Test: Class 12 Biology: CBSE Sample Question Paper- Term I (2021-22) - Class 12 MCQ

At the micropylar end out of the four nuclei, only three differentiate into two synergids and one egg cell. Together they are known as egg apparatus.  The egg apparatus consists of 2 synergids and one egg cell. The synergids have special cellular thickenings at the micropylar tip. These are together called the filiform apparatus. It helps to guide the pollen tubes into the synergid. Similarly, at the chalazal end, three out of the four nuclei differentiate as antipodal cells.  The remaining two cells (of the micropylar end and the chalazal end) move towards the center and are known as polar nuclei, which are situated in a large central cell.  Hence, at maturity, a typical mature angiosperm embryo sac (the female gametophyte) appears as a 7-celled structure, though it has 8-nucleate.

The tender coconut water is a free-nuclear endosperm (made up of thousands of nuclei) and the surrounding white kernel is the cellular endosperm.

Pollen grains are well preserved as fossils due to the presence of sporopollenin. Sporopollenin is a highly resistant organic material, which can withstand high temperature and strong acids and alkali. Enzymes cannot degrade sporopollenin.

The seed X is dicot and is non-endospermic whereas seed Y is monocot and is endospermic.

Placenta acts as an endocrine gland by secreting Human chorionic gonadotropin (hCG), human placental lactogen (hPL), oestrogens, progesterone and relaxin.

The label X represent uterus. Blastocyst gets attached at this site. The label Y is the fallopian tube where the process of fertilisation takes place. The label Z represents ovaries where egg is produced by the process of oogenesis. 

Penetration of the sperm in the ovum is followed by completion of meiosis II.

The correct sequence of hormone secretion from beginning of menstruation is  FSH, estrogen, and progesterone. 

In the dioecious aquatic plant shown, the flowers are small and white in colour. The pollen grains have a mucilaginous covering to protect from wetting.

Such fruits are called false fruits.

Two type of gametes will be produced: AB and aB.

Walter Sutton and Theodor  Boveri in  1902. They discovered that behaviour of chromosomes was parallel to behaviour of genes. Chromosome and genes both occur in pair. The two alleles of a gene pair are located on homologous sites of homologous chromosomes.

Sickle cell anaemia involves mutation in a single base pair in the beta globin chain of haemoglobin pigment in the blood. Glutamic acid in short arm of chromosome II gets replaced with valine at the sixth position.

In human beings, where genotype AABBCC represents dark skin colour, aabbcc represents light skin colour and AaBbCc represents intermediate skin colour; the pattern of genetic inheritance can be termed as polygenic and quantitative inheritance.

The female are diploid having 32 chromosomes and males are haploid i.e. having 16 numbers of chromosomes.

Rajesh is suffering from thalassemia. Thalassemia is an autosomal recessive disorder of the red blood cells.  In Thalassemia one of the components of the haemoglobin molecule is inadequately produced or not produced at all. Mahesh, on the other hand is suffering from sickle cell anemia, which is an autosome linked recessive disorder. It is caused by point mutation in the beta-globin chain of haemoglobin pigment of the blood.

A genetic material should

• Be able to generate its replica (Replication).
• Be chemically and structurally stable.
• Provide the scope for slow changes (mutations) that are required for evolution.
• Be able to express itself as ‘Mendelian Characters’.

The promoter site and the terminator site for transcription are located at 5′ (upstream) end and 3′ (downstream) end, respectively of the transcription unit 

The primary transcript (hn RNA) contain both exons and introns and required to be processed before they become translationally active (mRNA). The introns are removed by a process called RNA processing or RNA splicing.

In presence of lactose (inducer), it binds with repressor protein and inactivates it. So repressor protein cannot bind to operator gene. The operator gene becomes free and induces the RNA polymerase to bind with promoter gene. Hence, three structural genes express their product and respective enzymes are produced. These enzymes act on lactose so that lactose is metabolized into glucose and Galactose.

Oswald Avery, Colin MacLeod and Maclyn McCarty. They worked to determine the biochemical nature of ‘transforming principle’ in Griffith’s experiment. They discovered that

• DNA alone is transformed.
• Proteases and RNases did not affect transformation.
• Digestion with DNase inhibited transformation, suggesting that the DNA caused the transformation.

AUG is the initiation codon for methionine. So methionyl tRNA complex would have UAC at the Anticodon site.

Short stretches of DNA used to identify complementary sequence in a sample are called probes.

Anther is a sac-like structure that produces pollen grains. It is bilobed and each lobe has two theca i.e. dithecous.

Lactational amenorrhea is a natural method of contraception and  involves the prevention of conception by breast feeding the child.

Saheli is a non-steroidal preparation used as oral contraceptive pills. It is a ‘once a week’ pill with very few side effects and high contraceptive value.

Parturition is a neuroendocrine mechanism and is started by the signals from the developed foetus and the placenta. The signals originating from the foetus and placenta induce mild uterine contractions (fetal ejection reflex). This causes the release of oxytocin from maternal pituitary. Oxytocin causes stronger uterine muscle contractions which in turn stimulate further secretion of oxytocin. This process is continued leading to expulsion of the baby out of the uterus through the birth canal.

When two genes are located on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage.

Placenta is connected to the embryo by an umbilical cord. It transports substances to and from the embryo.