Understanding Genetics
Genetic Definitions
To understand the more detailed processes that take place in the nuclei of your cells you
need to understand some genetic definitions:
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Chromosomes ~ are paired structures that are made up of strands of chromatin
which contains DNA and proteins. In humans, there are 46 chromosomes (23
pairs) in the cell nucleus of regular cells of the body – called somatic cells – as
opposed to the gametes (sperm and egg cells), which contain only 23 unpaired
chromosomes. A chromosome has a short arm and a long arm, which are held
together by a centromere.
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DNA – deoxyribonucleic acid ~ is the chemical molecule that serves as genetic
material. A strand of DNA is a long chain (a polymer) of nucleotides. Each
nucleotide of DNA contains a nitrogenous base, a sugar with five carbon
molecules called deoxyribose, and a phosphate group. There are four kinds of
nitrogenous bases in DNA; adenine (A), thymine (T), cytosine (C) and guanine
(G). The nitrogenous bases (nucleotides) can be and are different throughout the
long chain of DNA. DNA exists inside the chromosomes.
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Genes ~ lie along the chain of DNA. They are made up of sections of nucleotides.
Some genes can have many nucleotides; others have only a few. Humans have
thousands of different genes, which reside on different chromosomes, but on the
same chromosomes in all people. For example the gene for cystic fibrosis (CF) is
always found in the same location on gene number 7 in all humans. However, not
all humans have CF (or any other genetic disorder). Some genes show the effect
(they are expressed), whereas other genes do not show an effect (they are
repressed).
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Alleles ~ are the different forms of a trait. They are alternative forms of a gene
or base pair sequence that occur on a chromosome. For example, the gene for
hair colour resides in a certain location on a certain gene in all humans. However,
humans can have many different shades of hair colour; the different shades are
represented by different alleles.
Mendel’s Law
Gregor Mendel was a monk in Austria in the mid 19 th century. To pass his quiet time, he
watched pea plants grow. As he observed subsequent generations of pea plants growing,
he noticed subtle changes and wondered how they occurred. He observed the colour of
seeds, flowers, and unripe pods; the shape of the seeds and pods, the length of the
stem, and the positions of the flowers. Most importantly, he kept accurate records and
an accurate account of what plants showed what traits.
During Mendel’s time, the general thought was that the traits of a father blended with
the traits of a mother. So a tall father and a short mother were expected to breed
average-sized children. Traits in offspring were expected to be averages of the traits in
the parents.
One day Mendel crossed a tall pea plant with a short pea plant, expecting to get
average-sized pea plants. However, three tall pea plants and one short pea plant grew.
He continued to cross tall with short over and over again, the results were similar every
time. What he discovered was that offspring carry all the traits of the parents but each
offspring is capable of expressing different ones. He figured that these traits were passed
from generation to generation by something that he called factors. What Mendel called
factors we now call genes.
After studying 28,000 or so pea plants and reviewing his data, Mendel was fairly
confident in his research, to be able to state that heredity followed specific patterns.
First, he stated that traits are inherited independently of each other – this is Mendel’s
Law of Independent Assortment. This law states that each trait or characteristic is found
on separate factors (genes), that each factor(or gene) comes in pairs, and that each pair
separates on its own.
Mendel also came up with the Law of segregation, which states that during cell division,
each allele of a gene pair will randomly move to different gametes. On one of the
number 2 chromosomes you have two H alleles; on the other number 2 chromosome
you have two h alleles. You have four alleles for hair colour (H, H and h, h) at the
locations of the genes for hair colour. When an egg or sperm cell are produced only half
of your genetic material goes into each gamete. So, you produce two gametes
containing H alleles and two alleles containing h alleles. But each of those alleles
separated randomly into the four gametes.
There a multiple genes and alleles for hair colour. Two of the genes may be next to each
other, at a particular position on the chromosome, and they will not separate randomly.
Their alleles on their paired chromosome will separate randomly though.
Genetic crosses
When a geneticist writes a genetic equation, the alleles are represented by letters. The
dominant traits are usually capitalized and the recessive traits are usually lowercase.
Writing the letters that represent the alleles of a gene is called a genotype. A genotype
can be written to represent a phenotype, which is the physical result of the expression of
a gene.
Phenotypes are created by crosses between different organisms. For example, if you
cross a red rose bush with a white rose bush, you would get a rose bush with red or
maybe even pink roses. If you were crossing the rose bushes explicitly to see the result
of flower colour, you would be performing a monohybrid cross, which cross examines
just one trait. Therefore, the phenotype for the first bush is ‘red flowers’ and so the
genotype would be RR. The phenotype for the second bush is ‘white flowers’ and so the
genotype would be rr.
Copying your DNA
Your DNA does not copy (replicate) itself – only when you create gametes and mate.
Every cell in your body needs to be replaced periodically. Cells never stop working and
eventually wear out. Cell turnover, as it is called, happens constantly, on any day your
body can be replacing some blood cells, skin cells, hair cells and mucous cells (just to
name a few types of cell!). Whatever needs to be replaced, the process of DNA
replication is the same.
DNA looks like a twisted ladder, with the nucleotide bases forming the ‘rungs’ of the
ladder. During replication the DNA strand must ‘unzip’ so that the rungs are split apart
with one nucleotide one each side of the strand. Each side of the original DNA strand
becomes a template strand upon which the new complementary strand forms. The
‘unzipping’ of the DNA helix is initiated by the enzyme known as helicase.
The entire DNA strand does not unzip all at one time. Only part of the original DNA
strand opens up at one time. When the top of the helix is opened, the original DNA
strand looks like a Y. This partly open/partly closed area where replication is going on is
called the replication fork.
The nitrogen bases that make up each nucleotide along the strand of DNA include
adenine (A), guanine (G), cytosine (C) and thymine (T). In a molecule of DNA, A always
pairs with T and C always pairs with G: A – T, C – G.
As the enzyme DNA polymerase moves along the template strand, of a base says A then
a T is added to the growing complementary strand. If a base on the template strand
says G then a C is added to the growing complementary strand.
The order of the bases is important because the order of bases delineate the genes, and
the genes dictate whet amino acids are produced, and the amino acids determine which
proteins are produced, and proteins are needed in every cell of your body. Proteins make
up cell structures themselves, as well as enzymes that initiate cellular processes that
keep you alive.
The DNA polymer works continuously on the side known as the leading strand The other
side looks messy because the process does not occur smoothly. On the lagging strand,
the DNA polymerase reads the template strand and assembles the new bases in
fragments. These fragments are called Okazaki fragments, and they are then joined
together by the enzyme DNA ligase to form the new complementary strand.
The replicating DNA strand needs energy to go through the steps of reading the
template, producing the complementary base, and joining the base to the growing
strand. The molecules of the sugar deoxyribose provide that energy. The phosphate
bonds that are broken apart when the original strand of DNA ‘unzips’ provides the
chemical energy needed to get the whole process started.
The following image shows how DNA replication takes place:
DNA mistakes
Believe it or not, the newly created DNA strand in cells are proofread before cell division
is finalized. If a mistake is detected, it’s back to the template strand. The nucleotide that
as inserted in error is removed, and the correct one is put in place. If the proofreading
function goes awry, mismatch repair enzymes are available to shore things up. Error
recognition and repair mechanisms exist in organisms with eukaryotic cells, the details of
how they function are nor as well understood.
If a mistake in a new strand of DNA goes undetected or unpaired, the mistake becomes
a mutation. A mutation is a deviation from the original DNA strand. The nucleotides are
not in the same sequence. Although mutations can and do cause serious defects, not all
mutations are bad.
The following list explains how mutations, which are usually caused by certain chemicals
or radiation affect humans:
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Substitutions ~ These types of mutations occur when the wrong nucleotide is
put in for another nucleotide, for example, if the code for a particular gene read
5’ –A-T-C-G-T-C-A-G-3’, the correct complementary sequence for the code on the
new strand of DNA would be 3’ –T-A-G-C-A-G-T-C- 5’.
Genetic code is written in a specific direction, because DNA is a double helix in
which two strands intertwine, confusion can easily be created when trying to keep
track of the ends of the strands. To avoid confusion, one strand of DNA is labelled
3’ (3 prime) and the other 5’ (5 prime), the strand should be read in the 5’ to 3’
direction.
In the example above, the third base over should be guanine (G) instead of
cytosine (C). That base could have been passed over during the ‘reading’ of the
strand of DNA, or a new cytosine base could have been put in instead of the
guanine base. In either case it is wrong, so it is classed as a mutation. Because
the mistake involves only one base it is known as a point mutation. There is a
chance that the protein that the gene creates would not be affected by this
mutation and is, therefore, called a silent mutation.
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Deletions ~ If during the creation of a new strand of complementary DNA, a
nucleotide is read but the complementary base is not inserted, the
complementary strand is missing a nucleotide, this type of mutation is called a
deletion. These mutations can cause serious diseases.
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Insertions ~ If an extra nucleotide is slipped into a newly developing
complementary strand, the rest of the strand is read wrong. This type of mutation
is called a frameshift mutation because the reading of the frames of genetic code
is shifted.
Producing Proteins
Ribonucleic acid (RNA) is very similar to DNA, except for these differences:
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RNA is single stranded
It contains the sugar ribose instead of deoxyribose
It uses uracil (U) as a nitrogenous base instead of thymine (T)
The nucleotides in RNA pair up as: A – U and C – G. RNA bases can pair up, even though
the RNA molecule is single-stranded, this is because RNA has a secondary structure and
can fold up and base pair with itself where complementary. RNA molecules are
important for the production of proteins, and, just after DNA has been replicated, the
complementary strands produce proteins.
DNA harbours the genes that code for what proteins will be produced in your body. But
the code buried in segments of DNA is not what initiates protein production. Firstly, the
DNA must be ‘rewritten’ into a strand of RNA, and the mRNA carries the information out
of the cell’s nucleus to the ribosomes. At a ribosome, the original message is translated,
and then the appropriate protein can be produced. Protein synthesis is initiated on the
ribosomes that exist free in the cytoplasm. The ribosomes that are attached to the ER to
make proteins to be secreted or transported to other organelles.