continuity of. cells

Cell cycle

G1 phase

organelles and other cytoplasmic components replicated/ are formed

 Cell increases in size

S phase

 DNA is duplicated

 Chromatids form in the nucleus

 Histones form

G2 phase

 Spindle fibres synthesised

 Chromosomes are checked for error

 2nd growth phase

When does G1 checkpoint take place

At the end of the G1 phase

• Before replication, what does this checkpoint ensure

Cell has reached critical size

Enough nutrients for the next stages

Necessary growth factors in place

No DNA damage

What happens if checkpoint requirements not met

Resting state or G0 phase

G2 checkpoin

At the end of the G2 phase.

 DNA replication taken place accurately

 No DNA damage

The inhibition of microtubule formation e.g. vincristine

Vincristine binds to tubulin in spindle fibres prevents them functioning – can’t contract to pull chromatids apart – anaphase can’t happen. Mitosis stops.

Antimetabolites that act of S phase inhibitors preventing DNA synthesis e.g. Fluorouracil.

. Inhibits and enzyme involved in making nucleotides with the base thymine – this prevents DNA synthesis and the S phase and therefore stops mitosis.

What are the disadvantages of using these chemotherapy drugs

Can affect normal cells and cause side effects.

What do Chromosomes consist of

extended DNA molecule supported by special proteins called histones.

What is a nucleosome

Histones folded into stacks with DNA coiled around them = nucleosome.

What is meant by the term diploid?

Chromosomes in homologous pairs

How are homologous chromosomes very similar to each other?

Carry the same genes in the same sequence along their length

How are they different to each other?

Different alleles for some genes.

What is a karyotype?

Shows chromosome arrangement.

The 23rd pair of chromosomes indicates the gender = female.

4 stages of mitosis

1. Prophase

2. Metaphase (middle)

3. Anaphase (a for apart)

4. Telophase (transport)

Prophase

• Chromatin in the nucleus becomes condensed and more visible

• Nucleolus disappears

• Nuclear membrane breaks down

• Centrioles involved in the formation of spindle fibres move to opposite poles of the cell.

• Spindle fibres form

• Each chromosome can be seen as 2 chromatids joined by a centromere.

Metaphase

• Chromosomes align along the equators

  Chromosomes attach to spindle fibres at their centromer

• Anaphase

• Spindle fibres contract to pull chromatids apart

• Centromere splits

• Chromatids move to opposite

  poles of the cell.

Telophase (transported)

• Chromatids reach opposite poles and are now referred to as chromosomes again.

• Chromosomes decondense

• Nucleolus reappears

• Nuclear membrane reforms

• Spindles fibres break down

• Cleavage furrow starts to form

After mitosis, cytokinesis occurs. What happens?

Cell divides to form 2 daughter cells. Each daughter cell is genetically identical to each other and the parent

cell.

Cytokinesis in animal cells

cleavage furrow forms and splits the cell

Cytokinesis in Plant cells

cell plate is laid down in the centre of the cell = splits it. No centrioles involved in mitosis in plant cells.

Mitotic cell division maintains the state of ploidy. What does this mean?

A diploid cell will produce 2 diploid daughter cells (2n)

46 parent 46 daughter chromosomes

Meiosis is the process of reduction division. What does this mean?

Halves the number of chromosomes in gametes. One chromosome from each homologous pair enters a gamete. When female and male gametes fuse together during fertilisation the diploid is restored.

• How does meiois differ to mitosis?

• Chromosomes arrangement in daughter cells – different from each other and the parent cell

• 2 divisions in meiosis producing 4 daughter cells. One division in mitosis = 2 daughter cells

• Meiosis only occurs in the reproductive organs (testes/ovaries)

What happens during the second meiotic division (Meiosis II)?

2 chromatids of each chromosome split and go into two new cells produced by each daughter cell from meiosis 1

Prophase I

Homologous chromosomes have paired up = bivalents (pairing up takes place during interphase)

rest of process the same as mitosis prophase

Chromatin in the nucleus becomes condensed and more visible

Nucleolus disappears

Nuclear membrane breaks down Centrioles involved in the formation of spindle fibres move to opposite poles of the cell.

Spindle fibres form

 Chromatin condenses bivalents become more visible

Metaphase I

Bivalents align along the equator

Attach to the spindle fibres at their centromeres.

Anaphase I

Spindle fibre contracts pulling chromosomes to opposite poles

2 chromosomes in each bivalent pulled apart.The

Telophase I

Chromosomes pulled to opposite poles of the cell same at mitosis

Cytokinesis

2 daughter cells produced after a cleavage furrow forms and the cell splits.

Following meiosis II what occurs?

Cytokinesis occurs – each of the original daughter cells produced in meiosis I produces 2 new daughter cells each.

Overall 4 haploid daughter cells are produced each genetically different to each other and the parent cell.

Meiosis What are the two process that allow variation to be produced?

Independent assortment

Crossing over

1. Independent assortment

Only one chromosome form each homologous pair/bivalent can enter a gamete

For one gamete it can be either of the 2 homologous chromosomes from any pair

The chromosome that enters a gamete is dependent on the random nature of how chromosomes align along the equator in metaphase 1

There are millions of possible chromosome arrangements for any one gamete.

2. Crossing over

Bivalents align side by side in interphase (4 chromatids)

2 non-sister chromatids (from different chromosomes) can break and exchange sections with each other (swap alleles)

Crossing over takes place during prophase I

Point where chromosomes cross over are called chiasmata (chiasma – singular)