OCR A Level Biology Cell Division

What is a chromosome?

A strip of tightly coiled DNA

How many pairs of chromosomes does a normal human have?

23

What does a gene do?

Code for a characteristic

For what 3 reasons does mitosis occur?

Growth, repair and asexual reproduction

What occurs during the G1 phase of the cell cycle?

1. Proteins are synthesised

2. ATP is produced

3. Key organelles (including centrioles) grow larger, then duplicate

What is the G1 checkpoint known as in animals?

The restriction point

What is the G1 checkpoint known as in yeast?

START

Why is the G1 checkpoint important?

The cell must be checked to make sure that it is ready for division, as once the chromosomes are duplicated the process cannot be reversed

What factors are checked at the G1 checkpoint?

1. Nutrients- are the necessary nutrients present in the cell and its environment

2. Size- is the cell large enough for division?

3. Molecular signals- Is the cell receiving positive cues (i.e. growth factors) from its neighbours?

4. Mechanical signals- Is the cell attached to a support/crowded by neighbours

5. DNA integrity- is any DNA damaged?

What happens if the cell is rejected at the G1 checkpoint?

It passes into a resting state called G0

What are some reasons for passing into G0?

Cell age/senescence, damage, lack of need for them to reproduce

Is G0 temporary or permanent?

It can be both

Which cells spend all of their existence in G0?

Neurons, although many cells spend much of their life in G0 performing functions

What else, apart from carrying out functions, can occur in G0?

Cell death

What happens if a cell passes the G1 checkpoint?

It passes into the S phase

What occurs during the S phase?

Each chromosome is duplicated in the nucleus

Which stages of the cell cycle make up interphase?

G1, S and G2

What occurs during the G2 phase of cell division?

The duplicated chromosomes are checked for damage, and repaired/replaced if possible and necessary

What is checked at the G2 checkpoint?

Cell size, DNA condition (integrity, i.e. is any damaged?, and replication, i.e. is it fully replicated?)

What happens if a problem is detected at the G2 checkpoint?

The cell will pause at the checkpoint to undergo repairs

What happens if any problem is fixed at the G2 checkpoint?

The cell re-enters the cell cycle, and proceeds with division as normal

What happens if damage found at the G2 checkpoint is irreparable, and why is this important?

The cell may undergo apoptosis, which is important in preventing cancer by stopping errors being passed to daughter cells

What is apoptosis?

Programmed cell death

What happens if a cell passes the G2 checkpoint without any problems?

It proceeds to mitosis

How many chromatids is a chromosome made up of?

Either 1, or two genetically identical ones

What is a chromatid?

A strand of DNA

How many chromatids does each chromosome have during G1?

1 chromatid

How many chromatids does each chromosome have from S phase to the beginning of mitosis?

2

How do chromosomes condense?

The DNA (in the form of chromatin fibres) is supercoiled around histone proteins

What are the 4 stages of Mitosis?

Prophase, Metaphase, Anaphase, Telophase

What 6 things occur during Prophase in mitosis?

1. Chromatin fibres begin to coil and condense to form chromosomes.

2. Nucleolus disappears

3. Nuclear envelope begins to break down, and fully disappears by the end of Prophase

4. Microtubules begin to form spindle fibres linking the poles of the cell

5. Centrioles migrate to opposite poles of the cell

6. Spindle fibres attach to centromeres, begin to move chromosomes to cell centre

What is a centromere?

The point on a chromosome onto which a spindle fibre attaches

What occurs in Metaphase in mitosis?

Chromosomes are moved by the spindle fibres to form a plane in the cell's centre, called the Metaphase Plate, and are then held in position

What occurs in Anaphase in mitosis?

Centromeres holding pairs of chromatids together in chromosomes divide. Sister chromosomes separated and pulled to opposite poles of the cell by shortening spindle fibres

What shape do chromatids tend to be in during Anaphase?

A 'v' shape because of them being pulled by the spindle fibres

What occurs during telophase during mitosis?

Chromatids, which have now reached the poles of the cell, are referred to as chromatids. Each new set of chromosomes assembles at its respective pole, nuclear envelope and nucleolus reform. Chromosomes start to uncoil.

When does cytokinesis start in both animals and plants?

During telophase

How does cytokinesis occur in animals?

The cytoskeleton pulls the cell surface membrane inwards, causing it to invaginate (pinch in). This causes a cleavage furrow to form, until the cell surface membrane pinches off and two new cells are formed

What is the difference between cytokinesis in plants and animals?

In plants, it begins from the middle of the cell, as opposed to the outside as it does in animals

How does cytokinesis occur in plant cells?

Vesicles from the Golgi Apparatus line up in the same position as the metaphase plate, then fuse with each other and the cell surface membrane to form a new cell surface membrane in the middle of the cell called the cell plate. The cell plate then has new cellulose deposited along it.

Why can't a new cell wall be formed along the cell plate during cytokinesis in plants before the separation of the two daughter cells?

Because otherwise the new cells would undergo immediate osmotic lysis

What is osmotic lysis?

When a cell membrane bursts (and so, essentially, does the cell) due to the movement of water into the cell via osmosis

What do homologous chromosomes have the same of?

They have the same gene loci, so they have genes which code for the same thing. They also have the same length

What happens to homologous chromosomes before cell division?

They are copied

What is a bivalent?

Where pairs of homologous chromosomes attach to each other at points called chaisma (singular) or chaismata (plural)

What occurs during Prophase 1 of meiosis?

1. Chromosomes condense

2. Homologous chromosomes pair up to form bivalents (resulting in crossing over of chromatids)

3. Nuclear envelope disintegrates

4. Centrioles migrate to poles of cell

5. Spindle fibres begin to form

What is independent assortment?

Where pairs of homologous chromosomes line up with random alignments along the metaphase plate during metaphase in meiosis

What occurs during Metaphase 1 during meiosis?

1. Homologous pairs assemble along metaphase plate with random orientations (independent assortment)

2. Spindle fibres from each centriole attach to centromeres

What occurs during Anaphase 1 during meiosis?

1. Homologous chromosomes pulled to opposite poles of cell by spindle fibres, sister chromatids stay attached.

2. Sections of DNA entangled during crossing over break off and rejoin, with recombinant chromatids being formed by this exchange of genes

What is a recombinant chromatid?

One with a different genetic makeup to either the father or the mother's chromatid due to sections of DNA entangled during crossing over being exchanged.

What occurs during Telophase 1 during meiosis?

1. Chromsomes assemble at the pole of each cell and uncoil

2. Nuclear envelope reforms

3. Centrioles move back into cytosol

4. Cell surface membrane invaginates and pinches off to form 2 daughter cells, each with the same amount of genetic information as the parent cell

What happens during Prophase 2 during meiosis?

1. Chromosomes condense and become visible again

2. Nuclear envelope disintegrates

3. Centrioles migrate to poles of cell, spindle fibres begin to form

What happens during Metaphase 2 in meiosis?

1. Individual chromosomes line up along metaphase plate, again with independent assortment

2. Spindle fibres attach to centromeres

What happens during Anaphase 2 in meiosis?

Chromatids of individual chromosomes pulled to poles of cell by shortening spindle fibres after division of centromeres

What happens during Telophase 2 in meiosis?

1. Chromosomes assemble at poles, then uncoil and form chromatin again

2. Nuclear envelope reforms

3. Each cell's cell surface membrane invaginates and pinches off to form two new daughter cells, each with half the genetic information of the original parent cell

Why is mitosis used in asexual reproduction?

Because it saves energy and allows quick colonisation in a relatively short time in adverse conditions

Why is mitosis used in repair?

Old and damaged cells can be replaced by new, identical ones

Why is mitosis used in growth?

Allows accelerated growth to increase the size of the organism

What is a diploid number of chromosomes?

The full number of chromosomes in a normal cell (46 in humans)

What is a haploid number of chromosomes?

Half the normal amount of chromosomes (23 in humans)

What is a gametophyte?

An organism which produces gametes

What is a sporophyte?

An organism which produces spores

What is a spore?

A cell which is adapted for survival and dispersed in unfavourable conditions

How does meiosis ensure variation within a species? (methods)

1. Crossing over between homologous chromosomes

2. Fertilisation (1/2 of genetic information from both mother's and father's gametes)

3. Independent assortment and segregation of both homologous and non-identical sister chromatids

What is a specialised cell?

A cell which has adaptations which help it carry out a specific function

What is a tissue?

A group of the same specialised cells, which work together to carry out a function

What is another name for an erythrocyte?

A red blood cell

What shape do erythrocytes have and why?

A flattened biconcave shape to increase their surface area and SA:V ratio

What are the 3 adaptations which erythrocytes have in order to help them transport oxygen round the body?

1. Flattened biconcave shape

2. Flexible, to fit through narrow capillaries

3. In mammals, they lack nuclei or other organelles, to leave more space for haemoglobin

What is the function of a sperm cell?

To deliver genetic information to the female gamete, the ovum

What adaptations does a sperm cell have?

1. A flagellum containing microtubules and many mitochondria, giving them the ability and energy to swim.

2. An acrosome on its head containing digestive enzymes, which are released to digest the protective layers around the ovum and allow the sperm to penetrate

3. Protein fibres to strengthen the tail

What type of nucleus does a sperm cell have?

A haploid nucleus

What adaptations does a palisade mesophyll have?

1. Contain many chloroplasts to absorb lots of light for photosynthesis.

2. Cells rectangular, so can be closely packed to form continuous layer

3. Thin cell walls, increasing rate of CO2 diffusion

4. Chloroplasts can move in cytoplasm to absorb more light

5. Large vacuole to maintain turgor pressure

What is a neutrophil a type of?

Type of white blood cell

Why do neutrophils have a multi-lobed nucleus?

To make it easier for them to squeeze through small gaps to get to the site of infections

What adaptations does a neutrophil have?

Multi-lobed nucleus, granular cytoplasm, many lysosomes in cytoplasm

What is the function of a palisade mesophyll cell?

Take in light via chlorophyll for photosynthesis

What is the function of a neutrophil?

Plays an important role in the immune system

What is the function of a root hair cell?

Uptakes water and minerals from the soil

Where are root hair cells found?

Surface of the roots near the growing tips

What adaptations does a root hair cell have?

Long, thin extension to maximise SA:V ratio, vacuole containing cell sap (solution of ions and sugars) to lower water potential and maximise concentration gradient to take up water by osmosis

What is the function of a pair of guard cells?

To open and close one stoma

What happens once a guard cell loses water?

It becomes less swollen and changes shape, closing the stoma to prevent further water loss.

Why does a guard cell not change shape symmetrically?

Because the cell wall is thicker on the inside and thinner on the outside

What is the hole between two guard cells called?

Stomatal aperture

What is a 7 day old embryo called?

A blastocyst

Where are totipotent stem cells found?

A fertilized egg, zygote or the first 8-16 cells made from the fertilized egg

What type of cells can totipotent stem cells divide into?

Any type of cell

Where are pluripotent stem cells found?

The blastocyst and early embryos

What can totipotent stem cells produce apart from cells for the embryo?

Cells for the amnion (amniotic sac) or umbilicus (umbilical cord)

What are undifferentiated cells called?

Stem cells

Why does the replication of stem cells have to be tightly controlled?

If they do not divide quickly enough then tissues will be replaced inefficiently, leading to aging, but if there is uncontrolled division then they can form tumours

What can be created from pluripotent stem cells?

Any type of tissue, but not whole new organisms

How can scientists produce pluripotent stem cells?

By genetically manipulating cells found in umbilical cords, and some parts of adults (these would be multipotent stem cells before manipulation)

Where are the stem cells which create all blood cells located?

The bone marrow

What can be created from multipotent stem cells?

A few cells types, such as erythrocytes

What is the name for the type of stem cell which blood cells are created from?

Haematopoetic stem cells

What are the new cells created from stem cells necessary for?

Growth, development and tissue repair

What is the name for the ability of a stem cell to differentiate?

Potency

What is the average lifespan of a human erythrocyte?

120 days