Chromosomes & Cells
What is the haploid number in humans?
23 (n = 23)
What key event distinguishes Meiosis I from Mitosis?
Separation of homologous chromosomes.
What happens to the sister chromatids in Anaphase II?
They separate and move to opposite poles.
What type of chromosomes are produced by crossing over?
Recombinant chromosomes.
How many daughter cells result from mitosis?
Two identical diploid cells.
What is a locus?
The specific physical location of a gene on a chromosome.
What is the structure formed when homologous chromosomes pair during Prophase I?
A tetrad (or bivalent)
Why is Meiosis II sometimes called "mitosis of a haploid cell"?
Because sister chromatids separate similarly to mitosis, but the cell starts as haploid.
At what stage does crossing over occur?
Prophase I.
True or false: Crossing over occurs in both mitosis and meiosis.
False; it occurs only in meiosis (specifically Prophase I).
What do we call different versions of the same gene?
Alleles
What holds homologous chromosomes together at chiasmata before separation?
Cohesin proteins along chromosome arms.
How many cells result from Meiosis II starting from two cells at the end of Meiosis I?
Four haploid cells.
What is the formula for calculating possible gamete combinations through independent assortment?
2ⁿ where n = number of homologous pairs.
What protects cohesin around the centromere in Meiosis I but not in mitosis?
Shugoshin proteins.
In a human somatic cell, how many autosomes are present?
44 autosomes (22 pairs)
Why must homologous chromosomes attach to opposite spindle poles during Prometaphase I?
To ensure correct separation into two haploid cells.
What is one major difference between the starting cells of Meiosis II and mitosis?
Cells entering Meiosis II are haploid with sister chromatids; cells entering mitosis are diploid.
If a species has 5 pairs of chromosomes, how many genetically different gametes can it produce?
2⁵ = 32 possible gametes.
List two events unique to meiosis and not found in mitosis.
Synapsis of homologs, crossing over, formation of tetrads, reduction in chromosome number.
Explain why homologous chromosomes are similar but not identical.
They carry genes for the same traits in the same locations, but the alleles may differ because they come from different parents.
Describe the process of synapsis and explain its importance.
Homologous chromosomes pair and align gene-by-gene, forming the synaptonemal complex; this alignment enables accurate crossing over.
Explain why Meiosis II is necessary even though homologous chromosomes already separated in Meiosis I.
Sister chromatids must be separated so each gamete receives a single complete set of unduplicated chromosomes.
Explain how the combination of crossing over and independent assortment creates exponentially more variation than either mechanism alone.
Independent assortment mixes whole chromosomes randomly, while crossing over mixes alleles within each chromosome. Combined, they generate massive unique allele combinations across all chromosomes.
Describe two ways meiosis contributes to evolution, while mitosis does not.
Produces genetic variation that natural selection can act on.
Ensures long-term stability of chromosome number while still allowing diversity.