Essay Question For Mitosis

Mitosis And Meiosis

Mitosis and Meiosis describes the way in which cells divide and reproduce. Cell reproduction is fundamental in understanding how species such as humans reproduce.


One of the ways in which cells reproduce is called Mitosis and it is a type of cell division, which means that a parent cell divides into two daughter cells. The process of mitosis is divided into four stages known as: prophase, metaphase, anaphase and telophase. Mitosis is a form of asexual reproduction used by single-celled organisms for reproduction, as well as being used for the growth of tissues, fibers and membranes. Mitosis is a process used in all organisms.

During the prophase, the membrane dissolves. In the metaphase proteins in the chromosome move the chromosome, placing it at the center of the cell. During the anaphase chromosomes are split and moved towards the end of the cell, the two portions of the cell are referred to as daughter chromosomes. During the final phase, telophase, the two chromosomes attach to their parent cell, and the previous stages are repeated- but in reverse. After the four stages are completed, a process known as cytokinesis takes place, in which the daughter chromosome becomes a daughter cell. Once this is complete two new cells are formed which are identical to the original parent cell.


Meiosis is a specific cell division that results in either a sperm or an egg, which carries one half of the chromosome found in the parent cell. There is only one main purpose to Meiosis, which is for sexual reproduction and the propagation of the species. This process occurs in humans, animals, plants and fungi. Like meiosis this procedure is also a multistage procedure.

The two phases of meiosis is called meiosis 1 and meiosis 2 and both stages have four sub-stages of their own. Meiosis 1 has phases consisting of prophase 1, metaphase 1, anaphase 1 and telophase 1. Likewise meiosis 2 has prophase 2, metaphase 2, anaphase 2 and telophase 2. In meiosis 1, a germ cells splits into two diploid cells and there is an exchange of genetic material, however the processes of meiosis 2 is similar to the process witnessed in mitosis

Scientists research these processes to find relationships within a cell’s structure and such knowledge can be applied in biomechanics and nanotechnology. It is hoped that a further understand of these processes can lead to better medical procedures and technology.

Previous IB Exam Essay Questions: Unit 7

Use these model essay question responses to prepare for essay questions on your in class tests, as well as the IB Examination, Paper 3. These questions have appeared on recent IB examinations, exactly as shown below. Following each question is the markscheme answer which was used to evaluate student answers on the examination paper.
1. Define the terms gene and allele and explain how they differ.4 marks
  • gene is a heritable factor / unit of inheritance
  • gene is composed of DNA
  • gene controls a specific characteristic / codes for a polypeptide / protein
  • allele is a form of a gene
  • alleles of a gene occupy the same gene locus / same position on chromosome
  • alleles differ (from each other) by one / a small number of bases(s)/ base pair(s)

2. Describe the consequences of a base substitution mutation with regards to sickle cell anemia.7 marks

  • the sequence of nucleotide bases in DNA codes for the sequence of amino acids in proteins
  • DNA is transcribed into mRNA, which is translated into amino acids of protein
  • normal (ß chain) hemoglobin gene / DNA produces normal (ß chain) hemoglobin protein / amino acids
  • substitution= the replacement of one (or more) nucleotide base with another
  • caused by a copying mistake during DNA replication
  • as a result of a mutagen / X-rays / chemical / UV radiation / other mutagen
  • mutation in normal (ß chain) hemoglobin gene alters the sequence of nucleotide bases
  • normal nucleotide sequence = CTC altered to CAC
  • resulting in altered mRNA (GAG to GUG) during transcription
  • resulting in altered sequence of amino acids in (ß chain) hemoglobin protein (glutamic acid to valine) during translation
  • causing red blood cells to change shape / sickle under low oxygen conditions
  • causing sickle cells anemia when two copies of the mutated gene are inherited
  • producing a sickle cell carrier when one copy of the mutated gene is inherited
  • sickle cells anemia reduces oxygen flow to organs, leading to their deterioration

3. Outline the formation of chiasmata during crossing over.5 marks

Accept the points below in an appropriately annotated diagram.

  • crossing over/chiasmata formed during prophase I of meiosis;
  • pairing of homologous chromosomes/synapsis;
  • chromatids break (at same point); (do not accept chromatids overlap)
  • non-sister chromatids join up/swap/exchange alleles/parts;
  • X-shaped structure formed / chiasmata are X-shaped structures;
  • chiasma formed at position where crossing over occurred;
  • chiasmata become visible when homologous chromosomes unpair;
  • chiasma holds homologous chromosomes together (until anaphase); 5 max

4. Explain how an error in meiosis can lead to Down syndrome.8 marks

Accept the points below in an appropriately annotated diagram.

  • non-disjunction;
  • chromosomes/chromatids do not separate / go to same pole;
  • non-separation of (homologous) chromosomes during anaphase I;
  • due to incorrect spindle attachment;
  • non-separation of chromatids during anaphase II;
  • due to centromeres not dividing;
  • occurs during gamete/sperm/egg formation;
  • less common in sperm than egg formation / function of parents' age;
  • Down syndrome due to extra chromosome 21;
  • sperm/egg/gamete receives two chromosomes of same type;
  • zygote/offspring with three chromosomes of same type / trisomy / total 47 chromosomes;

5. Karyotyping involves arranging the chromosomes of an individual into pairs. Describe one application of this process, including the way in which the chromosomes are obtained.5 marks

application of karyotyping {2 max}

  • find gender / test for Down's syndrome / other chromosome abnormality
  • identify sex chromosomes / numbers of chromosome 21 / other chromosomes counted
  • XX = female and XY = male / third chromosome 21 indicates Down's syndrome / other chromosome abnormality (e.g. Klinefelter's syndrome)

obtaining chromosomes {3 max}

  • fetal cells obtained from amniotic fluid / amniocentesis / other named source
  • white blood cells obtained
  • cells encouraged to divide
  • cells accumulated / blocked in metaphase
  • prepare slide / chromosomes examined

6. Compare the processes of mitosis and meiosis.6 marks

answers must be pair-wise comparisons to receive any marks.

  • Mitosis: one cell division & Meiosis: two divisions / reduction division
  • Mitosis: chromosome number does not change & Meiosis: converts diploid to haploid cells
  • Mitosis: products genetically identical & Meiosis: products genetically diverse
  • Mitosis: separation of sister chromatids in anaphase & Meiosis: separation of homologous chromosomes in anaphase I and sister chromatids in anaphase II
  • Mitosis: no crossing over & Meiosis: crossing over in prophase I
  • Mitosis: no formation of tetrads / no synapsis & Meiosis: formation of tetrads / synapsis
  • Mitosis: produce cells for growth/repair/asexual reproduction & Meiosis: produce sexual cells / gametes for sexual reproduction
  • Mitosis: two cells produced & Meiosis: four cells produced
  • Mitosis: daughter cells with both copies of chromosomes/random assortment does not occur & Meiosis: random assortment of maternal/ paternal chromosomes
  • Mitosis: replication of DNA in interphase & Meiosis: replication of DNA in interphase I
  • Mitosis: four phases: prophase, metaphase, anaphase, telophase & Meiosis: same four phases twice

7. Outline one example of inheritance involving multiple alleles.5 marks

  • multiple alleles means a gene has three or more alleles / more than two alleles
  • ABO blood groups / other named example of multiple alleles
  • ABO gene has three alleles / equivalent for other example
  • IA IB and i shown (at some point in the answer) / equivalent for other example
accept other notation for alleles if clear
  • any two of these alleles are present in an individual
  • homozygous and heterozygous genotye with phenotypes (shown somewhere)
  • all six genotypes with phenotypes given (shown somewhere)
  • example / diagram of a cross involving all three alleles

8. Describe the inheritance of ABO blood groups including an example of the possible outcomes of a homozygous blood group A mother having a child with a blood group O father.5 marks

  • example of co-dominance
  • multiple alleles / 3 alleles
  • (phenotype) O has (genotype) ii
  • B can be IB IB or IB i
  • A can be IA IA or IA i
  • AB is IA IB
  • (P are) i i x IA IA
  • (gametes) i and IA
  • (F1 genotype) IA i
  • (F1 phenotype) blood group A
accept other notations if used consistently and if phenotype and genotype are clearly distinguished

9. Outline sex linkage.5 marks

  • gene carried on sex chromosome / X chromosome / Y chromosome
  • inheritance different in males than in females
  • males have only one X chromosome therefore, only one copy of the gene
  • mutation on Y chromosome can only be inherited by males
  • women can be carriers if only one X chromosome affected
  • example of sex linked characteristics (e.g. hemophilia / color blindness)
  • example of cross involving linkage

10. Explain, using a named example, why many sex-linked diseases occur more frequently in men than women. 9 marks

  • named example of sex-linked disease
  • caused by recessive allele
  • on the X chromosome
  • example of pair of alleles (e.g. X H and X h) (reject if alleles do not correspond)
  • females are XX and males are XY
  • females have two alleles of the gene and males have only one
  • allele causing the disease is rare / uncommon
  • probability of femles inheriting rare allele twice as low
  • calculation of squaring the gene frequency
  • female would have to inherit the allele from her father
  • who would have suffered from the disease
  • so females can carry the gene but still be normal
  • but males (with the gene) will have the disease
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