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Genetics - The Story of How You Become You
This is DNA, which is short for DeoxyRibonucleic Acid.
DNA is a kind of molecule called a nucleic acid. Like the other important nucleic acid in our bodies, RNA,
the job of DNA is to carry the information for how to build and work our bodies.
The DNA molecule is shaped like a ladder that is twisted into a spiral shape. This shape is called a double
helix.
The steps of this ladder are made up of pairs of nitrogenous bases, so when we talk about DNA, we often talk
about base pairs. A base pair is two molecules that are connected to each other, forming a connection
between the two sides of the DNA double helix. (as shown in the left and center drawings above).
Because of the size and shape of the bases, they can only connect in one way –
Adenine always pairs with Thymine (using their initials A pairs with T)
Guanine always pairs with Cytosine (G pairs with C).
As I told you in class, to remember this, remember that the letters made with straight lines (A and T) pair
up, and the ones made with curves (C and G) pair up.
This is very important because the instructions for building our bodies and
making them work are written in these A’s, T’s, G’s, and C’s, you can think of
them as the letters of the alphabet that DNA is written in.
Genes are short pieces of DNA that decide what form of a genetic trait you
will have.
The average size of a gene is about 5000 base pairs, but they can be much
longer.
It is the order of the A’s, T’s, C’s and G’s in the gene that decide how it will
work.
COPYING DNA
Because only A and T, G and C match up with each other, it is easy for the DNA molecule to make a perfect
copy of itself.
In this drawing, you can see how the old DNA molecule, at the top, is split into two halves. Then, starting
at the bottom, a new partner for each base on the old strand comes in and pairs up to make a perfect copy of
the missing side. When the DNA is finished being copied, there are two identical DNA molecules, each with
half of the old DNA and half that was just made.
This is important, because every cell in your body has to have its own complete copy of your DNA. SO,
before a new cell can be made, a copy of all of your DNA, all of the instructions needed to make your body
work,
has to be made for the new cell.
Even a skin cell, that might only live for a week or two, has all of the information to build and run every part
of your body.
WHERE IS THE DNA FOUND?
DNA is in the nucleus of the cell. This is why, when we studied organelles, we called the nucleus the control
center of the cell, because the DNA holds all of the instructions for making it work.
Most of the time the DNA is loose in the nucleus, they call it chromatin because it
makes the nucleus look dark.
BUT, when the cell is ready to make a new copy of itself, the DNA winds itself up
very tightly. The DNA double helix winds itself around proteins called histones into
a dense rod-shaped package called a chromosome.
Chromatin in the
nucleus.
When you see chromosomes, they usually are in the shape of an X.
This is because the DNA only wraps itself up into these tight, easily
moved packages when the cell is ready to divide.
The copy of the DNA that will go into the new cell, and the copy
that stays in the old one, are the two sides of the X. The two copies
are identical twins to each other and we call them sister chromatids.
These sisters will separate when the new cell is made.
CHROMOSOMES
Chromosomes are just big pieces of DNA. Each chromosome
contains certain genes.
For instance a chromosome may have the genes with the
instructions to build hair proteins, the proteins in your muscles, to
control eye color, and many more. Another chromosome will have
genes controlling other traits and characteristics.
The colored bands in the drawing at right represent
locations of genetic traits on two pairs of chromosomes.
The location where a gene is found on a chromosome is called its
locus (plural loci).
Different organisms have different numbers of Chromosomes.
Humans have 23 pairs of chromosomes for a total of 46.
23 come from our mother and 23 from our father.
22 of those pairs are called somatic
chromosomes, from the word soma- meaning
body.
Somatic chromosomes look alike and have gene
loci, or locations, for the same traits.
This is why for every genetic trait, we have 2
alleles that combine to decide what we will
look like,
one allele from each chromosome, one from
each parent.
The 23rd pair of chromosomes are called the
sex chromosomes. The X chromosome is
much bigger than the Y chromosome. The X
chromosome has more genes on it than the Y
chromosome.
X chromosome
Y Chromosome
If you have you have two X chromosomes, you are female.
If you have one X and one Y chromosome, you will be male.
XX chromosomes = Female
XY chromosomes = Male
MITOSIS AND MEIOSIS
Making New Cells
Organisms grow and reproduce by making new cells. Since every cell of an organism
has its own copy of DNA, there has to be a good way to make a copy of the DNA from the
old cell, to put into the new cell.
For any cell that is not going to be a gamete (an egg or sperm cell), a new cell is made
when one cell grows, copies its DNA, makes a new nucleus for the DNA, and then the cell
splits in two. This is called Mitosis.
Definitions:
Mitosis – The process of dividing sets of sister chromatids into 2 sets of chromosomes in 2
nuclei. (nuclei is the plural of nucleus).
Or- making a copy of all your DNA, and then putting the two sets into two nuclei for
two new cells.
Cell Division - making two new cells from one original cell. Cell division includes mitosis.
Sister chromatids – the pairs of identical chromosomes that are made before the cell divides.
They are tied together by a centromere.
Spindle fibers and centrioles - the threads of protein that attach to the chromosomes and
move them around
during mitosis.
ProphaseThe Chromosomes
wrap themselves up
into tight packages.
The Nucleus dissolves.
Prophase- The spindle
fibers get organized
and attach to the
chromosomes
Metaphase- the
chromosomes are
moved to the center of
the cell.
Anaphase- The
centrioles pull on the
spindle and the
chromosomes are
pulled apart.
Telophase- a new
nucleus forms around
each set of
chromosomes, and the
cell divides into two.
P – M – A – T/ Prophase, Metaphase, Anaphase, Telophase
The main things I want you to know about Mitosis are these:
1)
Most of the life of a cell is called INTERPHASE. Inter- means between and it is the time between cells
dividing and making new cells.
2)
At the end of interphase, before Mitosis starts, the DNA is all spread out through the nucleus of the cell.
When the cell is ready to divide, it makes a copy of each of its chromosomes. The cell also makes the spindle fibers
and centrioles it will need to move those chromosomes around the cell.
3)
During mitosis, the chromosomes wrap themselves up tightly. The spindle fibers will attach to pairs
of sister chromatids (the two copies of each chromosome) and will line them up in the middle of the cell.
4)
After they are all lined up in the middle of the cell, the sister chromatids will be pulled apart by the
spindle fibers and one set of chromosomes will be pulled to each end of the cell.
5)
A new membrane will form around each set of chromosomes to form a new nucleus, and the cell will
divide itself into two new cells. Then it goes back to its regular life in Interphase and grows to full size.
MEIOSIS
Meiosis is the special kind of cell division that happens only in the testes or ovaries.
The order of P M A T steps, and what happens in the steps, is almost exactly the same as in
Mitosis.
BUT (Pictures and diagrams on 172-173 of your book)
1) The cell divides TWICE and makes FOUR cells.
These cells are called GAMETES and are eggs or sperm.
2) During the first cell division, the PAIRS OF CHROMOSOMES separate, not the sister
chromatids. This is where the number of chromosomes is cut in half.
3) During the second round of P M A T, the sister chromatids separate to form 4 cells.
4) The 4 cells that result from meiosis have only half the number of chromosomes as all of
your other cells.
(this means when the egg and sperm come together, they’ll make a baby with the right
number).