2.2 - Meiosis Flashcards
What is meiosis?
A type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell
- The production of gamete cells that are NOT genetically identical (for genetic variation purposes)
What is asexual reproduction?
Asexual reproduction occurs when an organism makes a copy of itself without undergoing meiosis and fertilization. Asexually produced offspring are “carbon-copies”, i.e. are genetically identical to the parents.
What is sexual reproduction?
Sexual reproduction involves the production of sex cells (gametes) during meiosis followed by their subsequent fusion (fertilization) to yield a zygote that is genetically different from its parents.
- Union of two haploid cells (gametes) resulting in recombinant (randomized) offspring
What are the two kinds of asexual reproduction?
Mitosis and binary fission
What are the steps of fertilization/syngamy?
- Fusion of haploid gametes (egg and sperm)
- Produces a diploid zygote
- The 2n zygote grows into the adult.
Define haploid
Haploid cells are cells with half of the chromosomes they should have (23 instead of the full 46, NOT PAIRS)
- Gametes
Define diploid
Diploid cells are cells with all of the chromosomes they should have (the full 46, 23 PAIRS)
- Zygote, and all other body cells
What are autosomes?
- Found in both males and females
- In humans there are 22 pairs of autosomes
- Autosomes that are the same size (& structure) are called homologues
- All of the chromosomes that are NOT sex-chromosomes
What are sex chromosomes?
Sex chromosomes are a pair of chromosomes that determine and individual’s gender (XX for women, XY for men)
What are the characteristics of the X chromosome?
The X chromosome is much larger than the Y chromosome and contains many genes
What are the characteristics of the Y chromosome?
The Y chromosome is much smaller than the X chromosome and contains only a small number of genes
T/F: The X and Y chromosomes are homologous
False: homologous chromosomes are identical in size and structure; the X chromosome is much larger than the Y, and contains way more genes than the Y chromosome.
Why are haploid cells necessary?
Haploid cells are necessary because the merging of two diploid cells would result in offspring with 92 chromosomes instead of the correct number, 46 (two parents = 46+46)
At the beginning of meiosis, how many chromosomes/cells are there?
At the start of meiosis (meiosis I):
- 1 cell which contains 2 chromosomes (or 1 homologous pair)
At the end of meiosis, how many chromosomes/cells are there?
At the end of meiosis (meiosis II):
- 4 cells which each contain 1 chromosome
What occurs during meiosis (general description)?
- Homologues exchange sections (crossing-over)
- Homologues separate
- Chromosome number halved
- Chromatids separate
What occurs during prophase I?
Chromosomes condense and homologous pair up
- Centrioles migrate to poles
- Nuclear membrane breaks down
- Centrioles still moving to poles
- Spindle fibers forming (microtubules)
- Kinetochores form on chromosomes
- Can last for days, 90% of meiosis is spent here
- A process called synapsis occurs (crossing over)
What is synapsis?
Tetrads form (chromosomes connected together)
- Non-sister chromatids cross over exchanging genetic information (chiasmata)
What occurs during metaphase I?
Chromosome arranged on metaphase plate
- Kinetochore microtubules are attached to one chromosome from each tetrad (opposite poles)
What occurs during anaphase I?
Spindle guides WHOLE chromosomes to poles
- Unlike mitosis – sister chromatids move to same pole
- There are many possible combinations (2nd source of genetic variation: random assortment of homologous pairs on metaphase plate)
What occurs during telophase I/cytokinesis?
Pairs of homologous chromosomes reach poles
- Each pole now has a haploid set (still with 2 sister chromatids)
- Cytokinesis occurs at same time
- Cleavage furrows (or cell plate) form
- There is no further replication although chromosomes will divide again at MEIOSIS II (sister chromatids will separate)
What is interkinesis?
Interkinesis is a pause between meiosis I and meiosis II
- Note: no synthesis (S) phase, therefore no DNA replication
What occurs during prophase II?
Spindle reforms and the chromosomes move to metaphase II plate
What occurs during metaphase II?
Chromosomes are lined up along plate
What occurs during anaphase II?
Centromeres begin separation of sister chromatids
- Sister chromatids become individual chromosomes.
- Chromosomes move towards opposite poles
What occurs during telophase II/cytokinesis?
- Separation of cytoplasm
- Cells completely split
- Nuclei reform, and spindle fibers break up
- You will now have 4 completely unique daughter cells (independently assorted)
What are the 3 forms of genetic variation?
Crossing over:
- After crossing over and synapsis, sister chromatids are no longer identical
Independent assortment:
- Each human can produce over 8.3 million different gametes by random shuffling of chromosomes in meiosis I
Fertilization:
- A couple can produce over 64 trillion (8.3 million x 8.3 million) different zygotes during fertilization*
- Note: this figure does not take into account diversity created by crossing over
How many crossover events occur in prophase I?
In prophase I, approximately 2-3 crossover events occur per CHROMSOME PAIR in humans
How does independent assortment work?
How one pair of homologues lines up on the equatorial plane is independent of how any other pair of homologues lines up.
- Lineup of homologous pairs will be random/variable (maternal and paternal will not necessarily alternate: could be MPMPMP, or MMPMPPM, or any other random order)
What are the differences between mitosis and meiosis?
- Meiosis occurs in germinal (germ line) tissues
- Animals: ovaries and testes
- Plants: pistils and anthers - Meiosis involves two successive nuclear divisions in which the chromosomes are duplicated only once. This is a reduction division. This is made possible by the way chromosomes uniquely pair and align in meiosis
- At the completion of meiosis, there are 4 daughter cells each with ½ the number of chromosomes as the mother cell while in mitosis there are 2 identical diploid sister cells
- The 4 daughter cells are genetically different due to crossing over and independent assortment
How does sexual reproduction ensure genetic variability?
Sexual reproduction ensures genetic variability by:
- Chromosomes swap segments (and genes) during synapsis (CROSSING OVER)
- Each reproductive cell receives only one of each kind of chromosome in any combination (INDEPENDENT ASSORTMENT).
- Sperm and egg that join are from two different individuals (RECOMBINATION)
