Chapter 13 - Meiosis Flashcards
Please give the definition of genes and heredity.
Genes
are the unit of inheritance, made up of segments of DNA.
Heredity
is the transmission of traits from one generation to the next.
Please give the definition of an autosome.
Autosomes
The 22 pairs of chromosomes that do not determine the sex of a person
How is the sex of a human determined by the chromosomes?
The sex chromosomes are called X and Y
Human females have a homologous pair of X chromosomes (XX)
Human males have one X and one Y chromosome
What is a karyotype?
Karyotypes
is the number and appearance of chromosomes in the nucleus of an eukaryotic cell
We can perform a microscopic analysis of someone’s chromosomes
The 46 chromosomes in a human somatic cell are two sets of 23:
one from mother and one from father.
How does meiosis work?
- At sexual maturity, ovaries and testes produce haploid gametes
- Gametes are the only types of human cells produced by meiosis, rather than mitosis.
- Meiosis results in one set of chromosomes in each gamete.
- Fertilization and meiosis alternate in sexual life cycles to maintain chromosome number.
- Alternation of meiosis and fertilization is common to all organisms that reproduce sexually.
- Humans are diploid most of our life cycle but not organisms work that way.
What is the process of meiosis going from Diploid to Haploid ?
- Before meiosis, chromosomes are replicated
- Meiosis has 2 rounds of cell divisions
- Meiosis I and
- Meiosis II
- Products of meiosis are four daughter cells. Each daughter cell has only half as many chromosomes as the parent cell.
In chapter 12 we discused Mitosis.
What is the difference between Mitosis and Meiosis?
Mitosis
- Conserves number of chromosomes sets
- 23 pairs –> 2 cells with 23 pairs
- Produces genetically indentical daughter cells that are identical to the parent cell.
Meiosis
- Reduces number of chromosomes sets from pairs (diploid) to single(haploid)
- 23 pairs –> 4 cells with 23 single chromosomes
- Produces 4 genetically different daughter cells, each with different from the parent cell.
Meiosis II and Mitosis are virtually Identical
What are the 5 stages to Meiosis?
Meiosis I: First cell division
- Separates homologous chromosomes
- Preceded by interphase, in which chromosomes are replicated to form sister chromatids. The sister chromatids are genetically identical and joined at centromere.
- The single centrosome replicates forming two centrosomes.
1. Prophase I
- Typically occupies more than 90% of the time required for meiosis
- Chromosomes begin to condense
- In synapsis, homologous chormosomes loosely pair up, aligned gene by gene.
- Crossing over: mixing up genes
- Each pair of chromosomes forms a tetrad, a group of four chromatids.
- Non sister chromatids exchange DNA segments
- Each tetrad usually has one or more chiasmata
2. Metaphase I
- Tetrads line up with the metaphase plate, with one chromosome facing each pole
- Microtubules from each pole attach to kinetochores of each chromosome in the tetrad
3. Anaphase I
- Pair of homologous chromosome separate
- One chromosome moves toward each pole, guided by the spindle apparatus
- Sister chromatid remain attached at the centromere and move as one unit toward the pole.
4. Telophase I and Cytokinesis
In the beginning of telophase I, each half of the cell has a haploid set of chromosomes, each chromosome still consists of two sister chromatids.
Cytokinesis usually occurs simultaneously, forming two haploid daughter cells.
In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.
No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II ecause the chromosomes are already replicated.
Meiosis II: Second cell division
- Four phases, VERY similar to meiosis I.
1. Prophase II
- A spindle apparatus forms
- In late prophase II, chromosomes (each still composed of two chromatids) move toward the metaphase plate.
2. Metaphase II
- Sister chromatids are arranged at the metaphase plate
- Because of crossing over in meiosis I, the two sister chromatids of each chromosome are no longer genetically identical.
- Microtubules from each pole extend and attach to kinetochores of sister chromatids.
3. Anaphase II
- The sister chromatids separate
- The sister chromatids are genetically distinct from each other because of crossing over that occurred in Prophase I.
4. Telophase II and Cytokinesis
- Chromosomes arrive at opposite poles
- Nuclei form, and the chromosomes begin decondensing
- Cytokinesis separates the cytoplasm
- At the end of meiosis, there are four daughter cells, each with a haploid set of unreplicated chromosomes
- Each daughter cell is genetically distinct from the others and from the parent cell.
briefly describe what happens in Meiosis I and Meiosis II
Meiosis I
- Crossing over at Prophase I
- Pairs of replicated chromosomes are split up
- Transition from diploid to haploid condition
Meiosis II
- Sister chromatids are split
- Production of 4 haploid gametes from one diploid germ cell.
Why go through meiosis? (*hint: how does it contribute to genetic diversity)
Mutations
(changes in an organisms DNA) are the original source of genetic diversity.
Mutations create different versions of genes called alleles.
Reshuffling of alleles during sexual reproduction produces genetic variation.
What are the three mechanisms that contribute to genetic variation in offspring?
1. Independent Assortment of Chromosomes
Homologous pairs of chromosomes orient randomly at metaphase 1 of meiosis.
Maternal and Paternal homologues are “sorted” into daughter cells independently of other pairs.
The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number.
For humans (n=23), there are 223 possible combinations of chromosomes.
2. Crossing over
Crossing over produces recombinant chromosomes, which combine genes inherited from each parent.
Crossing over begins early in prophase I, as homologous chromosomes pair up gene by gene.
Homologous portions of two non-sister chromatids trade places.
Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome.
3. Random fertilization
Random fertilization adds to genetic variation because any sperm can fuse with an ovum (unfertilized egg).
Fusion of two gametes produces zygote with any of about 70 trillion diploid combinations
Crossing over ADDS EVEN MORE genetic variation.
Independent assortment of chromosomes contributes to genetic variation how?
Independent assortment of chromosomes
- Homologous pairs of chromosomes orient randomly at metaphase 1 of meiosis.
- Maternal and Paternal homologues are “sorted” into daughter cells independently of other pairs.
- The number of combinations possible when chromosomes assort independently into gametes is 2n, where n is the haploid number.
- For humans (n=23), there are 223 possible combinations of chromosomes.
Crossing over (of chromosomes) contributes to genetic variation how?
Crossing Over
- Crossing over produces recombinant chromosomes, which combine genes inherited from each parent.
- Crossing over begins early in prophase I, as homologous chromosomes pair up gene by gene.
- Homologous portions of two non-sister chromatids trade places.
- Crossing over contributes to genetic variation by combining DNA from two parents into a single chromosome.
Random Fertilization (of chromosomes) contributes to genetic variation how?
Random fertilization
- Random fertilization adds to genetic variation because any sperm can fuse with an ovum (unfertilized egg).
- Fusion of two gametes produces zygote with any of about 70 trillion diploid combinations
- Crossing over ADDS EVEN MORE genetic variation.
