Chapter 9 Flashcards
Sexual Reproduction and Meiosis
Asexual reproduction
-one parent replicates its DNA and splits the contents of one cell into two
-generates genetically identical offspring (unless a mutation occurs)
-amoeba, bacteria, archaea, single celled
-plants, fungi, multicellular
-relatively rare
-single celled organisms such as bacteria, archaea and protists reproduce asexually
-among multicellular organisms, asexual reproduction isn’t very common
Sexual reproduction
-the DNA of the offspring comes from 2 parents
-an egg fuses with a sperm, yielding the first cell of the next generation (zygote)
-generates genetically different offspring, which increases the chance of offspring survival in changing environments
-humans, dogs, dolphins, etc
-mutations can occur
Define conjugation
-a form of horizontal gene transfer in which one cell receives DNA via direct contact with another cell
-one bacterial cell uses an outgrowth cell a sex pilus to transfer genetic material to another bacterium
-also helped evolve sexual reproduction
Define diploid cells
-cells that contain two full sets of chromosomes (one from each parent)
-aka 2n
-most cells in a sexually reproducing organism have two sets of chromosomes
(Contains 46 chromosomes)
Define haploid cells
-has only one set of chromosomes
-aka n
How many chromosomes do human cells have?
-23 homologous sets of chromosomes (46 total)
-22 homologous pairs are autosomes (same number for both males and females)
-one set are sex chromosomes, denoted by a letter
-X chromosome is much larger than the Y chromosome
-XX = female
-XY = male
What is a karyotype?
-it is a size-ordered chart that shows all the chromosomes from a diploid human cell
Define homologous chromosomes
-a pair of chromosomes that carry the same set of genes
-similar in size, structure, centromere position, pattern of light and dark staining bands
-DNA sequence isn’t exactly the same, but each chromosome in a homologous pair has genes for the same traits
-members of a homologous pair may carry different alleles (alternative versions of the same gene)
What happens to chromosomes after replication?
-each chromosome consists of identical chromatids
-when members of a homologous pair are replicated, identical alleles are on sister chromatids
-DNA replication doesn’t change the number of chromosomes, nor do they make cells diploid
Define chromosome
-a single molecule of DNA and its associated proteins
-a continuous molecule of DNA wrapped around protein in a nucleus of a eukaryotic cell
-also the genetic material of a prokaryotic cell
Define sex chromosome
-chromosome that carries the genes that determine the sex
Define autosomes
-non sex chromosomes
-numbered 1-22
Define allele
-one or two or more alternative forms of a gene (different versions)
Define gamete
-sperm and egg cells
-haploid sex cells
Define zygote
-the first cell of a new organism
-it divides by mitosis to build an organism with trillions of identical cells
Define meiosis
-division of genetic material that halves the number of chromosomes and yields genetically variable nuclei
-halves the chromosomes and scrambles the alleles
-DNA replicates once but divides twice
-homologous chromosomes pair up
Define fertilization
-union of two gametes, creating a new cell: diploid zygote
Define mitosis
-division of genetic material that yields two identical nuclei
-produces cells needed for growth, development, and tissue repair
-homologous chromosomes do not pair up
Define somatic cell
-body cell that isn’t part of the germline and doesn’t give rise to gamete
-leaf cell, root cell, muscle cells, and neurons (they only divide by mitosis)
Define germ cell
-specialized cells that give rise to gametes
-cell of the germline (the only cells that undergo meiosis, though some also divide by mitosis)
-testes and ovaries
Start of meiosis
-the germ cell starts off with two pairs of homologous chromosomes
-the gametes have two single, unpaired chromosomes per cell
Meiosis I
1.) prophase I (early)- chromosomes condense and become visible
2.) prophase I (late)- crossing over occurs, spindle fibers form, nuclear envelope breaks up, homologous chromosomes pair up and attach to the spindle fibers
3.) metaphase I- paired homologous chromosomes align along the equator of the cell in two rows
4.) anaphase I- homologous chromosomes separate to opposite poles of the cell, sister chromatids remain joined
5.) telophase I- chromatids remain together, nuclear envelope forms around chromosomes, which may temporarily decondense, spindle fibers disappear
-cytokinesis may divide cell into two
Meiosis II
1.) prophase II- spindle forms, chromosomes attach to the spindle, nuclear envelope breaks up
2.) metaphase II- chromosomes line up in one row along the equator of the cell
3.) anaphase II- centromeres split as sister chromatids separate to opposite poles of the cell
4.) telophase II- nuclear envelopes assemble around daughter cells, chromosomes decondense, spindles disappear
-cytokinesis divides cells
-four new nonidentical haploid daughter cells each have one set of chromosomes
Define spindle
-a structure of microtubules that aligns and separates chromosomes in mitosis and meiosis
Define kinetochores
-spindle attachment points
-grow on each centromere
-protein that attaches a chromosome to the spindle in cell division
Chromosomes are different in meiosis because of what?
-crossing over in prophase I
-independent assortment in metaphase I
-random fertilization
What happens during crossing over
-meiosis prophase I
-two homologous chromosomes pair up and exchange pieces of DNA, scrambling the genetic material
What happens in independent assortment?
-meiosis metaphase I
-chromosomes pairs align randomly, scrambling the combination of chromosomes for each gamete
-a germ cell with 3 pair of chromosomes has 4 possible arrangements during metaphase I, yielding 8 possible gametes (2^3 = 8)
-2^ a number of chromosomes= the total different gamete possibilities
Monozygotic twins are genetically identical
-sometimes, early in development, an embryo splits in 2
-each embryo develops independently
-one zygote splits into 2 individuals
Dizygotic twins have unique DNA
-fraternal twins are a testament to the variation among gametes
-2 sperm cells fertilized 2 separate egg cells
-offspring may look very different
-2 zygotes developed into 2 individuals
Define chiasmata
-singular: chiasma
-the length along a chromosome
Define recombinant chromatid
-chromatid containing genetic information from both parents as a result of crossing over
Parental chromatid
-chromatid containing genetic information from only one parent
How many different possible gametes for humans?
-humans have 23 pairs of chromosomes
-2^23= 8,388,608 possible chromosome combinations per gamete
-about 70.3 trillion per couple!
Different variations of chromosome structure
-chromosomal deletion
-chromosomal duplication
-chromosomal inversion
-chromosomal translation
Define chromosomal deletion
-condition in which one of more genes are lost from a chromosome
Define chromosomal duplication
-condition characterized by multiple copies of part of a chromosome
Define chromosomal inversion
-condition in which a portion of a chromosome flips and reinserts itself
Define chromosomal translation
-condition in which genetic material is exchanged between homologous chromosomes
-ex: parts of chromosomes 5 and 14 have switched places
Define nondisjunction
-chromosomes fail to separate properly
-spindles may not form properly or attach correctly to chromosomes
-45 or 47 chromosomes
-abnormal gametes form
-if it occurs in anaphase I, 2 zygotes have too many chromosomes and 2 have too few
-if it occurs in anaphase II, 2 zygotes are normal, 1 zygote has too many chromosomes and 1 has too few chromosomes
-a zygote with too many chromosomes may survive, bill will develop defects
-a zygote with too few chromosomes will be unlikely to survive
Define polyploid cell
-a cell with a few extra chromosomes sets (one or more)
-chromosome number may vary
Define aneuploid cell
-a cell with too few or too many chromosomes but not involving a full extra or missing set
-chromosome number may vary
Define trisomy
-an extra copy of a chromosome
-3 copies of a chromosome instead of 2
Down syndrome is caused by nondisjunction
-a extra copy of a chromosome (a trisomy) causes genetic disorders, such as down syndrome
-down syndrome individuals have 3 copies of chromosome #21, not 2
-chromosomes #13 and #18 are the next most common trisomy
Triplo-X
-sex disorder
-XXX
-47 chromosomes
-1 in 1,500 females
-symptoms: tall stature, menstrual irregularities, increased risk of giving birth of triplo-X daughters or XXY sons
Klinefelter, or XXY syndrome
-sex disorder
-XXY
-47 chromosomes
-1 in 750 males
-symptoms: variable, but often include sexual underdevelopment, long limbs, large hands and feet, development of breast tissue
Jacobs, or XYY syndrome
-sex disorder
-XYY
-47 chromosomes
-1 in 1,000 males
-symptoms: often few noticeable symptoms, tall stature, acne, problems with speech and reading
Turner syndrome
-sex disorder
-XO
-45 chromosomes
-1 in 2,000 females
-symptoms: short stature, sexual underdevelopment, infertility
Define alternations of generations
-sexual life cycle of plants and many green algae, which alternates between a diploid sporophyte stage and a haploid gametophyte stage
Define spermatogenesis
-produces 4 sperm cells for each germ cell in the testes
-inside testes, spermatogenesis (diploid germ cells) divides meiotically to produce: more spermatogenesis and primary spermatocytes
-meiosis I- 2 haploid cells called secondary spermatocytes
-meiosis II- 4 spermatids (which specialize into a mature, tad-pole shaped sperm)
-74 days for the entire process
Define oogenesis
-produces 1 egg cell for each germ cell in ovaries
-inside ovaries, start with a diploid germ cell (oogonium) divide meiotically to produce: more oogonia and a primary oocyte
-meiosis I- a small haploid cell with very little cytoplasm (called polar body) and a much larger haploid cell called secondary oocyte
-meiosis II- divides unequally to produce another polar body and the mature egg (ovum) which contains a large amount of cytoplasm
Investigating life: evolving parasites select for sex in worms
-why do animals spend so much energy sexually reproducing
-studying a worm that can reproduce either sexually or asexually might help answer
Investigating life: bacteria can impact worm reproduction
-exposure to evolving parasites, such as bacteria, favors sexual reproduction in the worms
-exposure to no evolving bacteria favors asexual reproduction in the worms
Investigating life: asexual reproduction isn’t enough
-worms that only reproduce asexually couldn’t survive with the evolving bacteria
-sexual reproduction, and therefore meiosis, increase their evolutionary fitness (the ability to survive and reproduce) when faced with a rapidly evolving parasite population
