cell expands)

Kinetochore spindle fiber

Non Kinetochore spindle fiber

Cytokinesis

Animal:

•Cleavage furrow– Made by microfilaments (actin fibers)– A drawstring around the middle of the

cell

•Cell pinches off into 2 daughter cells

Mitosis & Cytokinesis in Plant Cells• Plants have centrosomes w/out

centrioles

• No cleavage furrow– plant cells cannot separate due to cell

wall

• Cell plate grows through divided cell

Duration of the Cell Cycle

• Prokaryotic Cells – 20 minutes

• No DNA/Histone complex + fast DNA replication (500 NT/sec)

• No spindle fibers, no mitosis

• 2 DNA rings are attached to 2 spots of the plasma membrane which grows apart

• 1 plasmid

Duration of the Cell Cycle

• Eukaryotic Cells – 12-48 hrs

• Longest phase of the cell cycle is interphase

• Longest phase of mitosis is prophase

Frequency of Cell Cycles

• Depends on cell type:Cell type Life span/frequency

Esophagus, epidermis 2-3 days

Small intestine 1-2 days

Large intestine 6 days

Red blood cells 3 months

White blood cells Up to 10 years

Nerve cells Lifetime (G0)