Mitosis

The M phase of the cell cycle is the process known as mitosis. Some scientists classify cytokinesis as separate from mitosis, but still in the M phase. Mitosis is split into 4 distinct phases plus cytokinesis.

Prophase is the longest phase of mitosis. Chromatin condenses to form chromosomes, centrioles move to the poles, spindles form, the chromosomes attach to the spindle fibers at the centromere and the nuclear envelope breaks down.

Metaphase is usually brief. Chromosomes line up across the center of the cell and microtubules connect the centromere of each chromosome to the two spindles at the poles.

Anaphase is the next phase. The spindles split the sister chromatids and when they reach the poles and stop moving, anaphase is over.

Telophase if the final phase. The distinct, condensed chromosomes disperse, the nuclear envelope reforms around the chromosomes, and the spindles break apart.

Cytokinesis is the last step of cell division. This involves the division of cytoplasm and the cell membrane pinches shut, separating the two daughter cells.

This Krispy Kreme illustration is actually a great example of what this looks like:

Click through to see it full size.

Cell Specialization

Animal cells and plant cells.  Fungal cells and bacteria cells.  Turns out cell are even more diverse than that.  A unicellular organism, like a typical bacterium, grows and operates alone.  It may hang out with other cells, but they are all on their own.  Some organisms however, are multicellular, and in more complex organisms these cells specialize what they do.

Animal cells specialize in many ways.  Some cells transport oxygen (red blood cells), some help fight foreign invaders (white blood cells), and some signals from your hands to your spine (nerve cells).

Plant cells specialize in different ways also.  The best example is the guard cell which regulates the exchange of oxygen, water vapor, and carbon dioxide.

Another great example, which can be found in both plants and animals is the male and female sex cell.

A group of similar cells that work together is called a tissue.  A group of tissues working together form an organ and a group of organs working together form an organ system.

Muscle cell > Heart muscle tissue > Heart > Circulatory System

What is Life? II

To go just a little further in depth as to what we meant last time when we discussed the characteristics of living things, I wanted to make a follow up post.  I’ve been putting this off, but it’s time to bite the bullet.

• All living things are made up of 1 or more cells.  Cells are the basic unit of life.  Most are smaller than you can see with the naked eye, but they are composed of different parts called “organelles” that work together to allow the cell to function and reproduce.
• All living things reproduce.  Reproduction creates offspring, which are similar, but not identical to the parent(s).  Reproduction can be sexual (two parents) or asexual (one parent).
• All living things are based on a genetic code.  Usually that code is DNA (but sometimes RNA in the case of some viruses, which remember are technically not living), which is a molecule that tells your cells what to do in order to function.  Essentially the genetic code is a set of instructions for your cells.
• All living things grow and develop.  Some organisms simply grow larger and prepare for reproduction.  Other organisms may develop legs or wings for movement, or teeth for chewing, or breasts for feeding their young.
• All living things obtain and use energy.  Just as you need food, so do plants, fungi, and even bacteria.  The sum of all chemical reactions to build up and break down materials is called metabolism.
• All living things respond to their environment.  A stimulus is a signal to which an organism responds.  When you get pollen in your nose, you sneeze.  When soil is moist and warm, a seed germinates.  When you turn on a light, roaches run away!
• All living things maintain a stable internal environment.  No matter what goes on outside the body, all organisms keep their internal conditions stable.  The process to do this is called homeostasis.  When you get cold, your body tries to keep your internal temperature from dropping too much.  So you begin to move involuntarily.  We call this shivering.  Likewise, if you’re too hot, you’re body sweats to cool you off.

What is Life?

If Biology is the study of life, how do we define life?  Is there one characteristic that defines all life?  If you ask a room of students “how can you tell if something is alive” you’ll hear things like “poke it with a stick and see if it moves.”  Unfortunately, that isn’t right.  In fact, there is not one characteristic that defines all life.  Living things are defined by a few characteristics.

All living things:

• are made up of 1 or more cells.
• reproduce.
• are based on a genetic code .
• grow and develop.
• obtain and use energy.
• respond to their environment.
• maintain a stable internal environment.

Even nonliving things can meet some of these descriptions, so it’s important to be able to tell living from nonliving. For example, a car obtains and uses energy and maintains a stable internal environment.  Some can even respond to their (external) environment!  To be defined as living an object must meet ALL of these requirements.

Technically, viruses don’t meet this description of life and many scientists don’t  consider them to be living things.  However, since they act like living things, they are studied under the umbrella of biology in a field either specifically called Virology or as a broader biology field called Microbiology.