Edexcel A Level Biology `Topic 3 Exam Questions

Describe how DNA is organised in bacterial cell. (3)

• M1: (large) circular DNA

• M2: DNA not associated with histone proteins

• M3: (small/ several) plasmids

• M4: Located in the cytoplasm/ not inside a nucleus

Describe the roles of golgi apparatus and secretory vesicles in the formation and transport of extracellular enzymes. (4)

• M1: {Vesicle fuse with/ protein enters} golgi apparatus

• M2: Modification of protein inside golgi apparatus

• M3: {Protein/ enzyme} packaged into secretory vesicles

• M4: Vesicles fuse with cell surface membrane

Describe what happens to lysosomes once their contents have been digested. (2)

• M1: Lysosome fuses with the cell surface membrane

• M2: Contents of lysosome released from the cell/ exocytosis

Describe how can we distinguish an eukaryotic organism from a prokaryotic organism. (4)

• M1: eukaryotic organisms have membrane bound organelles

• M2: eukaryotic organisms contain a named membrane-bound organelle (golgi apparatus, mitochondria, RER, SER)

• M3: Size of ribosomes larger than in prokaryotes

• M4: Eukaryotic organisms contain DNA that is {linear/ associated with histone proteins}

• M4: Eukaryotic organisms do not contain plasmids in cytoplasm

Liver cells can absorb lactate from the blood.

Deduce what happens to the lactate in these cells. (2)

• M1: lactate is {oxidised to form/ converted to} pyruvate

• M2: Pyruvate is converted to {glucose/ glycogen}

• M3: (pyruvate/ glucose) used in respiration

Describe the features that could be used to identify the prokaryotic cells. (4)

• M1: Presence of a cell wall

• M2: circular DNA/ plasmids

• M3: Small/ 70S ribosomes

• M4: Pilli/ flagellum

• M5: Capsule/ mesosome

Explain why enzymes that are incorrectly folded cannot carry out their function. (3)

• M1: If proteins not folded correctly, {the tertiary structure/ 3D shape} will be different.

• M2: Therefore, the active site of the enzyme would not {fit/ bind} with the substrate/ it would not be able to form an enzyme substrate complex

• M3: Therefore, it would not be able to catalyse the reaction

Describe what happens to these proteins following the process of translation until they are released from the cell. (3)

• M1: The protein are folded in the rough endoplasmic reticulum

• M2: The proteins are {packaged into/ transported in} vesicles

• M3: The protein is modified in the Golgi apparatus

• M4: Exocytosis

Explain why the following techniques are used when producing a root tip squash to observe cell division. (4)

1. Adding hydrochloric acid to the root tip

2. Adding a stain to the root tip

• M1: HCl breaks down the middle lamella

• M2: Allowing cells to be separated/ producing a thin layer of cells

• M3: To allow light to pass through

• M4: To make the chromosome visible

• M5: So that stages of mitosis can be identified

At the start of fertilisation, many sperm cells will surround the ovum.

Describe the events of fertilisation that occur after the acrosome reaction. (3)

• M1: Fusion of sperm cell membrane with egg cell membrane

• M2: Cortical granules release contents into zona pellucida

• M3: Contents of the cortical granules react with zona pellucida/ Zona pellucida {thickens/ hardens}

• M4: Fusion of {sperm and egg/ haploid} nuclei

Explain what is meant by the term sex linked disorder. (2)

• M1: A disorder caused by a {mutated/ faulty} gene

• M2: Located on the X/ Y chromosome

• M3 Therefore the disorder is more likely in one gender than another

Describe how meiosis leads to genetic variation in the gametes produced. (2)

• M1: (Produces non identical gametes due to) independent assortment of chromosomes

• M2: Crossing over between non-sister chromatids of homologous chromosomes

Explain why some genes show linkage and others show sex-linkage. (3)

• M1: There are more genes than there are chromosomes

• M2: Linkage relates to genes for different characteristics located on the same non-sex chromosome.

• M3: Sex-linkage relates to genes on the same {sex/ X/ Y} chromosome

Explain why DNA is replicated before mitosis begins. (2)

• M1: To ensure that {diploid number of chromosomes/ one copy of each chromosome} in each daughter cell

• M2: To ensure daughter cells are genetically identical

Describe how the acrosome is involved in the digestion of the zona pellucida. (2)

• M1: The membrane of the acrosome fuses with the plasma membrane of the sperm cell

• M2: Releasing enzymes from the acrosome

• M3: By exocytosis

Describe the events that occur during prophase in an animal cell. (3)

• M1: {nucleus/ nuclear envelope/ nucleous} breaks down

• M2: Spindle fibres form

• M3: {chromosomes/ chromatids} condense

• Centrioles migrate to opposite poles of the cell

Describe the appearance of the chromosomes in the cells undergoing the metaphase stage of mitosis. (3)

• M1: Condensed/ visible

• M2: Seen as pairs of chromatids held together by a centromere

• M3: Joined to the spindle fibres

• M4: Aligned on the equator of the cell

Give the meaning of the term polygenic. (2)

• M1: A characteristic showing continuous variation

• M2: Caused by multiple genes at different loci

Explain how epigenetic changes affect the development of tissues in the embryo. (3)

M1: DNA in a chromosome is wrapped around histones

M2: {acetylation/ modification} of histone affects {binding of RNA polymerase/ chromosome unwinding}

M3: Methylation of DNA affects the {transcription of genes/ production of mRNA}

M4: Therefore gene expression is altered

Describe the decisions that society has to make about the use of these embryonic stem cells. (3)

• M1: Embryonic stem cells are totipotent and can be used in a wider range of therapies

• M2: Source of embryonic step cells has to be {considered/ regulated}

• M3: {moral/ ethical} issues as the use of embryonic stem cells destroys embryos

• M4: Need for research establishments to be {regulated/ licensed}

Give the meaning of the term totipotent cell. (2)

• M1: A cell that has the ability to differentiate

• M2: Into all cell types

Cell division of the fertilised cell produces a ball of totipotent cells. This ball of cells continues to divide to form the embryo. The cells of the embryo become specialised to form tissues and organs.

Describe how cells become specialised. (3)

• M1: Chemical signals cause some genes to be {activated/ switched on}

• M2: Only activated genes are {transcribed/ produce mRNA}

• M3: mRNA leads to synthesis of specific proteins which cause cell modification

State what is meant by the term stem cell. (2)

• M1: A cell that is undifferentiated

• M2: That can give rise to specialised cells/ differentiated cells

• M3: that can divide to produce more stem cells

Explain how epigenetic changes can cause differences in a characteristic. (2)

• M1: Histone modification/ DNA methylation

• M2: {affects activation of/ activates/ deactivates} genes

As pluripotent stem cells divide, epigenetic changes are passed on.

Explain how epigenetic changes affect the activation of genes in daughter cells. (3)

• M1: Genes {activated/ deactivated} in stem cells

• M2: Because of {methylation of DNA/ histone binding}

• M3: therefore the same genes will be activated in daughter cells

Describe how mesenchymal stem cells can give rise to different types of cell. (4)

• M1: Different stimuli activate different genes

• M2: Genes activated are transcribed/ mRNA produced from activated genes

• M3: mRNA translated to produce proteins

• M4: Proteins determine {structure/ function} of cells

• M4: Different {genes activated/ proteins produced} results in different types of cell being produced.