Question: 100% NCERT covered from Particular Keyword.Explanations are 100% from NCERT | [Difficult level: Easy] From NCERT NEET Derived Question.
What is true about male and female gametophyte in plant
kingdom ?
(a) In bryophytes and pteridophytes they have
independent free-living existence.
(b) In Gymnosperms and Angiosperms they have no
independent free-living existence.
(c) Both (a) and (b)
(d) In bryophytes, pteridophytes and angiosperms they
have free-living life. They remain in sporangia which
are retained on sporophytes.
Answer: c
Independent [ keyword from NCERT]
👉Unlike bryophytes and pteridophytes, in
gymnosperms the male and the female gametophytes
do not have an independent free-living existence. They
remain within the sporangia retained on the
sporophytes.
👉Diplontic life cycle :
wherein the diploid
sporophyte is the dominant, photosynthetic,
independent phase of the plant. The gametophytic
phase is represented by the single to few-celled
haploid gametophyte. This kind of life cycle is
termed as diplontic. An alga, Fucus sp.
👉Bryophytes and pteridophytes, interestingly, exhibit
an intermediate condition (Haplo-diplontic); both
phases are multicellular. However, they differ in their
dominant phases.
👉A dominant, independent, photosynthetic, thalloid or erect phase is
represented by a haploid gametophyte and it alternates with the shortlived
multicelluler sporophyte totally or partially dependent on the
gametophyte for its anchorage and nutrition. All bryophytes represent
this pattern.
👉The diploid sporophyte is represented by a dominant, independent,
photosynthetic, vascular plant body. It alternates with multicellular,
saprophytic/autotrophic, independent but short-lived haploid
gametophyte. Such a pattern is known as haplo-diplontic life cycle. All
pteridophytes exhibit this pattern
👉Most algal genera are haplontic, some of them
such as Ectocarpus, Polysiphonia, kelps are haplo-diplontic. Fucus, an
alga is diplontic.
👉Unicellular organisms are capable of (i) independent existence and
(ii) performing the essential functions of life.
👉Anything less than a complete
structure of a cell does not ensure independent living.
👉Passive symports and
antiports:
Some carrier or transport proteins allow
diffusion only if two types of molecules
move together. In a symport, both
molecules cross the membrane in the same
direction; in an antiport, they move in
opposite directions . When a molecule moves
across a membrane independent of other molecules, the
process is called uniport.
👉During mid-1960s, three independent researches reported the
purification and chemical characterisation of three different kinds of
inhibitors: inhibitor-B, abscission II and dormin. Later all the three were
proved to be chemically identical. It was named abscisic acid (ABA).
👉Seeds offer several advantages to angiosperms. Firstly, since
reproductive processes such as pollination and fertilisation are
independent of water, seed formation is more dependable
👉Law of Independent Assortment:
Based upon such observations on dihybrid crosses (crosses between
plants differing in two traits) Mendel proposed a second set of generalisations
that we call Mendel’s Law of Independent Assortment. The law states that
‘when two pairs of traits are combined in a hybrid, segregation of one pair
of characters is independent of the other pair of characters’.
The Punnett square can be effectively used to understand the
independent segregation of the two pairs of genes during meiosis and
the production of eggs and pollen in the F1 RrYy plant.
👉 RrYy plant:
The important thing to remember here is that segregation of 50 per cent R
and 50 per cent r is independent from the segregation of 50 per cent
Y and 50 per cent y
👉During Anaphase of meiosis I, the two chromosome pairs can align at
the metaphase plate independently of each other (Figure 5.9). To
understand this, compare the chromosomes of four different colour in
the left and right columns.
👉Linkage:
Morgan carried out several dihybrid crosses in Drosophila to study genes
that were sex-linked. The crosses were similar to the dihybrid crosses carried
out by Mendel in peas. For example Morgan hybridised yellow-bodied,
white-eyed females to brown-bodied, red-eyed males and intercrossed their
F1 progeny. He observed that the two genes did not segregate independently
of each other and the F2 ratio deviated very significantly from the 9:3:3:1
ratio (expected when the two genes are independent).
👉It was found that Mendel’s law of independent assortment does not hold
true for the genes that were located on the same chromosomes. These
genes were called as ‘linked genes’. Closely located genes assorted
together, and distantly located genes, due to recombination, assorted
independently. Linkage maps, therefore, corresponded to arrangement
of genes on a chromosome.
👉Heterogeneous nuclear RNA (hnRNA) :
hnRNA undergoes
additional processing called as capping and tailing. In capping an
unusual nucleotide (methyl guanosine triphosphate) is added to
the 5'-end of hnRNA. In tailing, adenylate residues (200-300) are
added at 3'-end in a template independent manner. It is the fully
processed hnRNA, now called mRNA, that is transported out of the
nucleus for translation
👉Providing proof that the codon was a triplet, was a more daunting
task. The chemical method developed by Har Gobind Khorana was
instrumental in synthesising RNA molecules with defined combinations
of bases (homopolymers and copolymers). Marshall Nirenberg’s cell-free
system for protein synthesis finally helped the code to be deciphered.
Severo Ochoa enzyme (polynucleotide phosphorylase) was also helpful
in polymerising RNA with defined sequences in a template independent
manner (enzymatic synthesis of RNA). Finally a checker-board for genetic
code was prepared
👉plasmids and bacteriophages have the ability to replicate
within bacterial cells independent of the control of chromosomal DNA.
Bacteriophages because of their high number per cell, have very high
copy numbers of their genome within the bacterial cells. Some plasmids
may have only one or two copies per cell whereas others may have
15-100 copies per cell.
