Campbell Biology Chapter 13 Review: Meiosis and Sexual Life Cycles

13.1 Genes, chromosomes and two ways to reproduce

pp. 248–249

On the AP exam? Yes

Topics 4.5, 5.1 and 5.2 build on this: mitosis copies cells for growth and asexual reproduction, while meiosis and fertilization power sexual reproduction. DNA in mitochondria and chloroplasts comes back in Topic 5.4, and why variety helps a population is Topic 7.11. You won't need any particular asexual species.

In the course: Topic 5.1 Meiosis, Topic 5.2 Meiosis and Genetic Diversity, Topic 4.5 Cell Cycle, Topic 5.4 Non-Mendelian Genetics, Topic 7.11 Variations in Populations (notes, videos and more questions)

Key points

You don't inherit a trait itself. You inherit genes, which are stretches of DNA holding instructions, mostly for building proteins, and those proteins shape your traits.
Each gene has a fixed address, its locus, on one particular chromosome. A chromosome is a single long DNA molecule wound up with proteins, and it can hold hundreds or thousands of genes.
Mitochondria and chloroplasts carry their own small DNA molecules, but the rest of a eukaryotic cell's genome is packed into the chromosomes in its nucleus.
Each species has a typical chromosome count. Human body cells, called somatic cells, carry 46.
Eggs and sperm (gametes) are how genes travel between generations. When two gametes fuse, the offspring gets genes from both parents.
In asexual reproduction, one parent makes offspring through mitosis. The offspring form a clone, genetically the same as the parent except for rare mutations.
With two parents, each offspring gets half its genes from each, in a combination that no brother, sister or parent shares exactly (identical twins aside).

Key terms (12)

heredity: How traits get handed down from parents to their young. Another word for it is inheritance.
variation: Differences among individuals, such as siblings who look alike but aren't the same.
genetics: The science of how traits are inherited and why individuals differ.
gene: A section of DNA that carries the instructions for one product, usually a protein. Genes are the units you inherit.
chromosome: One long DNA molecule packed with proteins. It carries many genes in a fixed order.
locus: The exact spot on a chromosome where a given gene sits. The plural is loci.
gamete: A sex cell, an egg or a sperm. Two gametes fuse to start a new individual.
somatic cell: A body cell, such as a skin, muscle or liver cell. Eggs, sperm and the cells that make them are the only cells that don't count.
asexual reproduction: Making offspring from a single parent without gametes fusing. The offspring are copies of the parent.
clone: A group of individuals with identical genes because they all came from one parent by mitosis.
sexual reproduction: Making offspring by fusing gametes from two parents. Each offspring ends up with a new mix of the parents' genes.
mutation: A change in a DNA sequence. Mutations are the only way a clone's members can come to differ genetically.

Check yourself: 13.1 Genes, chromosomes and two ways to reproduce

4 questions on 13.1 Genes, chromosomes and two ways to reproduce. Pick an answer to see if you got it, and why.

Question 1 of 4

Which statement best describes a gene's locus?

Question 2 of 4

A gardener finds that all 30 strawberry plants along a fence grew from runners sent out by a single plant. DNA tests at several markers show that all 30 match the original plant exactly. What best explains this result?

Question 3 of 4

A child has the same rare enzyme deficiency as her father. Which of the following, delivered by the father's sperm, accounts for the shared deficiency?

Question 4 of 4

A flatworm species can reproduce either by splitting in two or by mating. Researchers genotype one parent worm and three young worms found near it at three loci. Each number stands for one allele (invented data). Worm | Locus 1 | Locus 2 | Locus 3 Parent | 1/3 | 2/2 | 4/5 Young worm X | 1/3 | 2/2 | 4/5 Young worm Y | 3/3 | 2/6 | 5/5 Young worm Z | 1/3 | 2/2 | 4/5 Which conclusion do the data best support?

0 of 4 answered

13.2 Chromosome sets, karyotypes and life cycles

pp. 250–253

On the AP exam? Yes

Topic 5.1 covers diploid and haploid cells, homologous chromosomes and why meiosis must balance fertilization; sex chromosomes come back in Topic 5.4. The three kinds of life cycles, including alternation of generations, aren't in the current course.

In the course: Topic 5.1 Meiosis, Topic 5.4 Non-Mendelian Genetics, Topic 6.7 Mutations (notes, videos and more questions)

Key points

A human body cell holds 46 chromosomes. Sorted by size, centromere position and banding pattern, they form 23 matching pairs. A picture of the chromosomes lined up in pairs like this is a karyotype.
The two chromosomes of a pair are homologous chromosomes, or homologs. You got one from each parent. They carry the same genes at the same loci, but the versions of those genes can differ.
X and Y are the sex chromosomes. They match in only small regions and mostly carry different genes. The other 22 pairs are autosomes. Most females are XX and most males are XY.
A diploid cell (2n) has two full chromosome sets; a haploid cell (n) has one. In humans 2n = 46 and n = 23, and every sexual species has its own pair of numbers.
Fertilization joins two haploid gametes into a diploid zygote, which builds the whole body by mitosis. Meiosis in germ cells inside the ovaries and testes makes the haploid gametes, so the count doesn't double every generation.
Every sexual life cycle alternates meiosis and fertilization, but the timing varies. In animals only the gametes are haploid. Plants and many algae have both a many-celled diploid stage (sporophyte) and a many-celled haploid stage (gametophyte). In most fungi the zygote is the only diploid cell.
Mitosis works on any cell, however many chromosome sets it has. Meiosis has to pull homologous pairs apart, so a haploid cell, whose chromosomes have no partners, can't go through it.

Key terms (15)

karyotype: An image of a cell's chromosomes arranged in matching pairs from largest to smallest. It's used to check chromosome number and structure.
homologous chromosomes: The two chromosomes of a matching pair, one from each parent. They carry the same genes in the same order, though the alleles may differ.
sex chromosomes: The chromosomes that help decide sex. In humans these are X and Y.
autosome: Any chromosome that isn't a sex chromosome. Humans have 22 pairs.
diploid: Having two complete sets of chromosomes, written 2n. Human body cells are diploid, with 46 chromosomes.
haploid: Having one complete set of chromosomes, written n. Human eggs and sperm are haploid, with 23.
fertilization: The fusion of two gametes, including their nuclei. It turns two haploid cells into one diploid cell.
zygote: The single diploid cell formed at fertilization. Mitosis turns it into a whole organism.
germ cell: A cell set aside to make gametes. In animals, germ cells in the gonads are the only cells that go through meiosis.
gonad: An organ that makes gametes: an ovary or a testis.
life cycle: The repeating chain of stages, such as zygote, adult and gametes, that links each generation of a species to its offspring.
alternation of generations: A life cycle, found in plants and some algae, with both a many-celled diploid stage and a many-celled haploid stage.
sporophyte: The many-celled diploid stage in alternation of generations. It makes spores by meiosis.
spore: A haploid cell that can start a new organism on its own. It divides by mitosis to build a haploid body, with no partner cell needed.
gametophyte: The many-celled haploid stage in alternation of generations. It makes gametes by mitosis.

Check yourself: 13.2 Chromosome sets, karyotypes and life cycles

4 questions on 13.2 Chromosome sets, karyotypes and life cycles. Pick an answer to see if you got it, and why.

Question 1 of 4

A karyotype made from a male cat's skin cell shows 38 chromosomes arranged as 19 pairs, including an X and a Y chromosome. How many autosomes does the cell contain?

Question 2 of 4

Which statement correctly describes two homologous chromosomes in a person's liver cell?

Question 3 of 4

A mouse picks up the same new mutation in two places: in one cell of its liver and in one germ cell inside a testis. Which statement about passing the mutation to the mouse's offspring is correct?

Question 4 of 4

In a pond alga, every cell of the many-celled body holds 10 chromosomes, and so do its gametes. The only cell ever found with 20 chromosomes is the zygote. Which statement must be true of this alga's life cycle?

0 of 4 answered

13.3 Meiosis I and II, step by step

pp. 253–257

On the AP exam? Yes

This is the heart of Topic 5.1: know what separates in each division, why the products are haploid, and how meiosis differs from mitosis (Topic 4.5). Synaptonemal complexes and cohesins are background, and you won't need their names.

In the course: Topic 5.1 Meiosis, Topic 5.2 Meiosis and Genetic Diversity, Topic 4.5 Cell Cycle, Topic 6.7 Mutations (notes, videos and more questions)

Key points

DNA is copied once, in the S phase before meiosis starts, and then the cell divides twice. One diploid cell ends up as four haploid cells.
Prophase I: homologs line up gene by gene and are zipped together (synapsis), and non-sister chromatids trade matching segments (crossing over). The X-shaped crossover sites, chiasmata, keep each pair linked until anaphase I.
Metaphase I: homologous pairs, not single chromosomes, line up across the middle of the cell. Anaphase I: the two homologs of each pair are pulled toward opposite poles while sister chromatids stay together.
Meiosis I is the reduction step. Each of its two daughter cells has one chromosome set, but every chromosome still has two chromatids. No DNA copying happens before meiosis II.
Meiosis II works like mitosis in a haploid cell: single chromosomes line up, and then the sister chromatids split apart. The four final cells each hold one unduplicated copy of every chromosome, and because of crossing over they all differ genetically.
Proteins called cohesins hold sister chromatids together. They're removed from the arms in anaphase I, which frees the homologs, and from the centromeres in anaphase II, which frees the sisters.
When homologs or sister chromatids fail to separate (nondisjunction), gametes get an extra or a missing chromosome. The current course covers this in Topic 5.2; this book saves it for Chapter 15.

Key terms (13)

meiosis: Two rounds of cell division after one round of DNA copying. It turns one diploid cell into four haploid cells that differ genetically.
meiosis I: The first division of meiosis. Homologous chromosomes separate, so the chromosome number is halved.
meiosis II: The second division of meiosis. Sister chromatids separate, much as in mitosis.
sister chromatids: The two identical copies of a chromosome made in S phase, held together until they separate.
non-sister chromatids: Chromatids that belong to different homologs of a pair: one from the maternal chromosome and one from the paternal one.
synapsis: The close pairing of homologous chromosomes along their full length during prophase I.
synaptonemal complex: A protein framework that zips paired homologs together during synapsis.
crossing over: The exchange of matching DNA segments between non-sister chromatids in prophase I. It makes chromosomes with new combinations of alleles.
chiasma: An X-shaped point where non-sister chromatids crossed over. It helps hold the homologs together until anaphase I. The plural is chiasmata.
cohesin: A protein complex that glues sister chromatids together. Cutting it lets chromatids separate.
kinetochore: A protein structure at the centromere where spindle microtubules attach and pull.
allele: One version of a gene. Homologs can carry different alleles at the same locus.
nondisjunction: A failure of homologs (in meiosis I) or sister chromatids (in meiosis II) to separate. It gives gametes an extra or a missing chromosome.

Check yourself: 13.3 Meiosis I and II, step by step

4 questions on 13.3 Meiosis I and II, step by step. Pick an answer to see if you got it, and why.

Question 1 of 4

Lily plants have 2n = 24. A student looking at cells from a lily's anther sees a cell in which 24 chromosomes are arranged as 12 side-by-side pairs across the middle of the cell. Which stage is this cell in?

Question 2 of 4

Question 3 of 4

A drug stops cells from removing cohesin at the centromeres but still lets them remove cohesin along the chromosome arms. If a germ cell is treated just before meiosis begins, which step would fail first?

Question 4 of 4

Which statement correctly compares mitosis of a diploid cell with meiosis?

0 of 4 answered

13.4 Three sources of new gene combinations

pp. 257–260

On the AP exam? Yes

Topic 5.2 covers crossing over, independent assortment and random fertilization, and Topics 7.1–7.2 tie that variation to natural selection. You may need to work out 2ⁿ, but you won't be asked about particular asexual animals.

In the course: Topic 5.2 Meiosis and Genetic Diversity, Topic 6.7 Mutations, Topic 7.1 Introduction to Natural Selection, Topic 7.2 Natural Selection, Topic 7.11 Variations in Populations (notes, videos and more questions)

Key points

New alleles first arise through mutation. Sexual reproduction doesn't invent alleles; it reshuffles the ones that already exist into new combinations every generation.
Independent assortment: in metaphase I, each homologous pair faces the poles at random, without affecting any other pair. So each gamete gets its own random mix of chromosomes that came from the individual's mother and father.
With n homologous pairs, independent assortment alone allows 2ⁿ different chromosome combinations in gametes. For humans, n = 23, so that's 2²³ = 8,388,608.
Crossing over in prophase I swaps matching pieces between non-sister chromatids, creating recombinant chromosomes that carry alleles from both of the individual's parents. Nearly every homologous pair has at least one crossover, and long chromosomes often have several.
Random fertilization: any sperm can fuse with any egg, so the possibilities multiply. From independent assortment alone, two human parents could make about 7 × 10¹³ different zygotes, and crossing over raises that much further.
This variety is the raw material natural selection works on. When conditions change, some new combinations may survive and reproduce better than others.
Asexual reproduction is cheaper and passes a winning set of alleles on unchanged, which helps while the environment holds steady. Reshuffling alleles through sex helps when the environment shifts, which is one leading explanation for why nearly all animal species reproduce sexually at least some of the time.

Key terms (9)

genetic variation: Differences in alleles and gene combinations among individuals in a population.
independent assortment: The random, separate orientation of each homologous pair in metaphase I. Each gamete gets its own mix of maternal and paternal chromosomes.
recombinant chromosome: A chromosome carrying a new mix of alleles from both of an individual's parents, made by crossing over.
genetic recombination: Any process that produces offspring or gametes with combinations of alleles their parents didn't have.
random fertilization: The chance meeting of any one sperm with any one egg. It multiplies the number of possible offspring genotypes.
maternal chromosome: The member of a homologous pair that an individual inherited from its mother.
paternal chromosome: The member of a homologous pair that an individual inherited from its father.
natural selection: The process in which individuals with traits better suited to their environment survive and reproduce more, so their alleles become more common.
2ⁿ rule: The number of chromosome combinations independent assortment can produce, with n standing for the haploid number. For n = 3 it's 8.

Check yourself: 13.4 Three sources of new gene combinations

4 questions on 13.4 Three sources of new gene combinations. Pick an answer to see if you got it, and why.

Question 1 of 4

Ignoring crossing over, a geneticist calculates that independent assortment alone lets one moth make 512 different chromosome combinations in its gametes. What is this moth species' diploid number?

Question 2 of 4

Two brothers who aren't identical twins have the same mother and father. Which best explains why their genomes differ?

Question 3 of 4

DNA sequencing shows that one copy of chromosome 4 in a woman's egg carries her mother's alleles near one end and her father's alleles near the other end. Which process produced this chromosome?

Question 4 of 4

Twelve flightless beetles founded a population on a small island. Researchers sequenced preserved specimens of all twelve. Eighty years later, some beetles on the island carry an allele of a pigment gene that none of the founders had. What best explains where this allele came from?

0 of 4 answered