Lecture 10

Question 1

Review Anomalocaris

Answer 1

• large swimming predator discovered in stages
• the mouth parts were originally thought to be a fossil jellyfish (originally said that ‘jellyfishes’ had teeth)
• the grasping arms were originally thought to be large shrimps
• the soft body was originally thought to be part of a sponge

Question 2

Why are fossils so well preserved?

Answer 2

• Mid-Cambrian (geological setting of burgess shale)

levels:

• shallow sea
• fine sediment slides (turbidites) which results in rapid burial and low O2
• fossil deposits form in deep water at base of escarpment

Question 3

What are the 2 main oxygenation events?

Answer 3

• great oxygenation event (2.3 - 2.1 bya) -> iron ore deposits
• late precambrian oxygenation (~700 mya) -> “Cambrian Explosion” of marine animals

Question 4

review order from oldest to more recent

Answer 4

• First Chordates. Burgess Shale marine fauna Cambrian Period
• early Tetrapod transition to land or “FISHAPOD” Devonian Period
• highest oxygen, PLANTS diversify
• early mammals
• first angiosperms
• Mass Extinction, end of cretaceous period

Question 5

How many mya did most dinosaurs go extinct?

Answer 5

• ~66 mya

Question 6

Who filled niches “vacated” by dinosaurs?

Answer 6

• Cenozoic radiations of mammals and birds

Question 7

What are the 4 evidence for extraterrestrial impact?

Answer 7

• large crater by Yucatan, Mexico (180km in diameter)
• Microtektites in sediments around the crater. (these glass droplets form only under extremely high temperatures and pressures as the molten rock is splashed from an impact site)
• global dust layer with high iridium (very rare on earth but abundant in asteroids and meteors)
• shocked quartz grains (forms only under extremely high pressures created from events like impacts (not from volcanoes)

Question 8

What is the biggest extinction

Answer 8

End-Permian extinction

• background extinction
• At the end of the Permian, conditions became unsuitable for most life, and about 95% of marine species were eliminated as well as 70% of terrestrial species in a very short period of time, in geologic terms. It was also one of the few known mass extinctions of insects. Many plant species also went extinct

Question 9

which mass extinction event are we in on Earth? explain a lil

Answer 9

• 6th mass extinction
• current extinction rate accelerating
• seems “slow” by human lifetime standards, but would be considered very fast on geological timescales

Question 10

What is Meiosis also called?

Answer 10

• also called reduction division since chromosome number is halved (2n to n)

Question 11

Where does meiosis happen?

Answer 11

• only happens in the ovaries of females (producing eggs) and testes of males (produce sperm)

Question 12

What restores the diploid number after meiosis

Answer 12

fertilization

Question 13

What is asexual reproduction?

Answer 13

• some organisms regularly reproduce without sex
• parent passes 100% of its genes to all offspring (clones!)

Question 14

What is sexual reproduction

Answer 14

• occurs through fertilization of gametes produced by meiosis to produce genetically novel individuals
• offsprings are similar but also genetically distinct from both parents and siblings!
• each parent passes only ~50% of its genes to each offspring, that may include both males and females!

Question 15

the paradox of sex: why, in theory, should sex not exist?

Answer 15

• by breeding with a male, a female halves her genetic contribution to her offspring
• only half the female population is capable of bearing offspring - asexual competitors would win by out-reproducing sexual species
• if in a normal sexual population, some females can reproduce asexually, they will soon replace the sexuals by out-reproducing them: like some lizards

Question 16

What are the 2 explanations for the dominance of sexual reproduction (both are likely)?

Answer 16

1. Purifying Selection Hypothesis
2. Changing-Environment Hypothesis (accumulating variations fights pathogens)

• totally asexual species should ultimately be more prone to extinction compared to related sexual species! (both can co-exist in the short term)

Question 17

explain purifying selection hypothesis

Answer 17

• asexual organisms pass on all their genes to the next generation, including mutations that are deleterious
• they cannot permanently escape the damaging alleles which cause population reductions
• diploid sexual organisms have 2 chromosomes and “spare” copies of genes, and can avoid the effects of deleterious alleles through dominance, and “purify” their genomes by selecting out damaging alleles over time

Question 18

explain changing-environment hypothesis

Answer 18

• genetic variations produced by meiosis ensure the survival of some individuals in a population that may have rare mutations for immunity or resistance to new disease organisms

Question 19

What is the long-term advantage that sexual species have? Mutations may be what?

Answer 19

• harmful mutations (most are recessive) can be masked by a second copy of a normal gene on the homologous chromosome = DIPLOIDY (not in haploid prokaryotes)

Mutations may be:
- Deleterious (damaging)
- Neutral in Effect
- Beneficial (changed conditions)

• since many mutations are recessive in their expression, they are “stored” in the diploid condition, and add to genetic variation available for future selection! Darwin’s artificial selection confirmed this potential for change
• DIPLOIDY, SEX AND MEIOSIS ARE LINKED AND PROBABLY CO-EVOLVED

Question 20

what are homologous chromosomes?

Answer 20

• one of each chromosome pair is maternal (derived from the female parent), the other is paternal (from the male parent)
• a cell with one copy of each homologous chromosome (2 homologs) is diploid (2n)
• same size and shape and banding pattern (when stained)
• same genes in the same place (locus), but may have different ALLELES (alternate forms of the same gene)

Question 21

What is an unreplicated chromosome?

Answer 21

• a chromosome that consists of one double-helical molecule of DNA packaged with proteins for compactness

Question 22

What is a replicated chromosome?

Answer 22

• a chromosome after NDA replication
• consists of 2 identical chromatids, each containing one double-helical DNA molecule packaged with proteins for compactness
• consists of 2 joined sister chromatids

Question 23

What are sister chromatids?

Answer 23

• the 2 identical chromatid copies in a replicated chromosome

Question 24

4 things about human diploid (2n) karyotype (total chromosome summary)

Answer 24

• 2n (copies of each chromosome) = 46 (total # of chromosomes)
• 22 homologous pairs of autosomes (44) plus
• 2 sex chromosomes (X and Y)
• n=23 (haploid number) in gametes

Question 25

What are some errors in meiosis (nondisjunction)

Answer 25

• CHROMOSOME 21 is the smallest of the autosomes, and nondisjunction leads to an extra #21 (trisomy) leads to Down syndrome. It is theorized that trisomy of larger chromosomes with many more genes is likely fatal compared to #21
• eggs begin to undergo meiosis during embryology and halt the process until puberty and ovulation. The long period of exposure to environmental factors makes changes (including mutations) more likely in eggs rather than sperm, which are produced continuously after puberty
• age of the mother is a major risk factor

Question 26

What happens in prophase I

Answer 26

• replicated chromosomes condense, become visible
• spindle fibers form from centrioles
• nuclear membrane fragments
• each replicated chromosome joins (synapsis) with its homolog to form a tetrad
• crossing over between non-sister chromatids occurs in tetrads (4 chromatids together)

Question 27

What is crossing over

Answer 27

• non-sister chromatids cross over each other at the chiasma (constriction), break and exchange homologous segments of chromosomes
• after crossing over, a chromatid will include DNA segments and alleles from both parents
• the chromatid is now said to be a recombinant chromatid (one major source of new variations)

Question 28

What holds or glues the chromatids together?

Answer 28

• protein “glue”

Question 29

What is independent assortment

Answer 29

• randomly shuffles the positions of maternal and paternal chromosomes before cell division
• for 3 pairs of chromosomes there are 2^3=8 possible combinations
• for 23 human pairs, possible combinations in gametes are 2^23 = 8.4 million

Question 30

After fertilization, the number of chromosomes is 46. What is the number of possible combinations?

Answer 30

8.4 million * 8.4 million = 70.6 trillion combinations

• even without crossovers

Question 31

Meiosis I and II steps

Answer 31

1. uncondensed chromosomes replicate in the parent cell
2. chromosomes condense, spindle apparatus forms, the nuclear envelope breaks down, synapsis (pairing) of homologous chromosomes
3. Chiasmata (crossover point) visible, nuclear envelop broken down, often multiple chiasmata between non-sister chromatids
4. movement of bivalents to metaphase plate is complete
5. homologs separate and begin moving to opposite poles of the spindle apparatus
6. homologs finish moving to opposite poles, spindle apparatus disassembles
   The cell divides, the chromosome number is reduced from 2n to n
7. spindle apparatus reforms
8. chromosomes are lined up in the middle of the spindle apparatus (metaphase plate)
9. sister chromatids separate, begin moving to opposite poles of the spindle apparatus
10. daughter chromosomes finish moving to opposite poles, the spindle apparatus disassembles
    Cell divides, chromosome number is unchanged, n to n

Question 32

What are 3 genetic recombination methods?

Answer 32

1. crossing over during prophase I
2. independent assortment during metaphase I
3. random fertilization of any sperm from one parent with any egg from another parent

Question 33

why is genetic recombination important?

Answer 33

New combinations of alleles = GENETIC VARIATION increased in the species, (even without
new mutations) by recombining existing alleles on chromosomes
* “Raw material” for natural selection to sort!