12Biol quick review 3 (ch6-7)

A model of DNA is shown. Which row of the table correctly identifies the different components of the model?

• Row A

• Row B

• Row C

• Row D

Answer explanation

Row A correctly identifies the components of the DNA model, including the sugar, phosphate, and nitrogenous bases. W and X are part of the backbone of DNA and W is the part attached to the base so must be the sugar. Y and Z are bases but not complementary bases which rules out rows B and C.

A cell divides to produce gametes. This process will:

• supply cells for the replacement of damaged tissue

• produce cells that are genetically identical

• involve one nuclear division.

• produce haploid cells.

Answer explanation

The process of gamete formation, known as meiosis, results in haploid cells, which have half the number of chromosomes. This is essential for sexual reproduction, ensuring genetic diversity. Before meiosis I DNA replication occurs doubling the amount of DNA, by the end of meiosis I DNA amount has been halved and then halved again after meiosis II.

The cell cycle for a eukaryotic cell can be subdivided into the phases shown in the diagram. During the G1 and G2 phases of the cell cycle, most of the activity is directed towards:

• synthesis of organelles and growth of the cell.

• synthesis of the nuclear membrane.

• resting for the next phase.

• DNA replication

Answer explanation

During the G1 and G2 phases, the cell focuses on synthesizing organelles and growing in size, preparing for DNA replication and cell division. This makes 'synthesis of organelles and growth of the cell' the correct choice.

The following images show stages in meiosis in the order in which they occur. Which one of the following statements is correct?

• During the stage shown in image 9, chromatids separate.

• Cells after the stage shown in image 10 are haploid.

• During the stage shown in image 11, DNA will be replicated.

• Homologous chromosomes pair up in the stage shown in image 12.

Answer explanation

After the stage shown in image 10, the cells are haploid because meiosis reduces the chromosome number by half, resulting in gametes with one set of chromosomes. The images were shown in the order in which they occur during meiosis. Image 9. showed homologous chromosomes separating during meiosis 1. Haploid cells are produced at the end of meiosis 1. Chromatids separate during meiosis 2 (images 13. and 14.).

Name the enzyme that helps the two strands of DNA double helix unwind and separate.

Answer explanation

Helicase is the enzyme responsible for unwinding and separating the two strands of the DNA double helix during replication, making it essential for the process. Ligase and polymerase have different roles.