IGCSE Biology Paper-3: Specimen Questions with Answers 243 - 244 of 358

Question 243

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Mitosis and meiosis are types of nuclear division.

The Correct Letters of the Sex Chromosomes in a Female and a Male

Table 3.2

Complete Table 3.2 to show the correct letters of the sex chromosomes in a female and a male.

Sex Chromosomes

There are two types of sex chromosome, X and Y, as shown in Fig. 3.1.

A human body cell has 46 chromosomes including a pair of sex chromosomes.


Mitosis and Meiosis Both Involve Cells Dividing to Make New Cells

The sex chromosomes are also known as Allosomes or heterosomes. In females the sex chromosomes are XX and in males they are XY.

The Y chromosome in human determines the maleness. It carries a gene ‘Sry’ (sex determining region of Y-chromosome) which codes for a product called TDF (Testis determining factor) . TDF is required for the development of male sex and its absence leads to the femaleness.

Sex Determination in Human

Question 244


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Enzymes are proteins.

State the chemical elements that enzymes are made from.


Enzymes are made from amino acids, and they are proteins. When an enzyme is formed, it is made by stringing together between 100 and 1,000 amino acids in a very specific and unique order. The chain of amino acids then folds into a unique shape. That shape allows the enzyme to carry out specific chemical reactions – an enzyme acts as a very efficient catalyst for a specific chemical reaction. The enzyme speeds that reaction up tremendously.

For example, the sugar maltose is made from two glucose molecules bonded together. The enzyme maltase is shaped in such a way that it can break the bond and free the two glucose pieces. The only thing maltase can do is break maltose molecules, but it can do that very rapidly and efficiently. Other types of enzymes can put atoms and molecules together. Breaking molecules apart and putting molecules together is what enzymes do, and there is a specific enzyme for each chemical reaction needed to make the cell work properly.

Sugar Maltose

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