WSU home

Weber State University

      Department of Botany


BTNY LS1203 - Plant Biology


Meiosis

Mitosis results in two nucleus, no matter what the ploidy is.  Meiosis requires an even number ploidy (with chromosomes able to pair) to occur correctly.  The general description of the process focuses on a diploid nucleus, one with two sets of chromosomes.  The specific chromosomes from each set that each other are called homologous chromosomes.  The presence of homologous pairs of chromosomes is critical to the process of meiosis.  If the starting cell had two sets of chromosomes in the nucleus (diploid), by the end of meiosis there will be four haploid (1 set) nuclei.  Each of the four nuclei will be slightly different genetically from the other three because the events of meiosis will shuffle the genetic information present in the two starting sets of the original diploid nucleus.

During meiosis, there are two successive nuclear divisions (meiosis I and meiosis II), each with its own prophase, metaphase, anaphase, and telophase.  Note:  interphase, in which each chromosome was duplicated and now consists of two chromatids, has already occurred.



Prophase I

Remember that at the end of interphase, a nucleus has chromosomes that consist of two chromatids each.  During prophase I, the homologous chromosomes pair up and exchange segments with each other in a process called crossing over.  The chromosomes contract; the nucleoli and nuclear membrane disappear. 

Metaphase I
The homologous pairs align at the equator of the cell.  Each pair aligns independently of the other pairs.  So the pair of “A” chromosomes might have chromosome A facing “north” and chromosome a facing “south,” while the pair of “B” chromosomes might have chromosome b facing “north” and chromosome B facing “south.”  

Anaphase I
The homologous pairs separate and moved to opposite poles of the cell.

Telophase I
This step might not occur.  If it does, the chromosomes uncoil a little and the nuclear membrane reforms.

By the end of meiosis I, the ploidy of the cell has been cut in half.  

Prophase II
This phase will only occur if telophase I  took place.  Basically, if you made nuclei during telophase I, you need to undo them during prophase II.

Metaphase II
The chromosomes line up in the center of the cell, generally at right angles to the metaphase I alignment.

Anaphase II
The chromatids separate and move to opposite poles.

Telophase II
The chromosomes uncoil.  The nuclear membrane forms.  The nucleoli reappear.  Cytokinesis takes place.   The original diploid cell is gone.  In its place are four haploid cells that are genetically different from each other.  

Meiosis accomplishes two things as a prelude to sexual reproduction:
1.  Reduces the number of sets of chromosomes (ploidy) by half.  Why is this important?  During fertilization, two cells fuse to form a new individual, thus restoring the original number of chromosome sets.  If the ploidy had not been reduced, fertilization would result in an increase in chromosome sets.
2.  Genetic recombination.  The crossover exchanges of prophase I and the random assortment of homologous pairs during metaphase I result in new allele combinations.  This allows sexual reproduction to maintain the genetic variability in the population.  (Genetic uniformity could be - and is -  accomplished via asexual reproduction.)


Life Cycles


DNA Replication

A DNA molecule is composed of two polynucleotide strands held together by H-bonds.  The H-bonds occur between complementary bases, adenine with thymine and guanine with cytosine.  The two strands themselves are then referred to as complementary strands.

When Watson and Crick proposed their model of DNA structure, they also proposed a replication mechanism.  They suggested that during replication, the two DNA strands separate along the H-bonds holding the base pairs together.  Once synthesis was completed, there would be two new molecules of double stranded DNA, each consisting of an original (template) strand and one new strand.  This mechanism is known as semi-conservative replication.  Meselson and Stahl did a series of experiments that confirmed that DNA is indeed duplicated by the semi-conservative replication mechanism proposed by Watson and Crick. 

DNA is replicated during the S portion of interphase.  Therefore, replication occurs in all meristematic cells (prior to mitosis) and sporocytes (prior to meiosis).  Replication begins at an origin of replication; each chromosome has many origins of replication. The complementary strands of DNA separate, but the sequence of bases in each strand remains intact. Each strand then serves as a template, or pattern, during the synthesis of a new strand.  Enzymes called DNA polymerases match the complementary N-bases of nucleotides to the template strand and covalently bond these new bases together to make a new DNA strand.  The DNA polymerases proofread as they go; additional enzymes proofread when the replication in completed.  Some base-matching errors do occur and don't get corrected.  These errors are point mutations. 


Gene Expression

DNA contains information for making proteins.  The information carried by the nucleotides  in the double-stranded DNA  molecule needs to be converted in to a single stranded amino chain in a protein or polypeptide.   To do this, you need an intermediate molecule, RNA.  Actually, you need three distinct functional types of RNA.  All three types are made from a DNA template by the process of transcription.  Then amino acids are linked into polypeptide chains in translation as the nucleotide base sequence of one of the RNA molecules (mRNA) is translated into an amino acid sequence via the genetic code.  

RNA differs structurally from DNA in three ways:
1)  The sugar of RNA nucleotides is ribose rather than deoxyribose.
2)  Thymine is replaced by uracil.  In hydrogen bonding, U binds to A.
3)  RNA is usually single stranded and does not form a regular helical structure.

The three types of RNA needed for translation are messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).  All three types are synthesized the same way, by matching complementary bases of RNA nucleotides to the bases of one of the strands of the DNA molecule.  The strand that is used is the template strand.  The other is the nontemplate strand.  The nontemplate strand and the RNA molecule will have the same base sequence, with the exception of T in the nontemplate DNA strand and U in the RNA molecule.  The synthesis of RNA bases on a DNA template is called transcription. If the RNA made is tRNA or mRNA, it passes to the cytosol through the pores in the nuclear envelope. If it is rRNA, it is assembled into ribosome subunits in the nucleolus region of the nucleus. The ribosome subunits then exit the nucleus for the cytosol. Once all of the RNAs are in the cytosol, protein synthesis, or translation, can take place.


The Genetic Code

Gene expression via the central dogma:
DNA  --- transcription ---> RNA --- translation ----> protein

The gene expression system works because there is a genetic code: a set of nucleotide triplets that specify amino acids.  Each triplet is a codon for a specific amino acid.  There are 43 = 64 possible codons for 20 amino acids.

A role has been found for all 64 codons.
61 codons specifiy amino acids, with one of these codons also serving to show where translation should start on an mRNA molecule.
3 codons indicate the end of a message on mRNA and signal to stop.

Features of the genetic code:
triplet
commaless
non-overlapping
unambiguous
degenerate or redundant (the synonyms)
universal

Translation

The mRNA has the base sequence that specifies the amino acid sequence of the protein. The mRNA essentially serves as a working blue print of the gene in the DNA. The original plan, the base sequence in the DNA, remains in the nucleus. An expendable copy, the mRNA, is sent to the cytosol. This system also allows for amplification, for one DNA sequence can be used to make many mRNAs. The conversion of the RNA base sequence into an amino sequence is called translation. Translation takes place via the genetic code. An mRNA molecule is a string of codons. To translate the codons, you need anticodons. The tRNA molecules fold into a structure that has a sequence of three bases exposed. These three bases are the anticodon. They are complementary to the three bases that form the codon. Attached to one end of the tRNA molecule is the amino acid specified by the codon. To start translation, a tRNA binds to a start codon via complementary base pairing. Besides indicating the start of the message, the start codon also specifies the first amino acid. Ribosomes are made of two subunits, a large subunit and a small subunit. Once the first tRNA binds to the mRNA, the ribosomal small subunit attaches to the tRNA/mRNA complex. Then the large subunit attaches. Once the ribosome is assembled, translation can continue. Also with the start of translation, a reading frame is established, and the code is read in a non-overlapping manner. The ribosome moves along the mRNA, with the next tRNA bringing in the next amino acid, as the previous tRNA leaves its amino acid. Finally a stop codon is reached. Stop codons are sometimes called nonsense codons because they do not specify amino acids. When the stop codon is reached, the ribosome subunits disengage, releasing the mRNA and the newly synthesized protein. Translation, like transcription, allows cells to amplify genetic information. Many copies of a protein can be made from a single mRNA. In fact, most of the time, a second protein is started as soon as the first ribosome clears the area of the start codon.


Mutations

Changes to a DNA sequence can affect the ultimate protein product produced from a gene. A mutation is any changes in the DNA sequence. Mutations can be deleterious, an improvement, or not noticeable. In a substitution mutation, one base is exchanged for another. Some substitution mutations are silent because a change in the base sequence does not change the amino acid sequence due to the redundancy of the genetic code. Other substitutions in the DNA will change the amino acid in the protein, a sense mutation. Sometimes the new amino acid has similar properties of the one it replaces, or the amino acid change is in an area of the protein not critical for enzyme activity. In these cases, a mutation might not be apparent. In other cases, a sense mutation does lead to a significant amino acid change.  A third type of substitution mutation is a nonsense mutation, where the base substitution forms a stop codon. Two other types of mutations are deletions and insertions. In these mutations, bases are lost or added to the DNA sequence of a gene. These two mutations lead to shifts in the reading frame of translation and can greatly change amino acid sequences.


Genetics

Genes specify proteins. Proteins have many functions in organisms, notably as enzymes. Enzymes function in chemical reactions to build other types of molecules, like pigments, hormones, cellulose, starch, and membrane lipids. These molecules organize to form cells, tissues, organs, and ultimately an entire organism. Thus, an organism's physical appearance (phenotype) is due to its genes (gentotype). How genes are inherited and then interact with each other in an individual is the realm of genetics.

 

Gregor Mendel (1822-1884)
1856 began collecting data on hybridization experiments with peas
1865 presented his findings and conclusions at two consecutive meetings of the Brünn Natural History Society
1866 published his work (Versuche über Pflanzen-Hybriden = Experiments on Plant Hybrids) in the Proceedings of the Brünn Natural History Society.

Mendel was not the first person to attempt to formulate rules of heredity. He succeeded in establishing that inherited characteristics are determined by discrete factors that are passed from one generation to the next. The factors are shuffled and distributed each generation. Why was Mendel successful where so many others had failed?

1. Selected distinct, single gene characters that assorted independently of each other:
            seed morphology: round /wrinkled
            seed color: yellow/green
            flower color: purple/white
            pod morphology: inflated/constricted
            pod color: green/yellow
            flower position: axial/terminal
            height: tall/short
2. Tested specific hypotheses in a series of logical crosses
3. Obtained true-breeding lines before starting any hybrid crosses
4. Did a mathematical analysis of the outcome of crosses (counted offspring!!)
5. Studied multiple generations of known lineage for a particular trait
6. Kept accurate and meticulous records
 

Example of one of Mendel's crosses:

First crosses: P: tall x short
Possible outcomes:
        all tall
        all short
        all intermediate
        a mixture of tall and short
        a mixture of heights
Actual outcome: F1: all tall

Second crosses: self-fertilized the F1
Outcome:  F2: 787 tall, 277 short

Third crosses: self-fertilized the F2
        F3: short F2 (¼ of all F2) had only short offspring
            1/3 of the tall F2 (¼ of all F2) had only tall offspring
            2/3of the tall F2 (2/4 of all F2) had tall and short offspring in a 3:1 ratio

Subsequent self-crosses continued until an F7 generation was obtained.
P = parental generation
F1 = first filial generation
F2 = second filial generation

Today, we assign symbols to genetic traits. Let T represent the tall version of height and t represent the short version. A true breeding tall plants has two T alleles. A true breeding short plant has two t alleles.

P: TT x tt

gametes:
The TT parent produces gametes that only carry T.
The tt parent produces gametes that only carry t.
 
T
t Tt
tall

All of the F1 are Tt or heterozygous. Because this particular trait shows complete dominance, all of the offspring will be tall.

self-cross the F1: Tt x Tt (a monohybrid cross)
each parent produces the same two types of gametes: T and t
 
T t
T TT 
tall
Tt
tall 
t tT
tall 
tt
short 

The F2 show three genotypes (the allele combinations) and two phenotypes (the appearance):
        TT, Tt, and tt, in a 1:2:1 ratio
        tall and short in a 3:1 ratio
These ratios are the mathematically expected outcome of a monohybrid cross involving a trait that shows complete dominance.

Let's look at the same cross with the math.
 
½ T ½ t
½ T ¼ TT 
tall 
¼ Tt
tall 
½ t ¼ tT
tall 
¼ tt
short 

If we add up the fractions, there are ¼ TT, 2/4 Tt and ¼ tt among the genotypes (1:2:1) and ¾ tall and ¼ short (3:1) among the phenotypes.

This illustrates The Product Law of Probability: the probability of two or more independent events occurring simultaneously is equal to the product obtained by multiplying the probability of each individual event.

In the monohybrid cross, the probability of finding one gamete carrying the T allele is ½. The probability of finding another gamete carrying the T allele is also ½. Therefore, the probability of these two gametes occurring simultaneously (that is, meeting during fertilization) is ½ x ½ or ¼.

The same thing applies for the t allele. The probability of finding one gamete carrying the t allele is ½. The probability of finding another gamete carrying the t allele is also ½. Therefore, the probability of these two gametes meeting during fertilization is ½ x ½ or ¼.

The trick on the heterozygotes if that there are two ways for T and t to get together. The probability of finding one egg carrying the T allele is ½. The probability of finding one sperm carrying the t allele is also ½. Therefore, the probability of these two gametes meeting during fertilization is ½ x ½ or ¼. The reciprocal is also true. The probability of finding one egg carrying the t allele is ½. The probability of finding one sperm carrying the T allele is also ½. Therefore, the probability of these two gametes meeting during fertilization is ½ x ½ or ¼. The total probability for T and t getting together is ¼ + ¼ or 2/4.
 

Where do the different alleles come from? Mutations. In the pea plants studied by Mendel, the height allele that specifies tall codes for an enzyme that synthesizes gibberellic acid (GA), a plant hormone that stimulates cell elongation in stems. The short allele of the height gene has been mutated and cannot make the enzyme that synthesizes GA. Without GA, the stems do not elongate.  The two alleles for a particular gene that an organism possesses make up the genotype. If a plant has two identical alleles for height, either two tall alleles or two short alleles, its genotype is homozygous. If the two alleles are different, such as one tall allele and one short allele, the genotype is heterozygous. The outward, physical appearance of an organism is its phenotype. The plant with two tall alleles has a tall phenotype; that pea plant will be approximately 6 feet tall at maturity. The plant with two short alleles will have a short phenotype and be about 2.5 feet tall. What about the heterozygous plant? It will be 6 feet tall, showing the same phenotype as the plant with two tall alleles. The presence of one tall allele in the heterozygous plant enables that pea plant to make enough GA that the plant can grow to wild type height. Therefore, we call the tall allele dominant and the short allele recessive, because when both alleles are present in a heterozygous individual, the activity of the dominant allele completely masks the presence of the recessive allele. The genotypes of homozygous individuals are further designated as being either homozygous dominant or homozygous recessive.

Incomplete dominance
When incomplete dominance occurs, the heterozygote has its own phenotype that is intermediate between the two traits of the true-breeding lines.
A common example of incomplete dominance can be seen in the petal color of snapdragons.

P: red (R1R1) x white (R2R2)

F1: all pink (R1R2)

F2: 1/4 red (R1R1) + 2/4 pink (R1R2) + 1/4 white (R2R2)
 

In the flower color gene described above, the red allele is the original or wild type form of the gene. It codes for an enzyme that catalyzes a reaction in which a red pigment is synthesized. For the white allele, the DNA sequence was altered and no longer codes for a functional red pigment synthesizing enzyme. The flower petals are white because no red pigment can be made.  Plants that have two red alleles produce red flowers, and those that have two white alleles produce white flowers. Heterozygous plants that have one red allele and one white allele produce pink flowers.  A single red allele in the heterozygote cannot make enough pigment to turn the petals red. There is only enough pigment to make the petals pink. As a result, the heterozygote has a unique phenotype not displayed by either homozygote.

Dihybrid Cross

When a cross is made involving two traits that are heterozygous, that is a dihybrid cross. 

 G = green pods; g = yellow pods                T = tall; t = short           

P:  green tall (GGTT) x ggtt (yellow, short)

F1:  all green tall (GgTt)

F2:  9/16 green tall + 3/16 green short + 3/16 yellow tall + 1/16 yellow short

pod color in the F2:  3/4 green + 1/4 yellow            height in the F2:  3/4 tall + 1/4 short       

Multiply the odds for pod color and height (product law of probability):  9/16 green tall + 3/16 green short + 3/16 yellow tall + 1/16 yellow short


Genetic Maps

The product law of probability can only be used when the two traits assort independently of each other during meiosis, either because they are on different chromosomes or because they are vary far apart on the same chromosome.  Genes on the same chromosome form a linkage group.  The frequency at which they separate from each other during the crossovers during prophase I of meiosis allow us to make genetic maps of organisms.  Genes that are near each other separate infrequently; genes that are far apart separate often.  The frequency of crossover give us map distance.  By comparing the results of crosses with a variety of linked traits, we can put the genes in linear order along a chromosome.


19 January 2011