Meiosis and Sexual Life Cycles

• Meiosis 
• 2 parents
• Offspring vary genetically from both parents and siblings
• Sperm fertilizes egg to form zygote

• Mitosis/Budding/Fission
• 1 parent
• Creates clones (offspring genetically IDENTICAL to each other and parent)
• -Variations can arise through mutations 
• No sperm or egg required

DNA/Genes

• 46 total chromosomes (23 pairs)
• -22 pairs of autosomes (homologous chromosomes)
• -1 pair of sex chromosomes

• 2n=46
• Two copies of each chromosome (1 from mom and 1 from dad)
• Somatic cells

• n=23
• One set of chromosomes
• Gametes (sperm and eggs)

Visual representation of chromosomes in a cell

• Having more than two paired sets of chromosomes (triploid, tetraploid)
• Very common in plants (30-80%)
• Chromosome count doesn't mean complexity

The two species have different genes

• They are seedless because they are sterile
• There is an odd number of chromosomes so the gametes (seeds never form properly)

Is one duplicated chromosome

• Two nonsister chromatids
• A set from the mother and a set from the father

the region of a chromosome to which the microtubules of the spindle attach, via the kinetochore, during cell division.

E) They carry the same genes

• It reduces chromosomal content from 2 n to n
• Occurs in 2 steps (Meiosis I and Meiosis II)
• Results in the formation of 4 haploid cells (may become sperm or eggs)

You need to ensure that the zygote has the appropriate chromosome count

The pair of homologous chromosomes in diploid parent cell duplicate to become a pair of homologous chromosomes consistent of sister chromatids

• Homologous chromosomes pair and exchange segments (crossing over)
• Synapsis: pairing of homologs to form tetrad

The pairs of homologous chromosomes (bivalents) line up at the metaphase plate and the spindle fibers attach to the chromosomes

• The homologous pairs are separated. 
• Sister chromatids remain attached

• 2 haploid cells
• Each chromosome still has two sister chromatids

Homologous chromosomes

The chromosomes uncondenses and the nuclear envelope reforms

• When the cleavage furrow splits the cell
• When the cytoplasm splits off and the cell membrane pinches off
• Two haploid cells form; chromosomes are still double

• The 2 daughter cells, each with 23 chromosomes condense into X-shaped structures
• The membrane around the nucleus disolves and meiotic spindle forms again

The pair of (nonidentical) sister chromatids line up along the metaphase plate and the mitotic spindle fibers attach to each of the sister chromatids

• The (nonidentical) sister chromatids are then pulled to opposite poles 
• The separated chromatids are now individual chromosomes

• The chromosomes complete their move to the opposite poles of the cell
• A membrane forms around each set of chromosomes to create two new cell nuclei

• The cytoplasm splits creating 4 granddaughter cells
• With 23 chromosomes

• Synapsis and crossing over
• Line up of homologous pairs
• Separation of homologs not chromatids

• Independent assortment
• Crossing over
• Random fertilization

• Orientation of maternal and paternal homologs is random
• Each homologous pair is sorted independently of the others

• 2ⁿ
• n= # of chromosomes
• 2²³= 8.4 million possible gametes

• Produces recombinant chromosomes
• -contains DNA derived from two different parents
• 1-3 crossover events occur per homologous pair

Meiosis I at metaphase alignment

Homologous chromosomes

• Any sperm (~84 million possibilities) can fertilize any egg (~8.4 million possibilities)
• =70 trillion possible zygote combinations + variation due to crossing over

A) presence of dominant genes

• Gametes end up with extra or missing chromosomes
• -often results in spontaneous abortion of the fetus
• Better tolerated in plants than animals

• Products: n+1, n+1, n-1, n-1
• number of chromosomes

• Products: n+1, n-1, n, n
• number of chromosomes

The failure of one or more pairs of homologous chromosomes or sister chromatids to separate normally during nuclear division, usually resulting in an abnormal distribution of chromosomes in the daughter nuclei.

• Abnormal chromosome number (i.e one extra or one missing)
• Most are so disastrous that the embryo doesn't survive to term

Trisomy 21, 18 and 13

• Sex chromosomes
• XXY, XYY, XXX, XO
• XO- individual that only has 1 X so they are likely to be steril

• Down syndrome
• 1/700 children born in the US
• Symptoms range from mild to severe (characteristic facial features, developmental delays, short stature, steril)
• Life expectancy has increased to around 55

• As a women's age increases so does her risk of having a child with down syndrome
• This may be because of the way eggs develop
• -Meiosis I begins when female is in the womb (arrests in prophase I and restarts when puberty is reached)

There are more women having more kids under 35

D) produces offspring genetically identical to the parent

They both can occur in multicellular organizms

2

One from mother and one from father

• Asexual reproduction is more efficient
• -no males (who cannot give birth) needed
• -every individual capable of reproducing

• Purifying selection hypothesis
• Changing-environment hypothesis

• Genetically identical offspring not likely to thrive if the environment changes (i.e new pathogen)
• If off spring are genetically diverse, they are more likely to at least some will be resistant and can survive to pass on beneficial alleles

Natural selection against deleterious alleles (in asexual reproduction, all offspring would receive defective allele)