Meiosis And Mitosis Essay Questions

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

Essay Mitosis and Meiosis

926 Words4 Pages

Cell division may happen by either mitosis or meiosis, depending on what type of cell is invovled. Mitosis is a process by which a cell divides to form two daughter cells. They each have the same exact number and kind of chromosomes as the parent cell. Meiosis occurs in the primary sex cells leading to the formation of viable egg and sperm cells. They reduce the number of chromosomes to half in each gamete so that when they are getting furtilized, the species chromosome number is kept even.
Mitosis happens in the reproduction of unicellular organisms and in the addition of cells to a tissue or organ in a multicellular organism.
There are four stages of mitosis. The first stage is the prophase. In this stage the chromosomes become shorter…show more content…

The spindle fibers extending from the poles to the centromeres disappear and those fibers that lie in the plane between the rows of chromosomes remain for a longer time. A nuclear membrane reforms around each bundle of chromosomes at the poles. At the center of animal cells, the cytoplasm turns inward, pinching the old cell into two new cells.
Meiosis happens in the primary sex cells, which leads to the formation of viable egg and sperm cells. The purpose of meiosis is to cut the number of chromosomes in half.
There are two stages of meiosis, meiosis I and II. Meiosis I has five stages called prophase I, metaphse I, anaphase I, telophase I, and interkinesis I. Meiosis II has only four stages. They are prophase II, metaphase II, anaphase II, and telophase II.
The stages of meiosis I result in the reduction of the number of chromosomes.
In prophase I the chromosomes become shorter and thicker and the nucleolus disappears. The chromosomes pair with their homologues forming a group of four chromatids. The tetrads wrap around each other and may exchange like parts. The centrioles move and the spindle fibers appear and the nuclear membrane disappears.
The tetrads move as a group to the equator in metaphase I. The centromeres of each of the homologous pairs of chromosomes become attached to spindle fibers extending from opposite poles.
Each pair of double-stranded chromosomes, in anaphaseI, is pulled away from its homologue

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