Genetics and the Mechanics of Natural Selection

Genetics is unquestionably difficult for most people. It has some chemistry, lots of big, technical terms, and requires some concentration to understand., but important. Most of you are just entering your reproductive years where you will start thinking about reproducing. The well-being of your children depends on the outcome of genetics and how it works.

Cell structure (see figure 2.2 on page 41 of text) The body is composed of cells of two basic types: Somatic cells and gametes.
Somatic cells are the cellular components of body tissue: muscle, skin, bone, nerve, heart and brain.
Gametes are sex cells specifically involved in reproduction and not important structural parts of the body.
The most important cell structure for this class is the nucleus -- contains two molecules or nucleic acids that contain the genetic information that controls the cell's function: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

The cell contains the ribosomes and the mitochondria.

The ribosomes are involved with translating the instructions coded in the material of inheritance into proteins or chemical messengers that are used by the body's cells.

Mitochondria are involved with energy transfer within the cell, and are interesting because they contain their own set of hereditary material independent of that contained in the nucleus.

Chromosomes and DNA

The material of inheritance within the nucleus of a cell is arranged in long strands called chromosomes.
On the molecular level the chromosomes are actually nothing more than two long strings of DNA wound together in a spiral-like structure called a double helix.

Each of the two strands of DNA in a chromosome are composed of varying combinations of 4 simple molecules called bases.
The four bases are adenine, cytosine, guanine, and thymine labeled A, C, G, and T. A single DNA strand is composed of a string of bases, each of which can be one of 4 types: A, C, G, or T.
DNA strand -A-C-G-T-C-C-T-G-A-
The order in which the bases occur on the DNA strand is not random . Genes are nothing more or less than unique, specific sequences of the 4 bases.

"Gene" is a layman's term. Scientists avoid the use of the word gene, because it is not very specific. Locus (plural = loci) and allele are used instead.

A locus is the section of a DNA strand that contains the instructions for one specific product such as eye color or tongue rolling or the blood serum protein albumin. A locus is simply a section of the chromosome that holds a sequence of bases and is like an address.

The particular base sequence that resides at a given locus is called an allele. Only one allele can reside at a given locus in any one DNA strand.

Gene Expression

Alleles code for proteins which are either the desired product, or are messengers or controlling substances that produce the desired result.

A protein is simple a chain of amino acids. Just like a DNA strand is a chain of bases, a protein is a chain of amino acids.

There are 4 different bases that can be chained together to form a DNA strand, and there are about 20 different amino acids that can be chained together to form a protein.

Their are 20 amino acids and which sequence of 3 bases codes for each one.
Each set of three bases that codes for an amino acid is called a codon. So, there is a codon for the amino acid alanine, a different codon for serine, etc.

     base sequence      -A-C-G-  -T-C-C-  -T-G-A- 

     amino acid coded    {cys}    {arg}    {thr}

     for by codon

DNA replication

A DNA strand has a sticky part hanging off of it that gives it the power to replicate itself . That sticky part is capable of binding to another base. However, each of the 4 base pairs up with only one other of the 4 bases. Adenine and thymine always pair together, and cytosine and guanine always pair together.

These bonds between complimentary bases hold the two strands of DNA in a chromosome together. DNA strands in a chromosome are in some ways mirror images of each other.

     coding strand         -A-C-G-T-C-C-T-G-A- 

                            : : : : : : : : :

     anticoding strand     -T-G-C-A-G-G-A-C-T- 
The strand that actually has the base sequence coding for a protein is referred to as the coding strand, and its complimentary strand is called the anticoding strand. For every codon there is an anticodon.

Coding strands and anticoding strands have all the information contained in the other -- a built-in method for replication.

During DNA replication the two strands of the chromosome separate. The original coding strand causes new bases to be attracted to it and to form a new DNA strand that exactly duplicates the original anticoding strand. The original anticoding strand attracts bases that combine to form a new coding strand. This leaves two chromosomes each containing the same information.

A similar thing happens when the information contained within the codons of an allele is converted into amino acids in a protein. This process has two step, transcription and translation.

Transcription-- the information stored in the DNA is copied to a molecule called messenger RNA (usually abbreviated mRNA). RNA is very similar to DNA, but is slightly less stable.

DNA is used for long term storage of information, and the less stable RNA is used for short term storage of information. MRNA acts as a messenger and carries its information to the ribosomes, where protein is actually constructed.

a. First, the two strands of DNA separate at the locus.

                    original state:
     coding strand     -A-C-G-T-C-C-T-G-A- 
                        : : : : : : : : :
 anticoding strand     -T-G-C-A-G-G-A-C-T- 
                       opened up:
     coding strand     -A-C-G-T-C-C-T-G-A- 
 anticoding strand     -T-G-C-A-G-G-A-C-T- 

b. Second, mRNA makes a copy of the original DNA base sequence by pairing with the anticoding strand. (Mediated by RNA polymerase).

     coding strand     -A-C-G-T-C-C-T-G-A- 
       mRNA strand      A-C-G-U-C-C-U-G-A- 
 anticoding strand     -T-G-C-A-G-G-A-C-T- 

c. Third, the mRNA strand migrates to a ribosome, where translation takes place.

During translation, the ribosomes cause the information in the mRNA transcript to be translated into protein using another type of RNA called transfer RNA (usually abbreviated tRNA).

A tRNA molecule contains one anticodon and carries one amino acid - the correct amino acid corresponding to the codon the tRNA molecule is designed to match.

The tRNA anticodon bonds with a codon on the mRNA transcript. Ribosomes cause the tRNA anticodons to match up with the mRNA codons, then facilitate the attachment of the amino acid carried by the tRNA molecule to a chain of amino acids to eventually form a protein.

 Protein          -cys---arg---thr-
                    |     |     |
                  +-+-+ +-+-+ +-+-+
                  | | | | | | | | |
 tRNA             U G C A G G A C U
                  : : : : : : : : :
 mRNA strand     -A-C-G-U-C-C-U-G-A-

Chromosome Number

Each cell of the body (except sperm and ova) has two of each chromosome or diploid.

One chromosome came from the mother and one came from the father. So, the reproductive cells, the sperm and ova, must have only one chromosome or haploid. Haploid reproductive cells are called gametes.

Humans have 23 pairs of chromosomes, 46 altogether. For 22 of these chromosomes the two members of the pair are pretty much identical or autosomal chromosomes.

23rd pair of chromosomes is the sex chromosomes. The sex chromosomes come in two varieties, X and Y. Females have two X chromosomes, and males have one X chromosome and one Y chromosome. Generally speaking, if you have a Y chromosome you are a male.

All the gametes produced by females carry only X chromosomes. But, half of the gametes produced by males carry the X chromosome and half carry the Y chromosome. So, whether a given offspring is female or male depends on whether the father's sperm happens to have an X or a Y chromosome.

Most traits are autosomal traits, the loci carrying alleles for them are located on one of the autosomal chromosomes, but a few traits are sex linked, carried on one of the sex chromosomes - usually the X. One sex-liked trait is hemophilia, caused by a recessive gene on the X chromosome. It occurs mostly in men because they only have one X chromosome. If they have the recessive gene, then they will have hemophilia.

The Mechanisms of Inheritance

Cell division determines the nature of inheritance.

Mitosis and meiosis

Mitosis and meiosis are the two major division processes. The production of gametes (sperm and ova cells) entails producing cells with one copy of each chromosome from cells with two copies of each chromosome.

The process is relatively complex -- can't simply take any 23 chromosomes at random. There must be one of each of the 23 kinds of chromosomes.

Normal non-sex cell division is called mitosis. Cell division to produce gametes is called meiosis.
 

  1. During mitosis the chromosomes replicate themselves.
  2. Then one half of either the original or the replicate cell is drawn into the opposite half of the cell.
  3. The cell divides down the middle.
  4. Both new cells have the identical chromosome composition as the original.
See Chart 1 on the terms page for this lecture.

Meiosis is different.
1. The chromosomes replicate.
2. Series of 2 divisions resulting in 4 gametes, each with one of each pair of chromosomes.

See Chart 2 on the terms page for this lecture.

 

Recombination

Recombination occurs at fertilization when two haploid gametes merge to form a diploid cell again.

The mother contributes one haploid gamete and the father the other, and they are recombined to produce a unique offspring.

 

Mendelian inheritance

Gregor Mendel (1822-1884), a Czechoslovakian monk, worked out the laws of inheritance.
Mendel's work was lost, and not rediscovered until 1900.

Mendel experimented on peas because they come in lots of different varieties so he had some variation to work with.
He could control the fertilization of peas determining which pea plant was fertilized by which other pea plant.

Mendel discovered two major laws of inheritance, the principle of segregation, and the principle of independent assortment. Along the way he discovered some other principles of genetics.
Graphics are on the PowerPoint for this lecture. Follow and study them to get a better idea of the mechanics.

A. The principle of segregation


See Chart 3 on terms page for this lecture.

B. Sample Mendel experiment


1. Mendel's explanation
Represent the genes of both parents using a shorthand form of notation called a genotype. The round parent had 2 copies of gene "R" and therefore had the genotype RR. The wrinkled parent had two copies of the gene "r" and its genotype was rr. A genotype like RR and rr (both genes the same) called homozygous.

When the two genes are different, as in an Rr genotype, this genotype is heterozygous.

Crossing a homozygous round parent with a homozygous wrinkled parent t(he round parent had only round genes to contribute and the wrinkled parent had only wrinkle genes to contribut), produced offspring that were all Rr heterozygotes.

The mechanics of this are shown in this mating grid, Chart 4 on the lecture terms page.

Note: The order in which the genes are presented has no significance to a cell, so all heterozygous genotypes are the same, whether they are written as Rr or rR.

Expected might be that the offspring of a mating of round peas with wrinkled peas would have been intermediate between the two parents in some way - half round and half wrinkled, or partially wrinkled, or wrinkled on one half, or whatever.
 

The gene for round, and all genes that behave like it, are called dominant.
The gene for wrinkled and all genes that behave like it are called recessive. Some gene systems have several recessive alleles and others have several dominant alleles.

Example, the ABO blood group has at least 3 genes: A, B, and O. A and B are dominant, and O is recessive.
 

Mendel's experiment shows the difference between genotype and phenotype. Genotype is the genes possessed by an individual.
Phenotype is the actual condition expressed by the individual.

2. Mendel explained the fact that when he crossed the offspring with each other he got 3/4 round and 1/4 wrinkled as follows:

  1. If segregation of the genes in a pair occurs, then each gene in a pair has to be treated separately. See Chart 5 on the lecture terms page.
  2. The mating grid shows that there are 4 possible outcomes to the mating of two Rr individuals. In 3 of the 4 cases the offspring of this second generation would have at least one R gene and would be round. In 1 of the 4 cases the offspring would be rr and would be wrinkled.
  3. The only way these results could be obtained is if each gene of a pair is passed on separately. Shows that the principle of segregation is true.

C. Mendel's second law was the principle of independent assortment.
States that if you are working with more than one pair of genes, all pairs of genes separate randomly.
Example:
Some peas are yellow (dominant) and some are green (recessive).
When Mendel crossed peas with varying combinations of being yellow or green and round or wrinkled, he found that the condition of the offspring with respect to color had no relationship to whether or not they were round or wrinkled.
Color and roundness were completely independent systems. This is the principle of independent assortment.

Mendel's principle of independent assortment is usually, but not always true.

V. Mendel's laws, chromosomes, meiosis, and how genes are passed on

A. The principle of independent assortment says that alternative alleles are passed on separately.

Alleles reside at loci which are carried on chromosomes, and the members of each pair of chromosomes are separated during meiosis, so that each gamete contains only one allele.

Principle of segregation is a direct result of the mechanism of meiosis.

B. The principle of independent assortment says that alleles of different loci are inherited randomly with regard to each other.
The condition of one doesn't influence the condition of the other. This principle doesn't fare so well.
In most cases genes do sort independently so the principle is true in general.
But, loci come in packages, called chromosomes, and that there are only 46 such packages in a human being.

One phenomenon called that defies the principle of independent assortment:
Linkage refers to the phenomenon that some loci seem to travel together because they are carried on the same chromosome.
Linkage is not common because of a crossing over.
Crossing over reshuffles loci to the other chromosome of a pair.
Linkages between loci usually imply that the loci lie so close together on the chromosome that it is unlikely that crossing over will separate them.

Association also seems to defy the principle.
Refers to an apparent connection between specific alleles of different loci.
Example is ear wax type.
Ear wax can be either brown or gray and either wet or dry. Most people with brown ear wax also have wet ear wax, and most people with gray ear wax also have dry ear wax. The correlation is greater than chance, so there is said to be an association between ear wax color and wetness.
Causes of associations are poorly understood, but it may be that both the color and the wetness of ear wax are controlled by some third outside factor.
 


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