Chapter 12: Meiosis and Sexual Reproduction [Final Exam (30%)]

Sexual vs. Asexual Reproduction

A____ Reproduction: Generates g____ i____ o____ (c____) from a s____ p____.

S____ Reproduction: Mode of reproduction where o____ i____ g____ from t___ p____, resulting in u____ c____ of t____.

Site of Occurrence: M____ occurs specifically in s_____ e____ r____ c____ called g____ c____ located in g____ (ovaries and testes).

asexual genetically identical offspring, clones, single parent

sexual, offspring inherits genes, two parents, unique combinations, traits

meiosis, specialized eukaryotic reproductive cells, germ cells, gonads

Evolutionary Advantages of Sexual Reproduction

Sexual reproduction s____ e____ a____ from t___ p____ into n____ g____ c____.

• In a____ populations, n___ t____ c____ r___ s____ on r____ g____ m____.

• S____ reproduction generates n___ t____ c____ in f__ f____ g____ than m____ a____.

• An a____ t___ s____ much more r____ through a s____ r____ population.

• G____ d____ gives sexually reproducing populations a significantly b____ c____ of s____ e____ c____ compared to clonal populations

shuffles existing alleles, two parents, new genetic combinations

asexual, new trait combinations rely solely, rare genetic mutations

sexual, new trait combinations, far fewer generations, mutation alone

adaptive trait spread, rapidly, sexually reproducing

genetic diversity, better chance, surviving environmental changes

Somatic Cells vs. Gametes

Somatic cells / B____ cells are d____ (2n), containing t___ c____ s___ of h____ c____.

G____ (Sperm and Egg cells) are h____ (n), containing a s____ s___ of c____.

body, diploid, two complete sets, homologous chromosomes

gametes, haploid, single set, chromosomes

Human Chromosome Composition

• Total chromosome count in body cells = __ chromosomes, representing __ h____ p____

• Autosomes (Non-Sex Chromosomes): __ pairs of chromosomes that are m__ in s____, s____, and g____ c____.

• Sex Chromosomes: __ pair (X and Y) that d____ b____ s____ and d____ in s____ and g____ c____.

46, 23 homologous pairs

22, matched, size, shape, genetic composition

1, determines biological sex, differs, size, genetic composition

Homologous Chromosomes, Loci, and Alleles

H____ c____ refer to a pair of chromosomes (one maternal, one paternal) matched in length, centromere position, and gene location.

G____ l____ refers to the specific location of a gene on a chromosome.

A____ refer to the alternative/different versions of the same gene found at the same locus on homologous chromosomes.

homologous chromosomes

gene locus

alleles

Homologous Chromosomes, Loci, and Alleles

• Homologous Chromosomes: A p___ of chromosomes (one m____, one p____) matched in l____, c____ p____, and g____ l____.

• Gene Locus: The s____ l____ of a g____ on a chromosome.

• Alleles: A____ or d____ v____ of the s____ g____ found at the s____ l____ on h____ c____.

Key Principle: H____ chromosomes carry the s____ g____ in the s____ o____, but they may carry d____ v____ (a____) of those genes.

pair, maternal, paternal, length, centromere position, gene location

specific location, gene

alternative, different variations, same gene, same locus, homologous chromosome

homologous, same gene, same order, different versions, alleles

Chromosome Structure Breakdown

M____ d____ c____ contains two identical sister chromatids connected at the centromere, carrying specific gene alleles.

P____ d____ c____ contains two identical sister chromatids connected at the centromere, carrying corresponding gene alleles at matching gene loci.

maternal duplicated chromosomes

paternal duplicated chromosomes

Chromosome Structure Breakdown

Maternal Duplicated Chromosome: Contains t__ i____ s____ c____ c____ at the c____, carrying s____ g____ a____ (e.g. Gene A1, Gene B1).

Paternal Duplicated Chromosome: Contains t__ i____ s____ c____ c____ at the c____, carrying c____ g____ a____ (e.g. Gene A2, Gene B2) at m____ g____ l____.

two identical sister chromatids connected, centromere, specific genetic alleles

two identical sister chromatids connected, centromere, corresponding gene alleles, matching gene loci

Sexual Life Cycles: Alternation of Generations (n and 2n)

Sexual life cycles c____ between h____ (n) and d____ (2n) phases:

1. M____: C____ d____ (2n = 46) g____ c____ in o____ and t____ into h____ (n = 23) g____.

2. F____: U____ of a h____ s____ (n = 23) and e___ (n = 23), r____ the d____ s____ in a z____ (2n = 46).

3. M____ & D____: The d____ z_____ undergoes r____ m____ d____ to g____ into a m____ d____ a____ (2n = 46).

cycle, haploid, diploid

meiosis, converts diploid germ cells, ovaries, testes, haploid gametes

fertilization, union, haploid sperm, egg, restoring, diploid state, zygote

mitosis, development, diploid zygote, repeated mitotic divisions, grow, multicellular diploid adult

Overview of Meiotic Divisions

Meiosis is p____ by a s____ r____ of c____ d____ during i_____ (__ phase), followed by t___ s____ n____ d____:

1. M____ __ (Reductional Division): P___ u___ and s____ h____ c____ p____, r____ chromosome number from 2n to n and yielding t__ h____ c___ with d____ c____.

2. M____ __ (Equational Division): S____ s____ c____ w____ i____ D__ r____, producing a total of f___ h____ d____ c____ with u____ c____.

preceded, single round, chromosome duplication, interphase S, two sequential nuclear divisions

meiosis I, pairs up, separates homologous chromosome pairs, reducing, two haploid cells, duplicated chromosomes

meiosis II, separates sister chromatids without interrupting DNA replication, four haploid daughter cells, unduplicated chromosomes

Detailed Step-by-Step Tour of Meiosis

Pre-Meiosis: I____

• C____ r____ during the __ phase, c_____ s____-s____ c____ into d____ c____ with t___ s____ c____ held at the centromere.

Meiosis I: Homologous Chromosomes Separate

1. P____ I

• C____ c____ and c____.

• S____: H____ chromosomes c____ t_____ as physical p____.

• T____ F____: E____ p____ s_____ consists of f____ c____ (two sister chromatids per homologous chromosome).

• C____ O____: N_____ chromatids e____ c____ s____ at c____ s____ called c____.

• N____ e____ f____ and s____ a____ b____ to a____.

• Human Cell Counts: __ chromosomes, __ chromatids

2. M____ I

• H____ pairs (t____) a____ in a d____-f___ a____ along the m____ p____ (cell equator).

• S____ m____ a____ to k____ at the c____ of e____ h____ c____.

• Human Cell Counts: __ chromosomes, __ chromatids

3. A____ I

• H____ chromosome pairs s____ and m____ toward o____ p____ of the cell

• S____ c____ r____ a____ at their c____ and m____ t____ as s____ u____

4. T____ I & C____

• D____ c____ r____ the p____

• A n____ e____ r__-f____ around each chromosome set in some species

• C____ occurs, p____ the c____

• Result: __ h____ d____ c___, each containing o__ s___ of d____ c____

• Human Cell Counts: __ chromosomes, __ chromatids

Meiosis II: Sister Chromatids Separate (Occurs in both cells produced by Meiosis I, without any prior chromosome duplication)

1. P____ II

• C____ c____ a____ in relaxed; n____ e____ b____ d____

• S____ a____ f____ and m____ c____ toward the c___ m____

2. M____ II

• D____ c____ a____ in a s____-f___ along the m____ p____ at the cell equator

3. A____ II

• C____ s____; s____ c____ s____ and m____ toward o____ p____ as i____,u____ d____ c____.

4. T____ II & C____

• C____ r____ o____ p____; n____ e____ r__-f____ around f____ n___ n____.

• C____ d____ the c____.

• Result: __ g____ u____ h____ d____ c____ with u____ c____

• Human Cell Counts: __ chromosomes, __ chromatids

prophase I

chromosomes coil, condense

synapsis, homologous, come together, pairs

tetrad formation, each paired structure, four chromatids

crossing over, nonsister, exchange corresponding segments, contact sites, chiasmata

nuclear envelope fragments, spindle apparatus begins, assemble

46, 92

metaphase I

homologous tetrads align, double file arrangement, metaphase plate

spindle microtubules attach, kinetochores, centromeres, each homologous chromosome

46, 92

anaphase I

homologous, separate, move, opposite poles

sister chromatids remain attached, centromeres, move together, single units

telophase I, cytokinesis

duplicated chromosomes reach, poles

nuclear envelope re forms

cytokinesis, partitioning, cytoplasm

2 haploid daughter cells, one set, duplicated chromosomes

23, 46

prophase II

chromosomes condense again, nuclear envelope breaks down

spindle apparatus forms, moves chromosomes, cell middle

metaphase II

duplicated chromosomes align, single-file, metaphase plate

anaphase II

centromeres split, sister chromatids separate, move, opposite poles, individual, unduplicated daughter chromosomes

telophase II, cytokinesis

chromosomes reach opposite poles, nuclear envelopes re form, four new nuclei

cytokinesis divides, cytoplasm

4 genetically unique haploid daughter cells, unduplicated chromosomes

23, 0

How Meiosis Introduces Genetic Variation

T____ p____ m____ contribute to the v____ g____ d____ seen in sexually reproducing species.

1. C____ O____ (P____ I)

• Process: N____ c____ of p____ h____ c____ a____ t____, b____, and e____ c____ g____ s____ at p____ c____-o____ called c____.
  
  

• Outcome: Produces r____c____ that combine m____ and p____ a____ into n____ p_____ n___ p____ in either p____
  
  

• Products: Out of 4 chromatids in a tetrad, __ r____ p____ c____ and __ becoomes r____ c____

2. I____ O____ / A____ (M____ I)

• Process: E___ p____ of h____ chromosomes a____ r____ at the m____ p____, with e____ p____ that the m____ or p____ c____ f____ e____ p____.
  
  

• Formula: N____ of p____ chromosome c____ in g____ = 2ⁿ, where n = haploid chromosome number

• Human Calculation: For humans (n = 23), 2²³ = 8 388 608 (8.3 million+) possible combinations of homologous chromosomes per gamete—-even without considering crossing over

3. R____ F____

• A____ u____ s____ can fertilize a____ u____ e___
  
  

• 8.4 million x 8.4 million = ~70 trillion possible combinations

• Two parents c____ produce g____ i____ o____ in s____ f____

three primary mechanisms, vast genetic diversity

crossing over prophase I

nonsister chromatids paired homologous chromosomes align tightly, break, exchange corresponding genetic segments, physical cross-overs, chiasmata

recombinant chromosomes, maternal, paternal alleles, novel pattern not present, parent

2 remain parental chromosomes, 2 recombinant chromosomes

independent orientation / assortment, metaphase I

each pair, homologous, aligns randomly, metaphase plate, equal probability, maternal, paternal, faces either pole

random fertilization

any unique sperm, any unique egg

cannot, genetically identical offspring, separate fertilizations