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Tamil Nadu HSE Second Year Zoology study materials
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Tamil Nadu HSE Second Year Zoology practice questions

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Question 1Reproduction in Organisms and Human Reproduction

In human spermatogenesis, the primary spermatocyte undergoes meiosis I to produce which cells?

  1. One secondary spermatocyte and one polar body
  2. Two secondary spermatocytes Correct answer.
  3. Four spermatids
  4. Two spermatogonia

Explanation

Meiosis I is a reductional division, so each diploid primary spermatocyte divides into two haploid secondary spermatocytes. These then undergo meiosis II to give four spermatids, which mature into spermatozoa by spermiogenesis.

Question 2Reproduction in Organisms and Human Reproduction

The acrosome of a human spermatozoon is derived from which organelle?

  1. Golgi complex Correct answer.
  2. Ribosome
  3. Mitochondrion
  4. Centriole

Explanation

During spermiogenesis the Golgi complex condenses to form the acrosome, a cap-like vesicle over the nucleus containing hydrolytic enzymes. These enzymes digest the zona pellucida during fertilisation.

Question 3Reproduction in Organisms and Human Reproduction

Which hormone surge directly triggers ovulation in the human menstrual cycle?

  1. Follicle stimulating hormone
  2. Progesterone
  3. Prolactin
  4. Luteinising hormone Correct answer.

Explanation

A sharp mid-cycle surge of luteinising hormone from the anterior pituitary ruptures the mature Graafian follicle and releases the oocyte. This LH surge is itself provoked by the peak of oestrogen secreted by the mature follicle.

Question 4Reproduction in Organisms and Human Reproduction

The site of fertilisation in the human female reproductive tract is normally the:

  1. Vagina
  2. Ampullary-isthmic junction of the oviduct Correct answer.
  3. Cervical canal
  4. Fundus of the uterus

Explanation

Sperm ascend through the uterus into the oviduct and meet the oocyte at the ampullary-isthmic junction, where fusion of the gametes occurs. The zygote then travels to the uterus for implantation.

Question 5Reproduction in Organisms and Human Reproduction

Which germ layer gives rise to the notochord in the human embryo?

  1. Ectoderm
  2. Endoderm
  3. Mesoderm Correct answer.
  4. Trophoblast

Explanation

The mesoderm forms the notochord, which induces the overlying ectoderm to thicken into the neural plate. The notochord later contributes to the axial skeleton and persists as the nucleus pulposus of intervertebral discs.

Question 6Reproduction in Organisms and Human Reproduction

In human males, the epididymis performs which primary function?

  1. Production of seminal fluid
  2. Transport of urine
  3. Storage and maturation of spermatozoa Correct answer.
  4. Secretion of testosterone

Explanation

Spermatozoa leaving the testis are immotile and immature; the epididymis stores them while they acquire motility and fertilising ability. From here they pass into the vas deferens during ejaculation.

Question 7Reproduction in Organisms and Human Reproduction

Which structure in the human ovary secretes progesterone during the luteal phase?

  1. Primary follicle
  2. Zona pellucida
  3. Corpus luteum Correct answer.
  4. Corpus albicans

Explanation

After ovulation the ruptured follicle is reorganised into the corpus luteum, which secretes progesterone to prepare and maintain the endometrium. If pregnancy does not occur, it degenerates into the corpus albicans.

Question 8Reproduction in Organisms and Human Reproduction

Which type of placenta allows the closest contact between maternal and fetal blood without the two bloods mixing?

  1. Syndesmochorial
  2. Epitheliochorial
  3. Endotheliochorial
  4. Haemochorial Correct answer.

Explanation

In the haemochorial type, found in humans, fetal chorionic villi bathe directly in maternal blood in the intervillous spaces. Only a thin placental barrier separates the two circulations, yet the bloods never actually mix.

Question 9Reproduction in Organisms and Human Reproduction

A human oocyte completes its second meiotic division only when:

  1. A spermatozoon penetrates it Correct answer.
  2. The corpus luteum forms
  3. It is released from the Graafian follicle
  4. It reaches the uterus

Explanation

The secondary oocyte is arrested in metaphase II at ovulation and completes meiosis II only after sperm entry triggers activation. The second polar body is extruded at that moment, leaving the haploid female pronucleus.

Question 10Reproduction in Organisms and Human Reproduction

A couple is unable to conceive and the male's semen shows normal sperm number and motility but poor cervical mucus penetration. Which property of spermatozoa is most directly implicated?

  1. Number of Sertoli cells
  2. Acrosomal enzyme content and sperm surface changes during capacitation Correct answer.
  3. Testosterone concentration in blood
  4. Length of the vas deferens

Explanation

Penetration of cervical mucus and the zona pellucida depends on sperm capacitation and the release of acrosomal enzymes. A defect in these surface and enzymatic changes would impair penetration even when count and motility are normal.

Question 11Reproduction in Organisms and Human Reproduction

In the human menstrual cycle, the endometrium is shed when the levels of which hormones fall?

  1. Human chorionic gonadotropin alone
  2. Oestrogen and progesterone together Correct answer.
  3. Prolactin and oxytocin
  4. FSH and LH together

Explanation

Regression of the corpus luteum lowers progesterone and oestrogen, and this withdrawal removes the hormonal support of the endometrium. The resulting breakdown of the thickened lining constitutes menstruation.

Question 12Reproduction in Organisms and Human Reproduction

Which event marks the transition from the morula stage to the blastocyst stage in human development?

  1. Formation of the neural tube
  2. Formation of a fluid-filled cavity and differentiation into trophoblast and inner cell mass Correct answer.
  3. Disappearance of the zona pellucida
  4. Start of gastrulation

Explanation

The compact morula reorganises so that a blastocoel appears, and its cells separate into an outer trophoblast and an inner cell mass. This blastocyst then hatches from the zona pellucida before implantation.

Question 13Reproduction in Organisms and Human Reproduction

Which hormone is detected by a standard pregnancy test kit and why is it reliable?

  1. Progesterone, because it rises only in pregnancy
  2. LH, because it peaks during implantation
  3. Oestrogen, because it is absent in non-pregnant women
  4. hCG, because it is secreted by the trophoblast soon after implantation Correct answer.

Explanation

Human chorionic gonadotropin is secreted by the developing trophoblast shortly after implantation and appears in urine. Its presence is a specific indicator of pregnancy and it also sustains the corpus luteum.

Question 14Reproduction in Organisms and Human Reproduction

The primary function of the Sertoli cells of the seminiferous tubule is to:

  1. Store mature spermatozoa
  2. Nourish and support developing spermatozoa and secrete inhibin Correct answer.
  3. Produce seminal plasma
  4. Secrete testosterone

Explanation

Sertoli cells extend from the basement membrane to the lumen and provide nutrients and structural support to germ cells. They also secrete inhibin, which feeds back to suppress FSH release from the pituitary.

Question 15Reproduction in Organisms and Human Reproduction

During the follicular phase of the menstrual cycle, oestrogen exerts which feedback effect on the pituitary?

  1. No effect on gonadotropin secretion
  2. Stimulation of prolactin secretion only
  3. Negative feedback throughout, suppressing FSH and LH
  4. Positive feedback near mid-cycle, triggering the LH surge Correct answer.

Explanation

Low oestrogen early in the cycle suppresses gonadotropins, but rising oestrogen from the mature follicle switches to positive feedback. This produces the pre-ovulatory LH surge that causes follicular rupture.

Question 16Reproduction in Organisms and Human Reproduction

Which of the following correctly describes the type of egg and cleavage pattern seen in humans?

  1. Mesolecithal and meroblastic
  2. Centrolecithal and superficial
  3. Microlecithal and holoblastic Correct answer.
  4. Macrolecithal and discoidal

Explanation

The human egg has very little yolk and is therefore microlecithal or oligolecithal. Such eggs undergo complete holoblastic cleavage, producing blastomeres through the whole zygote.

Question 17Reproduction in Organisms and Human Reproduction

A woman's corpus luteum fails to regress after conception. Which structure is responsible for maintaining it?

  1. The trophoblast via hCG Correct answer.
  2. The thyroid via thyroxine
  3. The hypothalamus via GnRH
  4. The anterior pituitary via FSH

Explanation

The syncytiotrophoblast secretes human chorionic gonadotropin, which acts like LH to sustain the corpus luteum. Continued progesterone secretion from the corpus luteum prevents menstruation and supports early pregnancy.

Question 18Reproduction in Organisms and Human Reproduction

Which statement about the process of implantation in humans is correct?

  1. It takes place during the follicular phase
  2. It occurs in the fallopian tube within hours of fertilisation
  3. The blastocyst embeds in the endometrium about six to seven days after fertilisation Correct answer.
  4. It begins before the zona pellucida disappears

Explanation

After hatching from the zona pellucida, the blastocyst reaches the uterus and implants in the endometrium roughly six to seven days after fertilisation. Implantation occurs during the secretory or luteal phase, when the endometrium is receptive.

Question 19Reproduction in Organisms and Human Reproduction

Which of the following is a barrier method of contraception that also protects against sexually transmitted infections?

  1. Lactational amenorrhoea
  2. Copper-T intrauterine device
  3. Condom Correct answer.
  4. Oral contraceptive pills

Explanation

Condoms are physical barriers that prevent contact between semen and the female tract, thereby blocking both sperm and pathogens. Hormonal pills and intrauterine devices do not prevent transmission of infections.

Question 20Reproduction in Organisms and Human Reproduction

In the human male, the urethra serves which dual function?

  1. Transport of ova and urine
  2. Transport of sperm and passage of urine Correct answer.
  3. Secretion of testosterone and urine concentration
  4. Storage of sperm and production of seminal fluid

Explanation

The male urethra is a common passage for both semen during ejaculation and urine from the bladder. A sphincter mechanism normally prevents urine and semen from mixing.

Question 21Reproduction in Organisms and Human Reproduction

Which sequence correctly orders the stages of human embryonic development?

  1. Blastocyst, zygote, morula, gastrula
  2. Morula, zygote, gastrula, blastocyst
  3. Zygote, blastocyst, morula, gastrula
  4. Zygote, morula, blastocyst, gastrula Correct answer.

Explanation

Cleavage converts the zygote into a solid ball of cells, the morula, which then cavitates to form the blastocyst. Gastrulation follows implantation and produces the three germ layers.

Question 22Reproduction in Organisms and Human Reproduction

Which of the following is an example of asexual reproduction in animals that involves the formation of new individuals from a single parent without gamete fusion?

  1. Budding in Hydra Correct answer.
  2. Fertilisation in the frog
  3. Pollination in a flowering plant
  4. Internal fertilisation in humans

Explanation

Budding in Hydra produces a new individual by mitotic outgrowth from the parent's body wall, with no gamete fusion. It is a form of asexual reproduction typical of many coelenterates.

Question 23Reproduction in Organisms and Human Reproduction

A researcher observes that in a particular organism the offspring are genetically identical to the parent and no gametes are formed. Which reproductive mode is operating?

  1. Parthenogenesis involving haploid gametes
  2. Sexual reproduction with self-fertilisation
  3. Asexual reproduction by mitosis Correct answer.
  4. Sexual reproduction with cross-fertilisation

Explanation

Asexual reproduction relies on mitotic division, so the offspring are clones of the parent and gametes are not involved. Self-fertilisation and parthenogenesis still involve gamete formation or meiotic products.

Question 24Reproduction in Organisms and Human Reproduction

In the human ovary, a mature Graafian follicle differs from a primary follicle in which key respect?

  1. It has no oocyte
  2. It lies outside the ovary
  3. It contains an antrum filled with follicular fluid Correct answer.
  4. It secretes only progesterone

Explanation

As the follicle matures, fluid-filled spaces coalesce into a single antrum, and the oocyte sits on the cumulus oophorus. The primary follicle, by contrast, is a solid structure with a smaller oocyte and no antrum.

Question 25Reproduction in Organisms and Human Reproduction

Which of the following best explains why the human zygote is genetically unique even though it derives from two parents?

  1. The zygote is produced by mitosis from a somatic cell
  2. The zygote contains only maternal chromosomes
  3. Random segregation and crossing over during meiosis plus random gamete fusion create a new gene combination Correct answer.
  4. The zygote receives identical chromosome sets from both parents

Explanation

Crossing over in prophase I and independent assortment in meiosis I generate genetically varied gametes. Fusion of two such gametes produces a zygote with a novel combination of alleles, which is the essence of sexual reproduction.

Question 26Reproduction in Organisms and Human Reproduction

In the human male reproductive system, which structure stores spermatozoa and allows them to acquire motility and fertilizing ability?

  1. Seminal vesicle
  2. Vas deferens
  3. Prostate gland
  4. Epididymis Correct answer.

Explanation

The epididymis is a coiled tube where sperm undergo final maturation, gaining motility and the capacity to fertilize an egg. Sperm are stored there until ejaculation. The other structures are involved in transport or secretion but not in sperm maturation.

Question 27Reproduction in Organisms and Human Reproduction

Which layer of the human ovary contains the primary follicles and developing follicles?

  1. Tunica albuginea
  2. Germinal epithelium
  3. Ovarian cortex Correct answer.
  4. Ovarian medulla

Explanation

The ovarian cortex is the outer region of the ovary where follicles at various stages of development are located. The medulla contains blood vessels and connective tissue. The germinal epithelium covers the surface, and the tunica albuginea is a fibrous capsule.

Question 28Reproduction in Organisms and Human Reproduction

A woman with a regular 28-day menstrual cycle has a surge of luteinizing hormone (LH) on day 14. Which event is most directly triggered by this surge?

  1. Proliferation of the endometrium
  2. Ovulation Correct answer.
  3. Formation of the corpus luteum
  4. Menstruation

Explanation

The LH surge occurs just before ovulation and induces the rupture of the mature Graafian follicle, releasing the oocyte. Menstruation results from falling progesterone and oestrogen levels. The corpus luteum forms after ovulation, and endometrial proliferation is driven mainly by oestrogen.

Question 29Reproduction in Organisms and Human Reproduction

In human spermatogenesis, which cell division reduces the chromosome number from diploid to haploid?

  1. Meiosis I of primary spermatocytes Correct answer.
  2. Meiosis II of secondary spermatocytes
  3. Mitosis of spermatogonia
  4. Mitosis of spermatids

Explanation

Meiosis I is the reductional division in which homologous chromosomes separate, producing two haploid secondary spermatocytes from one diploid primary spermatocyte. Meiosis II is equational and separates sister chromatids. Mitosis of spermatogonia maintains the diploid number, and spermatids do not undergo mitosis.

Question 30Reproduction in Organisms and Human Reproduction

A man has a mutation that prevents the production of follicle-stimulating hormone (FSH) receptors on Sertoli cells. Which outcome is most likely?

  1. Impaired spermatogenesis due to lack of Sertoli cell stimulation Correct answer.
  2. Increased testosterone production by Leydig cells
  3. Normal sperm production because testosterone alone is sufficient
  4. Complete absence of sperm due to failure of meiosis in germ cells

Explanation

FSH acts on Sertoli cells to support spermatogenesis, including the production of androgen-binding protein and nutrients for developing sperm. Without FSH receptors, Sertoli cell function is impaired, leading to defective spermatogenesis. Testosterone alone cannot fully compensate, and Leydig cell function is primarily regulated by LH, not FSH.

Question 31Genetics and Molecular Genetics

In a dihybrid cross between two plants heterozygous for both seed shape and seed colour (RrYy × RrYy), what phenotypic ratio is expected in the offspring?

  1. 3:1
  2. 1:2:1
  3. 9:3:3:1 Correct answer.
  4. 1:1:1:1

Explanation

When two genes assort independently, each heterozygote produces four gamete types in equal frequency, and random fusion gives sixteen combinations that reduce to the classical 9:3:3:1 phenotypic ratio. The 3:1 ratio belongs to a monohybrid cross, and 1:1:1:1 arises from a test cross of a double heterozygote with a double recessive.

Question 32Genetics and Molecular Genetics

Which of the following best describes the law of segregation as proposed by Mendel?

  1. Alleles of different genes always enter the same gamete together
  2. Dominant alleles segregate while recessive alleles remain paired
  3. Both alleles of a gene are transmitted together to every gamete
  4. The two alleles of a gene separate during gamete formation so that each gamete carries only one allele Correct answer.

Explanation

The law of segregation states that the two alleles controlling a trait remain together in the somatic cell but separate during gamete formation, so each gamete receives only one of them. This separation occurs because homologous chromosomes are pulled to opposite poles during anaphase I of meiosis.

Question 33Genetics and Molecular Genetics

A cross between a red-flowered plant and a white-flowered plant produces only pink-flowered offspring. When these pink plants are self-pollinated, the offspring are 1 red : 2 pink : 1 white. What does this indicate about the alleles?

  1. The alleles show incomplete dominance, so the heterozygote has an intermediate phenotype Correct answer.
  2. The red allele is completely dominant over the white allele
  3. The white allele is lethal in the homozygous condition
  4. The trait is controlled by two independently assorting gene pairs

Explanation

In incomplete dominance the heterozygote shows a phenotype intermediate between the two homozygotes, and selfing gives a 1:2:1 genotypic ratio that is also visible as a phenotypic ratio. Complete dominance would produce a 3:1 ratio with only two phenotypes. The presence of viable white homozygotes rules out lethality.

Question 34Genetics and Molecular Genetics

A woman who is a carrier of haemophilia marries a man who does not have haemophilia. What proportion of their sons is expected to be affected?

  1. One quarter of the sons
  2. All of the sons
  3. None of the sons
  4. Half of the sons Correct answer.

Explanation

Haemophilia is X-linked recessive. The carrier mother (X^H X^h) passes X^h to half her sons and X^H to the other half, while the father contributes Y to all sons. Therefore half the sons inherit the recessive allele on their single X and are affected, whereas daughters receive the father's normal X and are not affected.

Question 35Genetics and Molecular Genetics

Which observation from a dihybrid cross supports the conclusion that the two gene pairs are linked rather than independently assorting?

  1. A 3:1 ratio among the offspring
  2. An excess of parental combinations and a deficiency of recombinant combinations among the offspring Correct answer.
  3. An exact 9:3:3:1 ratio among the offspring
  4. Equal numbers of all four phenotypic classes among the offspring

Explanation

Linked genes lie on the same chromosome and tend to be inherited together, so parental allele combinations appear more often than expected and recombinant classes are fewer. Independent assortment would give equal or 9:3:3:1 proportions. The excess of parentals is the diagnostic signature of linkage.

Question 36Genetics and Molecular Genetics

In Drosophila, crossing over between two linked genes occurs during which stage of meiosis?

  1. Pachytene of prophase I Correct answer.
  2. Telophase of prophase II
  3. Metaphase of mitosis
  4. Anaphase II

Explanation

Crossing over takes place during pachytene of prophase I, when homologous chromosomes are synapsed as bivalents and non-sister chromatids exchange segments. This recombination is what produces new combinations of linked alleles. Anaphase II merely separates sister chromatids.

Question 37Genetics and Molecular Genetics

A male with blood group AB marries a female with blood group O. Which blood groups are possible among their children?

  1. A, B, AB and O
  2. A and B only Correct answer.
  3. O only
  4. AB and O only

Explanation

The AB father produces gametes carrying either I^A or I^B, while the O mother produces only i gametes. Children therefore receive either I^A i (group A) or I^B i (group B). Group AB and group O are impossible because the father cannot donate an i allele and the mother cannot donate I^A or I^B.

Question 38Genetics and Molecular Genetics

In humans, the ABO blood group is controlled by three alleles of a single gene. What is this condition called?

  1. Polygenic inheritance
  2. Codominance
  3. Pleiotropy
  4. Multiple allelism Correct answer.

Explanation

Multiple allelism exists when more than two alleles of the same gene occur in a population, as with I^A, I^B and i for the ABO locus. An individual still carries only two of these alleles. Pleiotropy and polygenic inheritance describe different phenomena involving one gene affecting many traits or many genes affecting one trait.

Question 39Genetics and Molecular Genetics

A person with genotype I^A I^B has blood group AB. Which genetic principle explains why both A and B antigens are expressed?

  1. Incomplete dominance
  2. Complete dominance
  3. Codominance Correct answer.
  4. Epistasis

Explanation

In codominance both alleles are fully expressed in the heterozygote, so I^A I^B individuals show both A and B antigens on their red cells. Incomplete dominance would instead produce an intermediate phenotype, which is not the case here. Epistasis involves one gene masking the expression of another gene.

Question 40Genetics and Molecular Genetics

A polygenic trait such as human skin colour typically shows which pattern in a population?

  1. A 3:1 ratio of two phenotypes
  2. Only the extreme phenotypes appear
  3. Two sharply distinct phenotypes
  4. Continuous variation with many intermediate phenotypes Correct answer.

Explanation

Polygenic traits are controlled by several gene pairs acting additively, so the phenotypes grade continuously from one extreme to the other. Environmental influence further smooths the distribution, giving a bell-shaped curve. Single-gene traits, by contrast, tend to produce discrete classes.

Question 41Genetics and Molecular Genetics

In a pedigree, a trait appears in every generation, affects about half the offspring of an affected parent, and affects males and females equally. Which mode of inheritance is most consistent with this pattern?

  1. Autosomal recessive
  2. Y-linked
  3. X-linked recessive
  4. Autosomal dominant Correct answer.

Explanation

An autosomal dominant trait typically appears in every generation because each affected person has an affected parent, and an affected heterozygote transmits it to about half the children regardless of sex. X-linked recessive traits show a strong male excess and often skip generations, while Y-linked traits pass only from father to son.

Question 42Genetics and Molecular Genetics

Which of the following is the correct base-pairing rule in a DNA double helix?

  1. Adenine pairs with thymine by two hydrogen bonds and guanine pairs with cytosine by three Correct answer.
  2. Adenine pairs with cytosine by two hydrogen bonds and guanine with thymine by three
  3. Adenine pairs with guanine and thymine pairs with cytosine
  4. Adenine pairs with uracil and guanine pairs with cytosine

Explanation

In DNA, adenine pairs with thymine through two hydrogen bonds and guanine pairs with cytosine through three, which keeps the helix diameter uniform. Uracil occurs in RNA and pairs with adenine there, not in DNA. The pairing rules ensure complementary strands can be replicated faithfully.

Question 43Genetics and Molecular Genetics

The two strands of a DNA molecule are described as antiparallel. What does this mean?

  1. The strands are held together by covalent bonds between bases
  2. Both strands run in the same 5′ to 3′ direction
  3. One strand runs 5′ to 3′ while the other runs 3′ to 5′ Correct answer.
  4. One strand contains only purines and the other only pyrimidines

Explanation

Antiparallel means the two strands have opposite orientation, one running 5′ to 3′ and the other 3′ to 5′, so the sugar-phosphate backbones are reversed relative to each other. This orientation is essential for complementary base pairing and for the directionality of replication and transcription.

Question 44Genetics and Molecular Genetics

Which enzyme relieves the supercoiling ahead of the replication fork during DNA replication?

  1. DNA polymerase III
  2. DNA topoisomerase Correct answer.
  3. DNA ligase
  4. Primase

Explanation

As the fork advances, the unwinding of the helix creates torsional stress ahead of it, and topoisomerase relieves this by nicking and resealing the DNA. Ligase instead joins Okazaki fragments, polymerase extends the chain, and primase lays down RNA primers.

Question 45Genetics and Molecular Genetics

During DNA replication, the lagging strand is synthesised discontinuously. What is the direct reason for this?

  1. DNA polymerase can add nucleotides only in the 5′ to 3′ direction, away from the fork on that strand Correct answer.
  2. The lagging strand lacks a primer entirely
  3. The lagging strand contains uracil instead of thymine
  4. Ligase removes nucleotides faster than polymerase adds them

Explanation

DNA polymerase extends a new strand only by adding nucleotides to a free 3′ end, so on the template that runs 5′ to 3′ toward the fork the new strand must be made in short Okazaki fragments away from the fork. These fragments are later joined by ligase. The other strand is copied continuously toward the fork.

Question 46Genetics and Molecular Genetics

Which statement correctly distinguishes transcription from replication in a eukaryotic cell?

  1. Transcription copies only one strand of a gene into RNA, whereas replication copies both strands of the whole DNA Correct answer.
  2. Transcription uses DNA polymerase, whereas replication uses RNA polymerase
  3. Transcription produces a double-stranded RNA identical to the gene
  4. Transcription copies both strands into RNA, whereas replication copies only one strand

Explanation

Transcription is selective: RNA polymerase reads only the template strand of a particular gene to make a single-stranded RNA copy. Replication is genome-wide and semiconservative, producing two double helices from both parental strands. The enzymes and products also differ, with RNA polymerase making RNA.

Question 47Genetics and Molecular Genetics

In eukaryotes, which enzyme is responsible for synthesising the mRNA precursor from a DNA template?

  1. RNA polymerase III
  2. RNA polymerase II Correct answer.
  3. DNA polymerase I
  4. RNA polymerase I

Explanation

RNA polymerase II transcribes protein-coding genes in the nucleus to produce pre-mRNA, which is then processed by capping, polyadenylation and splicing. RNA polymerase I makes ribosomal RNA and RNA polymerase III makes transfer RNA and small RNAs. DNA polymerase I is a prokaryotic replication and repair enzyme.

Question 48Genetics and Molecular Genetics

A eukaryotic pre-mRNA contains introns and exons. What is the role of splicing?

  1. It adds a 5′ cap and a poly-A tail
  2. It converts mRNA into a polypeptide chain
  3. It separates the two strands of the DNA double helix
  4. It removes introns and joins exons to form mature mRNA Correct answer.

Explanation

Splicing, carried out by the spliceosome, excises the non-coding intron sequences and joins the coding exons together, producing a mature mRNA that can be exported and translated. Capping and polyadenylation are separate processing events, and translation occurs later in the cytoplasm.

Question 49Genetics and Molecular Genetics

Which feature of the genetic code allows a single amino acid to be specified by more than one codon?

  1. Universality
  2. Comma-less reading
  3. Non-overlapping nature
  4. Degeneracy Correct answer.

Explanation

Degeneracy means that most amino acids correspond to several synonymous codons, which buffers the effect of some point mutations. Universality refers to the code being nearly the same in all organisms, while the non-overlapping and comma-less properties describe how codons are read consecutively without shared bases.

Question 50Genetics and Molecular Genetics

A tRNA molecule carries the anticodon 3′ UAC 5′. Which mRNA codon does it recognise?

  1. 5′ GUA 3′
  2. 5′ CAU 3′
  3. 5′ UAC 3′
  4. 5′ AUG 3′ Correct answer.

Explanation

The anticodon pairs antiparallel and complementary with the codon, so 3′ UAC 5′ pairs with 5′ AUG 3′. Adenine pairs with uracil and uracil with adenine, while cytosine pairs with guanine, preserving the antiparallel orientation. The codon AUG also happens to be the start codon.