WJEC AS Biology 1.6 - Genetic information is copied and passed on to daughter cells

Chromosomes

• Long, thin structures of DNA and histone proteins, in the nucleus of eukaryotic cells, which contain the genes of the organism

• Only become visible when chromatin condenses prior to cell division

• singular before DNA replication

• Consist of two genetically identical sister chromatids joined by a centromere after DNA replication

Chromatid

one of 2 identical copies of a chromosome, joined at the centromere after DNA replication/prior to cell divisioncentromere

specialised region of a chromosome where 2 chromatids join, and to which the microtubules of the spindle attach at cell division

Haploid

• one complete set of chromosomes

• haploid number of chromosomes in the cell=n

• haploid number of chromosomes in human gametes=23 n=23

• diploid organism have haploid gametes

Diploid

• 2 complete sets of each chromosome

• called diploid sets

• 2n

• human body cells

• chromosomes occru in homolgous pairs as there are 2 corresponding of each chromoHomologous chromsomesome

Homologous chromosomes

• chromosomes in a homologous pair are identical in shape, size and carry the same gene loci, with genes for the same characteristics

• one chromosome of each pair comes from each parent

Mitosis

process in which daughter cells are provided with identical copies of genes

Stages of mitosis

(not a stage of mitosis); Interphase

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

(not a stage of mitosis); Cytokinesis

Cytokinesis

• the division of the cytoplasm to form 2 new daughter cells, each with a nucleus

• the cell pinches into 2

• 2 genetically identical diploid daughter cells are formed

• cell goes back into interphase

Interphase

• longest phase of the cell cycle

• cell grows

• making/synthesising/replicating new organelles

• synthesising ATP

• carrying out protein synthesis

• increasing in size

• replicating DNA

Prophase

• longest of 4 stages of mitosis

• chromosomes condense; become shorter and become visible as 2 sister chromatids joined by a centromere

• Animals cells; centrioles migrate to opposite sides of the cell. the spindle forms from microtubules radiating from each centriole. spindle fibres extend from pole to pole and from pole to centromere of each chromsome

• numclear membrane/envelope distintegrates

• nucleolus diappears

• pairs of chromatids can be seen lying free in the cytoplasm as chromosomes scatter throughout the cell

Metaphase

• the spindle completes

• the microtubules go from one side of the cel to the other

• chromsomes migrate to the equator of the cell and attach to spindle fibres by centromere

• each chromosome is attached to one spindle fibre

Anaphase

• a very rapid stage

• spindle fibres shorten, pulling the chromatids to the poles of the cell by the centromere

• centromere divides

• sister chromaids from one chromosome are pulled centromere first to opposite poles

• chromatids are called chromosomes again

Telophase

• chromosomes reach the poles and uncoil

• chromosomes uncoil and lengthen

• spindle fibres break down

• new nuclear membrane/envelope forms around each set of chromosomes

• nucleolus reappears

differences between mitosis in animal and plant cells

• Animal cells;

  • Shape; cell becomes rounded before mitosis

  • Centrioles; Present

  • Cytokinesis; cleavage furrow develops from the outside inwards

  • Spindle; degenerates at telophase

  • Occurrence; in adult mammals, in epithelia and bone marrow, hair follicles and nail beds for cell replacement, in other sites for tissue repair

• Plant cells;

  • Shape; no shae chnage

  • centrioles; absent from higher plant cells

  • Cytokinesis; cell plate develops from the centre outwards

  • Spindle; remains throughout new cell wall formation

  • Occurrence; in meristems

The significance of mitosis - chromosome number

• produces 2 cells that have the same number of chromosomes

• each chromosome in daughter cells is exact replica of parental cell

• produces cell genetically identical to parent, giving genetic stability

The significance of mitosis - Growth

• by producing new cells, an organism increase its cell number and can grow, repair tissues and replace dead cells

The significance of mitosis - Asexual reproduction

• produces complete offspring that are genetically identical to the parent

The significance of mitosis - Damage and disease

• Repaeated cell renewal

• Damage repeair and healing

• Uncontrolled mitosis leads to cancerous growth

Meiosis

• a two-stage cell division in sexually reproducing organisms that produces four genetically different dauhter cells, each with half the number of chromosomes of the parent cell/genetically different from the parent cell

• Diploid number (2n) is halved to haploid (n, 1 set, 23 pairs, 46 total)

• Ensures that the chromosome number of individuals remains the same each generation in sexually reproducing populations

• Reduction is achieved by two divisions. DNA content halved in first divisions in second division

Phases of meiosis

• (Interphase) - onky occurs once before meiosis I

• Meiosis I (cell is divided in 2)

  • Prophase I

  • Metaphase I

  • Anaphase I

  • Telophase I

  • (Cytokinesis)

• Meiosis II (the daughter cells divide again in 2)

  • Prophase II

  • Metaphase II

  • Anaphase II

  • Telophase II

  • (cytokinesis)

• 4 each genetically dufferent daughter cell formed

Prophase I

• Chromatin condenses and chromosomes become visible as 2 sister chromatids joined by a centromere

• Paternal and maternal chromosomes come together in homolgous pairs, forming bivalents (association of 2 chromosomes of a homologous pair at prophase I of meiosis in synapsis)

• Crossing over between the maternal and paternal chromsomes of the homologous pair takes place at chiasmata

• Animal cells; centrioles migrate to poles

• spindle forms and the nuclear membrane disintegrates

Metaphase I

• The bivalents migrate to the equator of the cell (lie at equator randomly, with either mother or father chromosome in the homologous pair facing either pole - independent/random assortment) and attach to the spindle by the centromeres

• Independent assortment LEADS TO GENETIC VARIATION produces new genetic combinations, with genes from both parents going into both daughter cells

Anaphase I

• The chromosomes in each bivalent separate

• The spindle fibres shorten

• Pulls the chromosomes to opposite poles

• Each pole reduces one of each homologous pair of chromosomes

• Each chromosome is still composed to 2 chromatids they are not genetically identical due to crossing over and this random arrangement at metaphase I means there is also a random mixture of maternal and paternal chromosomes

Telophase I

• Chromosomes reach the poles

• In some species, the nuclear envelope reforms around the haploid groups of chromosomes and the chromosomes do condense and are no longer gristle. Each nucleus is haploid as the chromosomes are not paired

• In most species; the chromosomes stays in their condensed form

Cytokinesis after Meiosis I

• Division of the cytoplasm to produce 2 new cells, each with a haploid nucleus

Prophase II

• Centrioles separate and organise a new spindle at right angles to the old spindle

Metaphase II

• Chromosomes migrate to and line up at the equator

• Each chromosome attaches to the spindle fibre by its centromere

• Random/independent assortment occurs as chromatids are not identical - each chromatid in the chromosome can orientate to face either pole randomly

Anaphase II

• Spindle fibres shorten

• Centromeres seperate, pulling the chromatids to opposite poles

• The chromatids are now called chromosomes

Telophase II

• Chromosomes reach the poles

• At the poles, the chromatids lengthen and can no longer be distinguished

• Spindle disintegrates

• Nuclear envelope and nucleoli reform

Cytokinesis after Meiosis II

• Each cell divided, forming 4 haploid, genetically different daughter cells

Significance of Meiosis

• keeps chromosome number constant between generations

• Generates genetic variation in the gametes and therefore zygotes they produce.

• Two ways this happens

  • Crossing over during Prophase I

  • Independent assortment at;

    • Metaphase I so that the daughter cells contain different combinations of maternal and paternal chromosomes

    • Metaphase II; so that the daughter cells have different combinations of chromatids

• Variation allows species to survive in a constantly changing environment and colonise new environments

Differences between Meiosis I and Meiosis II

Comparison of Mitosis and Meiosis

• Number of nuclear divisions in the process;

  • Mitosis; 1, Meiosis; 2

• Number of daughter cells formed;

  • Mitosis 2, Meiosis 4

• Ploidy of parental cells/nuclei;

  • Both 2n (diploid). Mitosis can be in n (haploid)

• Ploidy of daughter cells/nuclei;

  • Mitosis; 2n, diploid if parent cell is diploid. Meiosis; n, haploid

• Genetic nature of daughter cells/nuclei;

  • Mitosis; genetically identical to parent cell and each other. Meiosis; genetically different

• Pairing of homologous chromosomes;

  • Mitosis; do not associate in pairs. Meiosis; pair up to form bivalents

• Crossing over;

  • Mitosis; None. Meiosis; In prophase I

• Segregation of homologous chromosomes;

  • Mitosis; segregate independently. Sister chromatids separated. Meiosis; segregate to different daughter cells during Meiosis I

• Chiasmata;

  • Mitosis; Absent. Meiosis; Present

• Independent assortment;

  • Mitosis; None. Meiosis; In metaphase I and metaphase II

Explain what is meant by diploid and haploid.

Diploid organisms have 2 complete sets of chromosomes (2n)

Haploid organisms have 1 complete set of chromosomes (n). Can only divide by mitosis

What is meant by homologous?

referring to chromosomes; identical in size, shape and they carry the same gene loci, with genes for the same characteristics. one chromosome of each pair comes from each parent

Describe what happens in interphase; prophase; metaphase, anaphase and telophase.

• Interphase;

  • replication of organelles such as mitochondria and chloroplasts which have their own DNA. New organelles are made. synthesis of ATP and proteins. an increase in cell size. replication of DNA.

• Prophase;

  • Chromosomes condense and become visible. Centrioles move to opposite sides of cell in animal cells. Spindle forms from microtubules. Nuclear envelope disintegrates

• Metaphase;

  • Centromeres of chromosomes attach to spindle and line up on the equator.

• Anaphase;

  • Spindle fibres shorten. Centromere separates and individual chromatids are pulled to the poles centromere first.

• Telophase;

  • Spindle breaks down. Chromosomes uncoil . Nuclear envelope reforms

Why is interphase not counted as part of mitosis?

Preparatory stage of the cell cycle, rather than the actual nuclear division

What is cytokinesis (mitosis)?

division of the cytoplasm to create the 2 new genetically identical cells.

Explain why the DNA content of a cell doubles in interphase and halves in cytokinesis

DNA replication occurs during interphase, causing the DNA content to double. During cyotkinesis, the cytoplasm divides, allowing hte daughter cells to be genetically identical. each daughter cell has one set of DNA. Hlaf the doubled DNA content

Describe the products of cytokinesis after a mitotic division.

2 genetically identical daughter cells, each with their own diploid nucleus

What is the significance of mitosis?

• it produces cells which are genetically identical to the parent cell, giving genetic stability.

• Cell division occurs for a number of reasons, including:

  • growth

  • repairing damaged cells

  • replacement of old, worn-out cells

  • asexual reproduction.

What is the cause of cancer?

The cell cycle is controlled by genes. If the genes that regulate the cell cycle are damaged, uncontrolled mitosis can occur. This rapid replication of cells can form tumours, leading to a disease called cancer. These genes that cause cancer are called oncogenes.

(SCIENTIFIC DRAWING OF CELLS FROM SLIDES OF ROOT TIP TO SHOW STAGES OF MITOSIS) What is the stain used in a root tip squash?

propionic-orcein stain

(SCIENTIFIC DRAWING OF CELLS FROM SLIDES OF ROOT TIP TO SHOW STAGES OF MITOSIS) Why is a stain necessary?

Allows to clesrly oberve mitosis. makes chromosomes more visible, allowing for easier identification of the different stages of cell division

(SCIENTIFIC DRAWING OF CELLS FROM SLIDES OF ROOT TIP TO SHOW STAGES OF MITOSIS) Why is the root tip used to observe mitosis and not the shoot tip?

Meristem where mitosis is taking place. Allows to observe cell division taking place. Mitosis is actively taking place. Onions have only 16 large chromosomes

(SCIENTIFIC DRAWING OF CELLS FROM SLIDES OF ROOT TIP TO SHOW STAGES OF MITOSIS) What is the purpose of the acid?

To observe the stages of mitosis the root tissues must be well fragmented. Hydrochloric acid is used to separate the cells by breaking down the tissue which binds cells together (maceration).

(SCIENTIFIC DRAWING OF CELLS FROM SLIDES OF ROOT TIP TO SHOW STAGES OF MITOSIS) Why is the slide ‘squashed’?

single, thin layer of cells allows light to pass through to see the cells and chromosomes more clearly.

What is the formula to calculate mitotic index?

Mitotic Index = number of actively dividing cells in field of view / total number of cells in field of view

What does the mitotic index value indicate?

Percentage of cells in mitosis

How would you calculate the length of time taken to undergo each stage of mitosis using the root tip squash method?

Percentage of cells in that phase x total cell cycle time

What is meiosis for?

it produces cells which are haploid for sexual reproduction (gametes).

What is the significance of haploid gametes?

when 2 haploid gametes fuse at fertilisation, the zygote that is formed has 2 complete sets of chromosome, one from each gamete, restoring the diploid condition. if the number did not halve during meiosis or gamete formation, the number of chromosomes would double every generation

How many divisions are there in meiosis?

2 divisions

What is the sequence of the stages in meiosis?

Meiosis I; Prophase I, Metaphase I, Anaphase I, Telophase I

Meiosis II; Prophase II, Metaphase II, Anaphase II, Telophase II

What two processes in meiosis lead to genetic variation?

• Crossing over at chiasmata during prophase I

• Independent assortment at Metphase I and II

Why is variation important?

• keeps chromoskme number constant from one generation to the next

• allows species to survive in a constantly changing environment and colonise new environments

Write down 3 events that happen in interphase.

• DNA replication

• Protein synthesis

• Organelles replicated

Describe prophase I.

• Differs from prophase in mitosis as chromosomes form bivalents (pairs of homologous chromosomes).

• Arms of the chromatids may cross over forming chiasmata. Here, genetic material can be exchanged between homologous chromosomes, therefore increasing the variation in inherited genomes.

• Chromatin condenses and chromosomes become visible as 2 sister chromatids joined by a centromere

• Paternal and maternal chromosomes come together in homolgous pairs, forming bivalents (association of 2 chromosomes of a homologous pair at prophase I of meiosis in synapsis)

• Crossing over between the maternal and paternal chromsomes of the homologous pair takes place at chiasmata

• Animal cells; centrioles migrate to poles

• spindle forms and the nuclear membrane disintegrates

What is the effect of crossing-over on the genetic makeup of the chromatids?

• genetic material can be exchanged between homologous chromosomes, therefore increasing the variation in inherited genomes.

What attach to the spindles in metaphase I?

Centromeres of the chromosomes lined up at the equator

What event in metaphase I leads to genetic variation in the gametes?

the homologous chromosomes from parent 1 and parent 2 arrange themselves randomly along the spindle facing each pole.

Describe Anaphase I.

The chromosome bivalents separate as each chromosome is pulled by its centromere (which does not split) towards the opposite pole.

What is a polar body?

a small cell formed during meiosis that contains a nucleus but very little cytoplasm,

Draw a diagram to show metaphase II in a cell that at interphase had a diploid number of 2n = 6.

cell with 3 chromosomes lined up at equator

Draw a labelled diagram of a cross section of an anther.

How could you distinguish between cells about to divide; those that have undergone meiosis I and those that have undergone meiosis II.

• about to divide; diploid, homologous chromosomes pair up

• undergone meiosis I; haploid and each chromosome still exists as 2 sister chromatids joined at the centromere

• undergone meiosis II; 4 haploid daughter cells

How would you know if a cell was in metaphase?

Chromosomes attach to spindle and line up at equator

Meiosis; Homologous chromosomes arrange themselves in pairs along the equator. Independent assortment occurs here where the homologous chromosomes from parent 1 and parent 2 arrange themselves randomly along the spindle facing each pole.

How would you know if a cell was in anaphase?

Spindle fibres shorten. Centromere separates and individual chromatids are pulled to the poles centromere first.

Meiosis; The chromosome bivalents separate as each chromosome is pulled by its centromere (which does not split) towards the opposite pole.