Exam scores improved; will return them next Wednesday.
One makeup exam remains to be completed.
The final exam will cover the last third of the class, emphasizing workout problems involving Mendel's work and linking it to gene concepts, similar to lab exercises.
The review sheet will be less detailed compared to previous ones, focusing on fundamental skills like two-gene crosses and Punnett squares.
Topics from the previous exam not adequately covered, such as polyploidy and structural mutations, will be included in Exam 3.
The content of the last lecture may be reduced to essential information due to time constraints.
Polyploidy
Polyploidy arises from complete nondisjunction.
Normal organisms are diploid (2n).
Polyploidy involves multiple sets of chromosomes (n).
If 2n = 6, then n = 3. A triploid would be 3n = 9 instead of 6.
A tetraploid would be 4n = 12.
n represents the number of chromosomes in a haploid set (e.g., 23 for humans).
Polyploidy typically involves multiples of the haploid number, often seen in plants.
Euploidy refers to any deviation from the normal chromosome number, either less or more.
Mechanisms Leading to Polyploidy
Polyspermic fertilization: An egg (n) fertilized by two sperm (n + n) results in a triploid (3n) organism.
Eggs have mechanisms to prevent multiple sperm entries.
Complete nondisjunction:
Normal meiosis: 2n leads to n.
Abnormal meiosis: 2n leads to 4n (tetraploid) and null (no genetic material).
An egg created by nondisjunction (2n) fertilized by sperm (n) results in a triploid (3n).
A sperm created by nondisjunction (2n) fertilizing an egg (2n) establishes a tetraploid (4n) lineage.
Ploidy vs. Aneuploidy
Ploidy: Refers to the number of complete sets of chromosomes in a cell.
Triploid, tetraploid involve whole sets.
Aneuploidy: Refers to an abnormal number of individual chromosomes.
Nondisjunction can occur in either meiosis I (M1) or meiosis II (M2).
Meiosis I:
Abnormal: 2n, 2n
Meiosis II:
Produces half normal gametes (n) and half abnormal (2n and null).
The effect is the same: a triploid can result when a 2n gamete fertilizes a normal n gamete.
Complete nondisjunction is necessary to increase genetic material significantly.
Fertilization of two non-disjunction gametes (2n + 2n) results in an abnormal amount of DNA, but with equivalent amounts from each parent, potentially leading to a more balanced scenario than triploidy.
Triploids tend to be sterile due to difficulties in meiosis because chromosomes must align in pairs.
Nondisjunction in Mitosis
Mitosis is not typically associated with polyploidy, but nondisjunction can occur.
During anaphase, chromosomes may not split evenly, leading to aneuploidy (n+1, n-1).
This can result in rogue, dysfunctional cells typically dealt with by the immune system.
In cancerous tumors, rapid cell division increases the likelihood of mitotic nondisjunction, resulting in mosaicism.
Mosaicism: The presence of cells with different genetic makeups within the same organism.
A cell may fail to form a nucleus around all chromosomes, resulting in an n-1 scenario.
Every individual has some level of mosaicism due to errors in DNA replication and other factors.
Polyploidy in Nature and Artificial Selection
Euploidy: Deviations from the normal diploid number (umbrella term from n to however many polyploid).
Polyploidy: Focus on deviations in the high end of ploidy.
Artificial selection has led to polyploidy in crops.
Corn was originally small kernels with mixed colors and less nutrients. Selective breeding has resulted in larger kernels with more starch and nutrients.
Larger strawberries are a result of polyploidy.
Larger cell size in polyploids is due to the nucleus size; a larger nucleus leads to larger cells with more products.
Polyploidy results in larger flowers and kernels of wheat.
Endopolyploidy
Endopolyploidy: Polyploidy within cells in the human body.
Liver cells exhibit endopolyploidy, increasing DNA content for enhanced function.
Liver cells can regenerate and increase their DNA content without undergoing cytokinesis after replication.
Copies of chromosomes stay attached.
Leads to triploid, tetraploid, or octoploid cells.
This increases enzyme production for detoxification and secretion.
Liver cells increase transcription levels and gene copies.
There is a limit to how large a cell can grow, despite continued replication.
Uncontrolled expansion can lead to cancer if cell cycle checkpoints fail. Liver cancer occurs when normal checks are bypassed.
Polytene
Polytene chromosomes: Replicated chromosomes without cytokinesis in fly salivary glands.
Salivary glands in flies require a lot of enzymes for external digestion, which is why Polytene Chromosomes are needed.
Sterility in Triploids
Triploids are sterile because they have three copies of each chromosome, causing issues during meiosis.
During metaphase I, chromosomes must pair two by two, but the third copy interferes with proper alignment, leads to aneuploidy.
Examples include seedless watermelons and seedless flowers.
Seedless watermelons have small, white seeds due to failed attempts at meiosis.
Seedless flowers are larger but not propagatable through normal seed reproduction.
Propagation of Polyploids
Tetraploids (4n) can propagate if they cross with another tetraploid, because even numbers segregate evenly during meiosis (2n + 2n = 4n).
Crossing a tetraploid with a diploid (2n) results in a triploid (3n), which grows but cannot undergo meiosis.
Triploids can be propagated through cuttings (cloning).
Vines for grapes are often made this way.
Interspecies Hybridization and Polyploidy
Interspecies hybridization involves combining genetic material from two different species.
Auto- vs. Allo- prefixes:
Auto- = Self (within same species).
Allo- = Other (different species).
Autopolyploidy
Autopolyploidy arises from the same species.
When the same organism undergoing nondisjunction, generating a 2n gamete that unites with other same gamete generating 4n.
Complete nondisjunction results in a 2n gamete uniting with another to create a 4n scenario.
Easiest form of creating polyploidy; from genetically identical genetic material.
Allopolyploidy
Allopolyploidy involves combining chromosomes from different species.
Allotetraploid: Stable form where each species brings its own set of chromosomes, which then partner during meiosis.
Two 2n organisms make a 4n organism.
Allodiploid: Unstable form where the chromosomes from different species have different gene arrangements and cannot pair properly during meiosis.
Mules
Mules are a classic example of allodiploidy.
Result from the cross of a female horse and a male donkey, it matters which way that the species goes.
Horses have 64 chromosomes (2n = 64), and donkeys have 62 chromosomes (2n = 62).
Mules have 63 chromosomes (2n = 63) because the female gets 32 and the male gets 31.
Mules cannot reproduce with each other due to the odd number of chromosomes and the gene differences.
Ligers
Ligers (lion + tiger) are another example of interspecies hybrids with fertility issues.
Allodiploid Chromosome Pairing
Allodiploidy occurs when n+n is added together from species 1 and species 2.
However, the chromosomes found have genes in different places than the original.
The Mule is an example because can undergo fertilization, but when it realigns in meiosis, pairing isn't able to happen since the genes don't lineup.
Structural Mutations: Deletion and Duplication
Gene Dosage
The amount of genes and individual has, and how much gene dosage is the critical point.
Deletion (Deficiency):
Loss of a chromosome segment.
Can involve the loss of tens to hundreds of genes.
Not just frameshift mutations affecting single genes; involves losing large amounts of genetic material.
Partial monosomy: Lacking a chromosome from that tip.
Recessive lethal genes may be expressed if the functional copy is lost.
The location of the deletion (end or middle) is irrelevant; its effect on gene content is the main concern.
Pseudo dominance: The expression of a recessive allele due to the deletion of the dominant allele.
If one allele has a dysfunction, and a gene is deleted, a disorder will occur.
Pseudoautosomal
Pseudoautosomal refers to a sex chromosome behaving like an autosome.
Cri du Chat Syndrome
Cri du chat (cry of the cat) syndrome is caused by a deletion of part of chromosome 5.
Characterized by a distinctive, high-pitched cry that sounds like a cat.
The nurse reviewing newborns can hear the specific sound and recognize the disease and its dysfunction.
Duplication
Duplication is the opposite of is where more material is gained.
Duplication is the presence of an extra copy of a chromosome segment.
Trisomy of a particular location.
More survivable than deletions.
The size of the duplication and the genes duplicated determine the impact.
Unequal Crossing Over
Unequal crossing over during recombination can result in duplications and deletions.
Misalignment of chromosomes during meiosis can lead to crossover events in the wrong position, producing gametes with either extra or missing segments.
One chromatid gains material, the other loses material.
If at any point a gene gets deleted, there has to be a time where the opposite gains said material.
Evolution of Genes
Gene duplication followed by mutation drives molecular evolution.
Duplicated genes can acquire new functions over time as they mutate.
Molecular Evolution of Globin Genes
Tracing back evolutionary time, there was an ancestral globin gene. Molecular Evolution happens from ancestral genes that duplicated.
The globin gene duplicated, creating myoglobin and hemoglobin.
Hemoglobin can exchange oxygen while also exchanging carbon dioxide. This makes it easier for aerobic organisms.
Myoglobin has a higher affinity to bind to oxygen, compared to hemoglobin, by the muscles that need it.
The beta and alpha subunits of these oxygen proteins all come from the original subunit that combined gene duplication.