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Summary Biology, a global approach chapters 13,14,15,20,21,22,25,26,27,31
Summary of chapters 13,14,15,20,21,22,25,26,27,31 and 28.1 28.6 from Biology A Global Approach (11th edition)
[Meer zien]Voorbeeld 2 van de 11 pagina's
Voorbeeld 2 van de 11 pagina's
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Samenvatting DT1 Functionelebiologie
Hoofdstuk 13
- A gene’s specific location along the length of a chromosome is called the gene’s locus.
- In asexual reproduction, a single individual is the sole parent and passes copies of all its
genes to its offspring without the fusion of gametes.
- A life cycle is the generation-to-generation sequence of stages in the reproductive history of
an organism, from the conception to production of its own offspring.
- The two chromosomes of a pair have the same length, centromere position, and staining
pattern: these are called homologous chromosomes (or homologs). Both chromosomes of
each pair carry genes controlling the same inherited characters.
- The union of gametes, culminating in fusion of their nuclei, is called fertilization. The
resulting fertilized egg, or zygote, is diploid because it contains two haploid sets of
chromosomes bearing genes representing the maternal and paternal family lines.
- Plants and some species of algae exhibit a second type of life cycle called alternation of
generations. This type includes both diploid and haploid stages that are multicellular. The
multicellular diploid stage is called the 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 the gametophyte. Cells
of the gametophyte give rise to gametes by mitosis. Fusion of two haploid gametes at
fertilization results in a diploid zygote, which develops into the next sporophyte generation.
- A third type of life cycle occurs in most fungi and some protists, including some algae. 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 or a haploid multicellular adult
organism. Subsequently, the haploid organism carries out further mitoses, producing the
cells that develop into gametes. The only diploid stage fount in these species is the single-
celled zygote.
- Page 310/311
- After interphase, the chromosomes have been duplicated and the sister chromatids are held
together by proteins called cohesins. Early in prophase I, the two members of a homologous
pair associate loosely along their length. Each gene on one homolog is aligned precisely with
the corresponding allele of that gene on the other homolog. The DNA of two nonsister
chromatids, is broken by specific proteins at precisely matching points. Next, the formation
of a zipper-like structure called the synaptonemal complex holds one homolog tightly tot the
other. During this association, called synapsis, the DNA breaks are closed up so that each
broken end is joined to the corresponding segment of the nonsister chromatid. Thus, a
paternal chromatid is joined to a piece of maternal chromatid beyond the crossover point,
and vice versa. These points of crossing over become visible as chiasmata after the
synaptonemal complex disassembles and the homologs move slightly apart from each other.
The homologs remain attached because sister chromatids are still held together by sister
chromatid cohesion, even though some of the DNA may no longer be attached to its original
, chromosome. At least one crossover per chromosome must occur in order for the
homologous pair to stay together as it moves to the metaphase I plate.
- Meiosis reduces the number of chromosome sets from two (diploid) to one (haploid),
whereas mitosis conserves the number of chromosome sets. Therefore, meiosis produces
cells that differ genetically from their parent cell and from each other, whereas mitosis
produces daughter cells that are genetically identical to their parent cell and to each other.
- Because each pair of homologous chromosomes is positioned independently of the other
pairs at metaphase I, the first meiotic division results in each pair sorting its maternal and
paternal homologs into daughter cells independently of every other pair. This is called
independent assortment.
- Because crossing over produces recombinant chromosomes, individual chromosomes carry
genes (DNA) from two different parents.
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- True-breeding: breeding with homozygote parents
- Crossing of two true-breeding varieties is called hybridization. The true-breeding parents are
referred to as the P generation, and their hybrid offspring are the F1-generation.
- Law of segregation: the two alleles for a heritable character segregate (separate from each
other) during gamete formation and end up in different gametes.
- Law of independent assortment: two or more genes assort independently – that is, each pair
of alleles segregates independently of any other pair of alleles – during gamete formation.
- In epistasis, the phenotypic expression of a gene at one locus alters that of a gene at a
second locus.
Hoofdstuk 15
- Chromosome theory of independence: Mendelian genes have specific loci along
chromosomes, and it is the chromosomes that undergo segregation and independent
assortment.
- The phenotype for a character most commonly observed in natural populations, is called the
wild type. Traits that are alternatives to the wild type, are called mutant phenotypes
because they are due to alleles assumed to have originated as changes, or mutations, in the
wild-type allele.
- A gene located on either sex chromosome is called a sex-linked gene. The human X
chromosome contains approximately 1100 genes, which are called X-linked genes, while
genes located on the Y chromosome are called Y-linked genes.
- The inactive X in each cell of a female condenses into a compact object called a Barr body,
which lies along the inside of the nuclear envelope. Most of the genes of the X chromosome
that forms the Barr body are not expressed. In the ovaries, however, Barr body
chromosomes are reactivated in the cells that give rise to eggs, resulting in every female
gamete having an active X after meiosis.
- Selection of which X chromosome will form the Barr body occurs randomly and
independently in each embryonic cell present at the time of a mosaic of two types of cells:
those with the active X derived from the father and those with the active X derived from the
mother.
- Genes located near each other on the same chromosome tend to be inherited together in
genetic crosses; such genes are said to be genetically linked and are called linked genes.
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