Meiosis and Linkage Flashcards
Explain what meiosis is and explain what briefly happens in the 2 stages of meiosis
Meiosis consists of two successive nuclear divisions (Meiosis I and Meiosis II).
Meiosis I: two daughter nuclei.
Meiosis II: each of the two daughter nuclei divides, resulting in four nuclei.
Chromosomes duplicate at the start of Meiosis I but do not duplicate in Meiosis II.
Explain what happens in Prophase I
Chromosomes thicken and become visible.
Centromeres begin to move toward opposite poles.
Homozygous chromosomes enter synapsis.
The synaptonemal complex forms (SC).
Synapsis is complete
Crossing over – genetic exchange between nonsister chromatids of a homologous pair – occurs.
Synaptonemal complex dissolves.
A tetrad of four chromatids is visible.
Cross-over points appear as chiasmata, holding non-sister chromatids together.
Chromatids thicken and shorten.
The nuclear membrane breaks down; the spindle forms
Explain what happens in synapsis
Each chromosome seeks out its homologous partner.
The matching chromosomes become zipped together in a process known as synapsis.
The ‘zipper’ itself is a protein structure called the synaptonemal complex (SC).
SC aligns the homologs with remarkable precision, juxtaposing the corresponding genetic regions of the chromosome pair.
Explain what the synaptonemal complex (SC) is
The joining of homologous pairs of chromosomes is called synapsis
A macromolecular assemblage directs it called the synaptonemal complex
Explain what happens in crossing over
Structures called recombination nodules appear at the SC.
The exchange of parts between non-sister chromatids (father and mother) occurs at these nodules.
Such an exchange is known as crossing-over.
Results in the recombination of genetic material.
Chromatids may no longer be of purely maternal or paternal origin.
No genetic information is lost or gained, all chromatids retain their original size.
The sites of crossing-over are called chiasmata (singular, chiasma).
Explain what happens in Metaphase I
Tetrads line up along the metaphase plate
Each chromosome of a homologous pair attaches to fibers from the opposite poles
Sister chromatids attach to fibers from the same pole
Explain what happens in Anaphase I
Centromere does not divide
The chiasmata dissolves
Homologous chromosomes move to opposite poles
Explain what happens in Telophase I and Interkinesis
Nuclear envelope re-forms
Resultant cells have half the number of chromosomes, each consisting of two sister chromatids
Interkinesis: interphase between Meiosis I and Meiosis II
Interkinesis is similar to interphase with one important exception: no chromosomal duplication takes place (no S phase)
Number of chromosomes reduced to half of the normal diploid number
Meiosis I = reductional division
Explain what happens in Prophase II
Chromosomes condense
Centrioles move toward the poles
The nuclear envelope breaks down at the end of prophase II
Spindle apparatus reforms
Explain what happens in Metaphase II
Chromosomes align at the metaphase plate
Sister chromatids attach to the spindle fibers from the opposite pole
The two sister chromatids are no longer identical due to recombination in Meiosis I
Sister chromatids carry the same genes but may carry different combinations of genes
Explain what happens in Anaphase II
Centromeres divide, and sister chromatids move to opposite poles
Explain what happens in Telophase II and Cytokinesis
Chromosomes begin to uncoil
Nuclear envelope and nucleoli re-form
The cytoplasm divides, forming 4 new haploid cells
Explain why Meiosis II is called equation division
Does not reduce the number of chromosomes; similar to Mitosis)
Why do mistakes in meiosis produce defective gametes?
Segregation errors during meiotic division can lead to aberrations, such as trisomies, in the next generation
If for example, the homologs of a chromosome pair do not segregate during meiosis I (a mistake known as non-disjunction), they may travel together to the same pole and eventually become part of the same gamete.
Most trisomies are lethal in utero; one exception is Trisomy 21.
What is the role of crossing over in meiosis?
Produces new allelic combinations
What does it mean if genes are on the same chromosome?
Linkage: inherited together
Explain what and how genetic linkage works
Two genes are linked if they are physically so close that meiosis occurs with no cross-overs between them.
The two linked loci tend to be inherited together.
The recombination frequency (RF) indicates how often two genes are transmitted together.
Genes with recombination frequencies less than 50 percent are on the same chromosome = linked.
Linkage group = all known genes on a chromosome.
Two genes that undergo independent assortment have a recombination frequency of 50 percent and are located on nonhomologous chromosomes or far apart on the same chromosome = unlinked.
The distance between two loci (specific location of a gene) is measured in centimorgans and 1 centimorgan is defined as the genetic distance between two loci with a recombination frequency of 1%.
1.0% recombinants = 1.0 map unit = 1.0 centiMorgan (cM).
Why is linkage analysis important?
Linkage analysis is a method used to map the relative positions of two or more loci using genetic markers.
Diseased genes are often passed to offspring along with specific genetic markers.
A genetic marker is a DNA sequence representing a specific locus, that comes in identifiable variations. These allelic variations segregate according to Mendel’s laws, which means it is possible to follow their transmission as you would for any gene.
Genetic markers can help link an inherited disease with the responsible diseased gene because linked genes (i.e. genetic marker + diseased gene) are inherited together.
Genetic markers are used to track the inheritance of a nearby gene that has not yet been identified, but whose approximate location is known.
What is the difference between a genetic map and a physical map?
Genetic map distances (measured in m.u.) based on recombination frequencies are not a direct measurement of physical distance (bp of DNA) along a chromosome.
Recombination of ‘hot spots’ (small regions of DNA where the frequency of recombination is much higher than average) overestimates physical length.
Low rates in heterochromatin and centromeres underestimate actual physical length.
What does meiosis result in:
4 haploid daughter cells
Which of the following cells undergo meiosis?
Brain cell
Liver cell
Unicellular organisms
Sperm Cells
All of the above
Sperm cells
Which of the following distinguishes prophase 1 of meiosis from prophase of mitosis?
Spindle forms
Homologous chromosomes pair up
The nuclear membrane breaks down
Chromosomes become visible
Homologous chromosomes pair up
A cell with a diploid number of 24 undergoes meiosis. How many chromosomes are in each daughter cell (Meiosis I)?
12
During which stage of meiosis does crossing over occur?
Prophase I
An organism has 56 chromosomes in its diploid stage. How many chromosomes are present in somatic cells?
56
An organism has 56 chromosomes in its diploid stage. how many chromosomes are present in metaphase (mitosis)
112
An organism has 56 chromosomes in its diploid stage. How many chromosomes are present in metaphase I?
112
An organism has 56 chromosomes in its diploid stage. How many chromosomes are present in metaphase II?
56
An organism has 56 chromosomes in its diploid stage. indicate how many chromosomes are present in gametes
28
True or False recombinants are any combination of alleles that are the same as the parental combinations
False
True or False recombination frequency increases with increasing distance between genes; however, no matter how far apart two genes may be, the maximum frequency of recombination between any two genes is 50%
True
