Chapter 13: Meiosis and Sexual Life Cycles Flashcards
What happens when meiosis goes wrong?
Gametes end up with extra or missing chromosomes that often results in spontaneous abortion of the fetus (miscarriage)
What is synapsis?
The pairing of homologous chromosome pairs in prophase I to form a tetrad
Overview of meiosis I
Homologous chromosome pairs separate into two cells that are now haploid cells with duplicated chromosomes
What are the 2 ideas on why the risk for Down syndrome increases with maternal age?
- Body releases the good eggs first
2. Amount of cohesin changes so chromatids aren’t held as tightly together
What does the diploid number represent?
Two copies (homologs) of each chromosome (1 from mom and 1 from dad)
Offspring vary genetically from parents and siblings
Sexual reproduction
No sperm or egg required
Asexual reproduction
What is random fertilization?
Any sperm (8.4 million possibilities) can fertilize any egg (8.4 million possibilities)
What does the changing-environment hypothesis state?
If offspring are genetically diverse, it is more likely at least some will be resistant in a changing environment (like a new pathogen) and can survive to pass on beneficial alleles
What is a karyotype?
Chromosomes frozen in order to be observed
What is the final result of meiosis?
The formation of 4 haploid cells that may become sperm or eggs
What are somatic cells?
Non-sex cells
What happens in interphase I?
Chromosomes duplicate resulting in pairs of homologous chromosomes
What happens immediately after meiosis I?
The cell goes directly to meiosis II. There is no DNA replication
What 3 events are unique to meiosis only?
Synapsis and crossing over, line up of homologous pairs, separation of homologous pairs (homologs) and not chromatids
Sperm fertilizes egg to form zygote
Sexual reproduction
What is aneuploidy?
Abnormal chromosome number (extra or missing)
What happens in anaphase II?
Sister chromatids of chromosome pairs separate and are pulled to opposite ends
What happens in metaphase II?
Microtubules attach to kinetochores and and line up chromosome pairs at the metaphase plate
Symptoms of trisomy 21 (Down syndrome)
Characteristic facial features, developmental delays, short stature, heart defects, sexually underdeveloped (or sterile)
Creates clones (offspring genetically identical to each other and parent)
Asexual reproduction
What are sister chromatids?
Duplicated chromosomes from one parent
Human haploid number
n=23
Meiosis. Requires 2 parents
Sexual reproduction
What is the life expectancy of trisomy 21 (Down syndrome)?
55
Overview of meiosis II
Sister chromatids of homologous chromosome pairs separate to create 4 haploid cells
Overview of interphase
Pair of homologous chromosomes in diploid parent cell get duplicated in s phase. Sister chromatids form in duplicated pairs of homologous chromosomes
What happens in prophase I?
Homologous chromosome pairs pair and exchange segments (crossing over). The pairing of homologous chromosomes is called a tetrad (bivalent)
How many crossover events occur per homologous pair?
1-3
Why is meiosis important for organisms who reproduce sexually?
Offspring would continue to gain more and more chromosomes if it did not happen
What is a tetrad and when does it form? What is the result of the tetrad?
The pairing of homologous chromosome pairs during prophase I. It results in crossing over
Mitosis/budding/fission. Requires 1 parent
Asexual reproduction
What is the only viable monosomy?
XO
What are the advantages of asexual reproduction?
More efficient, no males needed, every individual capable of reproducing, more offspring, all offspring can reproduce independently
How does variation occur in asexual reproduction?
Mutations during DNA replication
What are recombinant chromosomes?
Chromosomes that contain DNA derived from two different parents
What is polyploidy?
Having more than two paired sets of chromosomes (triploid, tetraploid, etc)
What is gametogenesis?
The process of the 4 haploid gametes resulting from meiosis becoming sperm or eggs
What happens in telophase II?
Four haploid daughter cells separate each of which contains a single chromosome (haploid)
What does the haploid number represent?
One set of chromosomes (gametes)
How do eggs develop?
Meiosis I begins when the female is in the womb, then arrests in prophase I, and restarts when puberty is reached
What aneuploidys are the only ones seen in live births?
Trisomies of 21, 18, 13 and sex chromosomes
What are non sister chromatids?
The chromosomes of paired homologous chromosomes (one from each parent). When the homologous chromosomes pair
What are the 3 sources of genetic variation?
Independent assortment, crossing over, and random fertilization
How does the time of nondisjunction relate to end problems of meiosis?
The earlier it happens, the more cells get messed up in the end (gametes have wrong numbers)
What happens in prophase II?
Chromatin is condensed back into chromosomes
How many chromosomes do humans have? Pairs? Autosomal and sex?
46 chromosomes
23 pairs
22 autosomal pairs
1 sex pair (XY or XX)
Two hypotheses why sexual reproduction exists
Purifying selection hypothesis and changing-environment hypothesis
How does meiosis occur?
In two steps (meiosis I and meiosis II)
Human diploid number
2n=46
What happens in anaphase I?
Pairs of homologous chromosomes split up but sister chromatids remain attached
What have haploid numbers of chromosomes?
Gametes (sperm and eggs)
Where is meiosis going wrong more tolerated?
More tolerated in plants than animals
What happens in telophase I and cytokinesis I?
The cell splits into 2 haploid cells with 2 sister chromatids each (have duplicated chromosomes)
What does crossing over produce?
Recombinant chromosomes (chromosomes that contain DNA derived from two different parents)
What is trisomy 21?
Down syndrome
What happens to the risk of Down syndrome as maternal age increases?
It begins to increase exponentially
What does meiosis do?
Reduced chromosomal content from 2n to n
What does the purifying selection hypothesis state?
Natural selection occurs against deleterious alleles. Not all offspring will inherit the bad allele like in asexual reproduction. At least 50% won’t inherit allele so they are able to survive and pass on good genes
What happens in metaphase I?
Microtubules attach to kinetochores and line up tetrads (bivalents) at the metaphase plate
What is the result of meiosis I?
2 haploid cells but each chromosome still has 2 sister chromatids (have duplicated chromosomes but are haploid)
What is the result of random fertilization?
70 trillion possible zygote combinations + variation due to crossing over
What does chromosome count not show?
The complexity of the organism
Triploid, tetraploid, and octaploid number examples
3n, 4n, 8n
Where is polyploidy most common?
Plants (30-80%)
What is nondisjunction?
The failure of paired chromosomes to separate
What is independent assortment of chromosomes?
Maternal and paternal copies don’t have to be on the same side. Orientation of maternal and paternal homologs is random and each homologous pair is sorted independently of the others
What do most aneuploidys result in?
The embryo not surviving to term
What are homologous chromosomes?
Same chromosomes from different parents
What is a centromere?
The center of a chromosome
