Our Journey To Now: Outcome 7

What is DNA replication?

It is the process of making a copy of DNA

Step 1: DNA Helicase

• Double helix unwinds with the help of enzyme DNA helicase

• DNA helicase opens the double helix by breaking the hydrogen bonds that limit the complementary nitrogen bases between two strands

Step 2: Replication fork

• Once the strands are separated, proteins attach to each strand to hold them apart

• they form a ‘Y’ shape

Step 3: Dna Polymerase

• At the replication fork, DNA polymerase enzymes move along each of the DNA strands

• DNA polymerase adds nucleotides to the exposed nitrogenous bases.This caused two new double helixes to form

• all DNA is copied and the polymerase is signalled to be detached

What is the new DNA composed of?

Each new DNA strand is composed of a new and original strand of DNA that are identical to the original DNA molecule.

If the polymerase pairs the wrong nucleotide, it will backtrack and fix it.

What is mitosis

the division of cells

Why does Mitosis occur

• So living things can grow

• to repair damaged tissue

• If the cell cannot get enough nutrients

• Or if the cell can’t expel waste

What is the original cell called and what are the two new cells called?

• Original cell: parent cell

• New cells: daughter cells

Before a cell can divide, what needs to occur?

The DNA must replicate

Mitosis cell cycle:

• Interphase

• Prophase

• Metaphase

• Anaphase

• Telophase

• Cytokinesis

Interphase

• The period of growth

• DNA replication

• Cell spends most of its life in Interphase

Prophase

• Chromosomes coil up

• Nuclear envelope disappears

• Spindle fibres form

Metaphase

• Chromosomes line up in the middle of the cell

• Spindle fibres connect to chromosomes

Anaphase

• Chromosomes copies divide

• Spindle fibres pull chromosomes to opposite poles

Telophase

• Chromosomes uncoil

• Nuclear envelopes form

• 2 new nuclei are formed

• Spindle fibres disappear

Cytokinesis

• The division of the rest of the cell (cytoplasm and organelles) after nucleus divides

• After mitosis and cytokinesis, cell returns to interphase

Meiosis

• Sexual reproduction

• Sex cells divide to produce gametes (sperm or egg)

• meiosis can only occur in gonads (testes or ovaries)

Who many divisions does Meiosis have?

2 - Meiosis I and Meiosis II

What is the division of male sex cells called?

Spermatogenesis

What is the division of female sex cells called?

Oogenesis

Fertilisation

the fusion of a sperm and egg to form a zygote (fertilised egg)

Meiosis I: Interphase I

• Chromosomes replicate

• Centriole pairs also replicate

Meiosis I: Prophase I

• longest and most complex phase

• Chromosomes condense

• Synapsis: homologous chromosomes come together to form a tetrad

• Homologous chromosomes → a pair of chromosomes (maternal & paternal) that carry genes controlling th esame inherited traits

Sex chromosomes

XX - female

XY - male

Crossing over

Segments of non-sister chromatids break and reattach to the other chromatid.

During the meiosis process, crossover occurs causing genetic diversity.

Meiosis I: Metaphase I

• Shortest phase

• Tetrads align on the metaphase plate

Meiosis I: Anaphase I

• Homologous chromosomes separate and move towards the poles

• Sister chromatids remain attached at the centromeres

Meiosis I: Telophase I

• Each pole has haploid set of chromosomes

• Cytokinesis occurs and two haploid daughter cells are formed

Meiosis II: Prophase II

• Same a prophase in mitosis

Meiosis II: Metaphase II

• Same as metaphase in mitosis

Meiosis II: Metaphase II

• Same as anaphase in mitosis

• Sister chromatids split

Meiosis II: Telophase II

• Same as telophase in mitosis

• Nuclei form

• Cytokinesis occurs

• Four haploid daughter cells produced