5 .continuity of. cells

Cell cycle

G1 phase

• organelles and other cytoplasmic components are replicated

• Cell increases in size

S phase

• DNA is duplicated

• Chromatids form in the nucleus

• Histones form

• G2 phase

• Spindle fibres synthesised

• Chromosomes checked for error

• 2nd growth phase

When does G1 checkpoint take place

end of G1 phase

• Before replication, what does this G1 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 end of G2 phase.

• DNA replication taken place accurately

• No DNA damage

Last checkpoint

Metaphase checkpoint – chromosomes have attached to spindle fibres

cancer preventative drugs vincristine

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

Antimetabolites preventing DNA synthesis e.g. Fluorouracil. Cus cancer

Fluorouracil inhibits enzyme involved in making nucleotides with base thymine –

prevents DNA synthesis and S phase and therefore stops mitosis.

disadvantages of using chemotherapy drugs

Can affect normal cells and cause side effects.

What do Chromosomes consist of

extended DNA molecule supported by histones.

What is a nucleosome

Histones folded into stacks with DNA coiled around them

What is meant by the term diploid?

Chromosomes in homologous pairs

How are homologous chromosomes similar to each other?

Carry same genes in same sequence along their length

How are homologous chromosomes 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 nucleus becomes condensed and more visible

• Nucleolus disappears

• Nuclear membrane breaks down

• Centrioles involved in formation of spindle fibres move to opposite poles of 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 centromere

• 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 parent cell.

Cytokinesis in animal cells

cleavage furrow forms and splits the cell

Cytokinesis in Plant cells

cell plate is laid down in centre of cell = splits it.

No centrioles involved in mitosis in plant cells.

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

A diploid cell will produce 2 diploid daughter cells

46 parent 46 daughter chromosomes

Meiosis is process of reduction division. meaning?

Halves no. of chromosomes in gametes. One chromosome from each homologous pair enters a gamete. When female and male gametes fuse together during fertilisation diploid is restored.

How does meiois differ to mitosis?

• in meiosis - Chromosomes arrangement in daughter cells is 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)

Prophase I in meiosis

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

Chromatin condenses bivalents become more visible

Nucleolus disappears

Nuclear membrane breaks down

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

Spindle fibres form

Metaphase I

Bivalents align along the equator and Attach to spindle fibres at their centromeres.

Anaphase I

Spindle fibre contracts pulling chromosomes to opposite poles

2 chromosomes in each bivalent pulled apart.

Telophase I

• Chromosomes pulled to opposite poles of cell

• Chromosomes decondense slightly

• Nucleolus reappears

• Nuclear membrane reforms

• Spindles fibres break down

• Cleavage furrow starts to form

What happens duringMeiosis II

Spindle fibres form at right angles to those in meiosis I and pull sister chromatids apart; two chromatids of each chromosome separate and go into new cells produced by each daghter cell from meiosis 1

Following meiosis II what occurs?

Cytokinesis occurs – each daughter cell produced in meiosis I divides again after a cleavage furrow forms, producing 2 new daughter cells each

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

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

Independent assortment

Crossing over

1. Independent assortment

Only one chromosome from each homologous pair can enter a gamete

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

Homologous chromosomes pair up to form bivalents (4 chromatids) in prophase I

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

crossing over occurs during prophase I

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

Why is crossing over significant

Two chromosomes or a homologous pair have identical genes but some alleles may differ

Crossing over consequence

two chromatids of same chromosome are no longer identical

Some chromatids now have unique sequences or alleles that did not exist in potential chromosome

The 2 chromatids are now referred to as recombinants

What is a bivalent

A pair of homologous chromosomes (4 chromatids)

Crossing over diagram