Chapter 13 Flashcards
Heredity (inheritance)
The transmission of traits from one generation to the next
Genetics
Scientific study of heredity and hereditary variation
Genes
Coded information in the form of hereditary units
Gametes
Reproductive cells (vehicles that transmit genes from one generation to the next)
Somatic cells
All cells of the body, except the gametes and their precursors
Locus
A gene’s specific location along the length of a chromosome
Genome
Genetic endowment
Asexual reproduction
A single individual is the sole parent and passes copies of all its genes to its offspring without the fusion of gametes
The genomes of offsprings in asexual reproduction
Is virtually exact copies of the parent’s genome
Clone
An individual that reproduces asexually gives rise to a clone, a group of genetically identical individuals
Changes occur in asexually reproducing organisms as a result of
Mutations
Sexual reproduction
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents
Life cycle
Generation to generation sequence of stages in the reproductive history of an organism, from conception to production of its own offspring
In humans, somatic cells have
46 chromosomes
Karyotype
When images of chromosomes are arranged in pairs, starting with the longest chromosomes
Resulting, ordered display
Homologos chromosomes (homologs)
Two chromosomes of a pair have the same length, centromere position, and staining pattern
Both chromosomes of each pair carry genes controlling the same inherited characters
Sex chromosomes
The X and Y chromosomes
They determine an individual’s sex
Autosomes
All other chromosomes other than sex chromosomes
Our 46 chromosomes are from
Two sets of 23 chromosomes Maternal set (23) and paternal set (23)
The number of chromosomes in a single cell set is represented by
n
Diploid cell
Any cell with two chromosome sets (2n)
Gametes contain
A single set of chromosomes
Haploid cell
Single set of chromosomes (half) (n)
Fertilization
When a haploid sperm from the father fuses with a haploid egg from the mother
Culminating in fusion of their nuclei
Zygote
Fertilized egg
A zygote is a
Diploid because it contains two haploid sets of chromosomes with genes representing the maternal and paternal family lines
The only cells not produced by mitosis are
Gametes, which develop from specialized cells called germ cells in the gonads (ovaries and testes)
Meiosis
A type of cell division
Happens in sexually reproducing organisms
Gamete formation
Reduces the number of sets of chromosomes from two to one in the gametes
Counterbalances the doubling that occurs in fertilization
After fertilization, the diploid zygote
Divides by mitosis, producing a multicellular organism that is a diploid
Alternation of generations
Plants and some species exhibit this
Second type of life cycle
Includes both diploid and haploid stages that are multicellular
Multicellular diploid stage
Sporophyte
Meiosis in the sporophyte produces haploid cells called
Spores
Unlike a gamete, a haploid spore doesn’t fuse with another cell but divides mitotically, generating a multicellular haploid stage called
Gametophyte
Cells of this give rise to gametes by mitosis
Sporophyte generation produces a gametophyte as its offspring, and
The gametophyte generation produces the next sporophyte generation
Third type of life cycle
After gametes fuse and form a diploid zygote, meiosis occurs without a multicellular diploid offspring developing
Meiosis produces not gametes but haploid cells that then divide by mitosis and give rise to either unicellular descendants of a haploid multicellular organism
Either haploid or diploid cells can
Divide by mitosis, depending on the type of life cycle
Only diploid cells can
Undergo meiosis, because haploid cells have only a single set of chromosomes that cannot be further reduced
Divisions of meiosis
Meiosis I and Meiosis II
Result in 4 daughter cells, each with only half as many chromosomes as the parent cell
Sister chromatids
Two copies of one chromosome
Sister chromatids make up
Sister chromatid cohesion
The two chromosomes of a homologous pair are
Individual chromosomes that were inherited from different parents
Different versions of a gene
Allele
After interphase, the chromosomes have been duplicated and the sister chromatids are held together by proteins called
Cohesions
Each gene on one homolog is aligned precisely with
The corresponding gene on the other homolog
The DNA of two non sister chromatids (one maternal, one paternal) is broken by
Specific proteins at precisely corresponding points
Synaptonemal complex
Zipper like structure
Holds one homolog tightly to the other
Synapsis
The DNA breaks are closed up so that each broken end is joined to the corresponding segment of the nonsister chromatid
A paternal chromatid is joined to a piece of maternal chromatid beyond
The crossover point, and vice versa
Meiosis reduces
The number of chromosome sets from two (diploid) to one (haploid), whereas mitosis conserves the number of chromosome sets
Meiosis produces cells that
Differ genetically from their parent cell and from each other
Mitosis produces cells that
Are genetically identical to their parent cell and each other
Events during meiosis I
- Synapsis and crossing over
- Homologous pairs at the metaphase plate
- Separation of homologs
- Synapsis and crossing over
During prophase I, duplicated homologs pair up and crossing over occurs
Do not occur during prophase of mitosis
- Homologous pairs at the metaphase plate
At metaphase I of meiosis, chromosomes are positioned at the metaphase plate as pairs of homologs, rather than individual chromosomes, as in metaphase of mitosis
- Separation of homologs
At anaphase I of meiosis, the duplicated chromosomes of each homologous pair move toward opposite poles, but the sister chromatids of each duplicated chromosome remain attached. In anaphase of mitosis, sister chromatids separate
Sister chromatids stay together due to
Sister chromatid cohesion, mediated by cohesion proteins
Chiasmata hold
Homologs together as the spindle forms for the first meiotic division
At the onset of anaphase I, the release of cohesion along sister chromatid
Arms allows homologs to separate
At anaphase II, the release of sister chromatid cohesion at the
Centromeres allows the sister chromatids to separate
Meiosis I is the
Reductional division because it reduces the number of chromosome sets from two to one
Three mechanisms contribute to the genetic variation arising from sexual reproduction
Independent assortment of chromosomes
Crossing over
Random fertilization
Independent assortment of chromosomes`
Each homologous pair may orient with either its maternal or paternal homolog closer to a given pole
Independent assortment
Because each pair of homologous chromosomes is positioned independently of the other parts of metaphase I, the first meiotic division results in each pair sorting its maternal and paternal homologs into daughter cells independently of every other pair
Each daughter cell represents
One outcome of all possible combinations of maternal and paternal chromosomes
Crossing over
Produces recombinant chromosomes – individual chromosomes that carry genes derived from two different parents
During meiosis, each of us produces a collection of gametes differing greatly in
Their combinations of chromosomes we inherited from our two parents
Crossing over produces
Chromosomes with new combinations of maternal and paternal alleles
Random fertilization
Adds to the genetic variation arising from meiosis
Natural selection results in
The accumulation of genetic variations favored by the environment
