bio exam - year 12 !

Unit 3

What does continuinity of life require?

The replication of genetic material and its transfer to the next generation through processes, including binary fission, mitosis, meiosis and fertilisation

Why do cells need to divide?

For growth and repairment of damaged body cells

What is Binary fission

A process of asexual reproduction where a prokaryotic cell divides into 2 identical daughter cells

The DNA segregates and the cell divides all in one step and relatively faster

What is meiosis?

A type of cell division that produces gametes (sperm and egg cells) with half the number of chromosomes as the original cell.

This ensures that when fertilisation occurs, the offspring has the correct number of chromosomes

What are the steps of phase 2 in meiosis?

Prophase II:

• Chromosomes condense again in born cells

• Spindle fibres form

Metaphase II:

• Chromosomes line up at the centre of the cell

Anaphase II:

• Sister chromatids (identical halves of a chromosome) are pulled apart to opposite sides

Telophase II & Cytokinesis:

• 4 haploid cells form, each genetically unique

What are the steps of phase I of meiosis?

Prophase 1:

• Chromosomes condense and become visible

• Homologous chromosomes (matching pairs) pair up

• Crossing over: sections of DNA are exchanged between homologous chromosomes, increasing genetic diversity

Metaphase 1:

• Homologous chromosome pairs line up in the middle of the cell

• Independent assortment: chromosomes are randomly arranged, leading to variation

Anaphase 1:

• Homologous chromosomes are pulled apart to opposite ends of the cell

Telophase 1 & Cytokinesis:

• 2 new cells form, each with half the original chromosome number (haploid)

• Nuclear membranes may reform temporarily

Why is meiosis important?

• Genetic variation: creates variation through crossing over and independent assortment

• Maintains chromosome number: halves chromosome number in gametes so when fertilisation occurs the normal number is restored

• Essential for sexual reproduction: without meiosis, offspring would have double the chromosomes with each generation

Draw the steps of meiosis

diagram

what are the sources of genetic variation in meiosis?

Crossing over (prophase I): exchange of DNA between homologous chromosomes

Independent assortment (metaphase I): random arrangement of chromosomes leads to different combinations of gametes

Random fertilisation: any sperm can fertilise any egg, leading to even more variation

What is the difference between haploid and diploid?

Haploid (n): a cell with the half the normal chromosome number, e.g. gamete

Diploid (2n): a cell with a full set of chromosomes, e.g. a body cell

What is mitosis?

Type of nuclear division in somatic cells that maintains the parental diploid number of chromosomes in the daughter cells

Forming two identical daughter cells

Make a comparison table between mitosis and meiosis

make one in onenote and copy photo here

What are the stages of mitosis?

Prophase:

1. Chromatin threads condense to form chromosomes (two sister chromatids held together by centromere)

2. Nuclear membrane disintegrates and nucleolus disappears

3. Spindle begins to form, fibres attach to each chromosome at its centromere

4. The two centrosomes move towards opposite poles of the cell

Metaphase:

1. The chromosomes move to the centre of the cell and line up along the equator

2. Centromeres of chromosomes are aligned on the equator

3. Centrioles are located at opposite poles of the cell

4. Spindle fibres attach to the centromere

Anaphase:

1. Sister chromatids separate

2. Sister chromatids are pulled to opposite part of the cell

3. Each pole has complete identical set of maternal and paternal chromosomes

4. Sister chromatids are now chromosomes

Telophase

1. Chromosomes decondense (forms chromatin)

2. Two new nuclear envelopes form (one for each new daughter cell)

3. Spindle disappears

4. Cell elongates and is ready for cytokinesis to separate.