Slide 1
To understand the evolution of life on earth - we must gain an
understanding of genetics. To understand genetics, we must understand
cell division.
There are two types of cell division in eukaryotic organisms - mitosis and meiosis. We will start by discussing mitosis. Mitosis is cell division that is essentially a cloning process - during mitosis a cell divides and produces two cells that are genetically identical. Mitosis gives rise to two daughter cells that have the same ploidy number as the original parent cell. Ploidy number refers to the number of copies of each type of chromosome - haploid organisms have one copy of each chromosome, diploid organisms have two copies of each chromosome, and triploid organisms have three copies. Most of the organisms that we will be discussing during this course are diploid and have two copies of each chromosome. A diploid cell that goes through mitosis creates two daughter cells that are also diploid.
In single-celled organisms, mitosis is used for reproduction. However, we will mainly be looking at multi-cellular organisms and multicellular organisms use mitois for growth and repair of damaged tissues.
Slide 2
The division of a cell is an aspect of a cell's cycle. In
general, the cell cycle is comprised of two main stages - interphase and
M-phase. Interphase is the stage in which all of the cell activities
other than division take place. The M-phase refers to either mitosis or
meiosis and is the stage during which the cell divides.
Slide 3
For this course, you do not need to know details about mitosis
other than the movement of the chromosomes
Chromosome movement during cell division is key to understanding genetics - our next topic
Slide 4
Mitosis is a continous process but we can divide the process into
stages by looking at key landmarks during cell division. These landmarks are
so obvious, they can be seen by using a simple light microscope and a cell
stain, as seen in the microscope images at the top of this image.
Slide 5
Interphase is the longest stage in the cell cycle and it is the
time when a cell makes the decision to divide. Once the decision to
divide has been made, the cell prepares for division. During S-phase of
interphase, the chromsomes are replicated. S stands for synthesis and
refers to the process of DNA replication which we will learn more about later
in the course.
Prior to chromosome replication, the chromosome is comprised of a single strand. After the chromosome is replicated, the chromosome is comprised of two strands known as sister chromatids. The sister chromatids are identical to each other and they are attached at a location known as a centromere. A replicated chromosome is still considered a single chromsome - but it is a chromosome comprised of two identical sister chromatids.
Slide 6
Remember, as we discuss mitosis we are only focusing on the
movement and behavior of the chromosomes. You may have learned mitosis
in a previous class and focused on other aspects of cell division such as the
behavior of the nuclear envelope or formation of the mitotic spindle. We
will not discuss these features during the course. So the one thing you
need to know about prophase is that the chromosomes condense into distinct
structures (during interphase the chromosomes are diffuse and cannot be
observed as distinct structures - they need to condense for cell division to
avoid damaging the chromosome).
Slide 7
During metaphase, the chromosomes line up along the equator of the
cell - a region known as the metaphase plate. This is essentially an
organizational stage - cell division is a well-orchestrated process and
requires careful movement of the chromsomes.
Slide 8
During anaphase, the centromere regions of each replicated
chromosome splits and each chromatid becomes a distinct chromosome.
Remember, the sister chromatids are genetically identical to each other - so
two genetically identical sets of chromosomes are separating from each other
during this phase. The now-distinct chromosomes move to opposite pole
regions of the cell.
Slide 9
During telophase, the last stage of mitosis, the cell prepares to
become two new genetically identical cells. A nucleus forms at each pole
of the cell. Remember, the chromosomes at each pole used to be sister
chromatids and are genetically identical.
Slide 10
The process of mitosis is completed when cytokinesis takes
place. Cytokinesis is the division of the cytoplasm to produce two new
distinct cells. Prior to mitosis all of the cell organelles were also
being replicted so each new cell has a complete set of organelles and is a
complete cell. Mitosis is now complete and our original parent cell has
given rise to two new genetically identical daughter cells.