meiosis , mitosis , reproduction

G1 Phase (First Gap Phase):

It’s a period of cellular growth. During this phase, the cell increases in size and makes new proteins and organelles and cytoplasm increases in size. The G1 phase is also when the cell synthesizes mRNA and other types of RNA, as well as proteins necessary for DNA

S Phase (Synthesis Phase):

where DNA replication (Semi-conservative) occurs. The entire genetic content of the nucleus, i.e., all chromosomes are replicated during this phase. Each chromosome now consists of two identical sister chromatids, which are joined at the centromere. DNA mass doubles, but number of chromosomes stays the same.

G2 Phase (Second Gap Phase)

: This where further growth and protein synthesis occur in preparation for mitosis (cell division). During this phase, the cell continues to grow and produce new proteins.

Centrioles duplicate and spindle fibers start to be formed

prophase

The chromosomes condense, getting shorter, and fatter. They appear as two chromatids.

• The centrosomes start moving towards opposite ends of the cell, the spindle fibers form from centrosomes.

• The nuclear envelope breaks down and disappears and the nucleolus disappears, which allow spindle fibers to interact with the chromosomes.

metaphase

• All Chromosomes align at the equator of the cell

• After the centrosomes reach the opposite poles, spindle fibers attach to the centromeres of each chromosome, preparing to pull them apart during anaphase.

anaphase

• The spindle contract and shorten , pulling the centromere and breaking them. This makes the chromatids appear v-shaped.

• Sister chromatids are separated, and they’re pulled away by spindle fibers to the opposite poles of cells, centromere first

telophase

• The chromatids reach the opposite poles.

• They uncoil and become long and thin again (Chromosomes decondense) becoming chromatin fibers.

• A nuclear envelope forms around each group of chromosomes again and the nucleolus reform.

What’s meiosis? And why does it include 2 stages?

Meiosis is a type of cell division that occurs in sexually reproducing organisms.

It involves two rounds of cell division, resulting in the production of four haploid cells which aren’t genetically identical (cells with half the number of chromosomes as the parent cell)

. Meiosis is essential for sexual reproduction because it ensures genetic diversity in offspring and formation of gametes

the seperation of meiosis

The first round, called Meiosis I, separates the homologous pairs of chromosomes after they pair up, into two separate cells.

The second round, called Meiosis II, separates the sister chromatids (the duplicated chromosomes) into four separate cells.

• Homologous Chromosomes:

- Homologous chromosomes are a pair of chromosomes that have the same genes at the same loci, but may have different alleles.

- They are similar in size, shape, and genetic content.

- One homologous chromosome is inherited from the mother and the other from the father. - Homologous chromosomes pair up during meiosis I and exchange genetic material through crossing over.

- They separate during meiosis I, resulting in haploid cells with one copy of each homologous chromosome.

• Sister Chromatids:

- Sister chromatids are two identical copies of a single chromosome that are joined together by a centromere.

- They are produced during DNA replication in the S phase of interphase.

- Sister chromatids remain attached to each other until they are separated during cell division. -

They are separated during mitosis or meiosis II, resulting in two identical daughter cells.

prophase1

The chromosomes condense, getting shorter, and fatter. They appear as two chromatids.

• The centrosomes start moving towards opposite ends of the cell, the spindle fibers form from centrosomes.

• The nuclear envelope breaks down and disappears and the nucleolus disappears, which allow spindle fibers to interact with the chromosomes.

• Homologous chromosomes pair up to form Bivalents.

• Crossing over and exchange of alleles takes place between non-sister chromatids of the same homologous pair. This gives a different combination of alleles (Not new alleles)

metaphase1

• The centrosomes reach the opposite poles, spindle fibers attach to the centromeres of each chromosome, preparing to pull them apart during anaphase.

• Random independent assortment of maternal and paternal homologous chromosomes above and below the equator takes place which results in a different combination of alleles.

anaphase1

• The spindle contract and shorten , pulling the centromere. (No breaking of centromere here!) This makes the chromatids appear v-shaped.

• Homologous maternal and paternal chromosomes are separated, and they’re pulled away by spindle fibers to the opposite poles of cells, centromere first

telophase1

• The chromosomes reach the opposite poles.

• They uncoil and become long and thin again (Chromosomes decondense).

• A nuclear envelope forms around each group of chromosomes again and the nucleolus reform.

• Two daughter cell which are genetically different haploid cell

prophase2

• The chromosomes condense, getting shorter, and fatter. They appear as two chromatids.

• The centrosomes start moving towards opposite ends of the cell, the spindle fibers form from centrosomes

. • The nuclear envelope breaks down and disappears and the nucleolus disappears, which allow spindle fibers to interact with the chromosomes.

metaphase2

• All Chromosomes align at the equator of the cell

• After the centrosomes reach the opposite poles, spindle fibers attach to the centromeres of each chromosome, preparing to pull them apart during anaphase.

anaphase2

• The spindle contract and shorten , pulling the centromere and breaking them. This makes the chromatids appear v-shaped.

• Sister chromatids are separated, and they’re pulled away by spindle fibers to the opposite poles of cells, centromere first.

telophase2

• The chromatids reach the opposite poles

. • They uncoil and become long and thin again (Chromosomes decondense) becoming chromatin fibers.

• A nuclear envelope forms around each group of chromosomes again and the nucleolus reform.

• This results in 4 haploid genetically different cells. (Half number of chromosomes)

whats the significance of mitosis

growth and develpoment

cell replacement

repair of tissues

Asexual reproduction

whats the significance of meiosis

sexual reproduction

genetic diversity

Similarities between Mitosis and Meiosis:

1. Both mitosis and meiosis are forms of cell division, which result in the separation of existing cells into new ones.

2. Both processes involve the duplication of DNA before the cell divides.

3. Both mitosis and meiosis involve stages such as prophase, metaphase, anaphase, and telophase

Differences between Mitosis and Meiosis:

1. Purpose: Mitosis results in two genetically identical daughter cells for growth and repair, while meiosis produces four genetically different cells for sexual reproduction.

2. Number of Divisions: Mitosis involves one round of cell division, while meiosis involves two rounds of cell division

. 3. Genetic Composition: The daughter cells produced by mitosis are genetically identical to the parent cell, while those produced by meiosis are genetically different due to crossing over and independent assortment

. 4. Number of Chromosomes: Daughter cells from mitosis maintain the same number of chromosomes as the parent cell (diploid), while those from meiosis have half the number of chromosomes (haploid)

. 5. Crossing Over: Crossing over (exchange of genetic material between homologous chromosomes) occurs during prophase I of meiosis, but does not occur in mitosis.

sperm structure

• Head: Haploid nucleus to restore the diploid full set of chromosomes after fertilization and to avoid doubling of chromosomal number. Acrosome which is a one large lysosome that contains digestive enzymes for acrosomal reaction.

• Midpiece: contains mitochondria which are responsible for Aerobic respiration, which produces ATP, as a source of energy for moving the tail for swimming

• Tail: consists of many microtubules needed for swimming to reach the 2ry oocyte in the oviduct.

egg cell

• Cortical granules: It helps in forming fertilization membrane to prevent entery of more than 1 sperm. • Zona pellucida: It hardens after fertilization to prevent polyspermy.

• lipid droplets : To provide energy for the development of embryo.

• Huge number of mitochondria: They are responsible for aerobic respiration which is for energy release for embryo development, and so these mitochondria can be passed to developing embryo.

• Haploid nucleus: To restore the diploid full set of chromosomes after fertilization and to avoid doubling of chromosomal number after fusion with sperm to form zygote

Pollen tube nucleus

which contains instructions and genes that code for digestive enzymes needed to break down the style and ovary down for transfer of male gametes, this allows formation of pollen tube

• Generative nucleus

which divides by mitosis in pollen tube to become 2 male gametes.

How does male gametes reach the ovary to fertilize the female gamete?

Through the pollen tube, The pollen tube grows from the stigma towards the ovary, carrying two male gametes providing them with a pathway to reach ovary, The pollen tube tip secretes digestive enzymes to digest and break down the style tissues.

• The tube penetrates the micropyle of the ovule and releases the two male gametes into the embryo sac

. • One male gamete fuses with the egg cell to form a zygote, which develops into an embryo

• The other male gamete fuses with the two polar nuclei to form a triploid nucleus, which develops into endosperm, the endosperm provides nutrients for the developing embryo.

• (generative nuclus divides by mitosis

state what is meant by organ system

a group of organs working together to preform a specific function

suggest why fertilized damaged egg has 3 nuclei

when a sperm enetrs the egg cell , crotical granules fuse with the cell surface membrane to harden the zona pellucida, as some of the egg cell is harden there are areas where it is not hardened causeing polyspermy

explain the role of the spindle in mitosis

• the {attachment/binding} of centromeres (to the spindle fibres) (1)

• to allow the separation of the {(daughter) chromatids / chromosomes} (1)

• so that each daughter cell gets identical genetic material (1)

sperm cells

1 the mid piece contains mitochondria for AEROBIC respiration

2 the acrosome is a membrane sac that contain digestive enzymes

the nucleus contain LINEAR DNA mot circular

describe the function of the tube nucleus

 controls the growth of the pollen tube (1)

 contains the genes to make {digestive / hydrolytic} enzymes (1)

 (which allows) the male {nucleus / nuclei / gamete(s)} to {enter ovule / enter ovary / enter egg cell / fertilise the egg cell / fertilise the polar nuclei / fertilise female gametes} (

explain why is it possible for chromatide A to be genetically different from chromatide B

crossing over has occured

resulting in b recombinant chromatide contains genetic information from chromatide D

Give two differences in the arrangement of the DNA in a cell at the beginning of interphase and at the end of prophase I.

• (at the beginning of) interphase the DNA is {in the nucleus / surrounded by nuclear membrane / surrounded by nuclear envelope} whereas it is not at the end of prophase (I) (1)

• DNA is uncoiled at the beginning of interphase whereas it is condensed at the end of prophase (I) • DNA has not been replicated at the beginning of interphase whereas it has been replicated at the end of prophase (I)

i) Explain how meiosis causes this genetic variation.

• (due to) crossing over / {independent / random} assortment (1)

• (crossing over) results in different allele combinations (in the chromosomes) (1)

• (independent assortment) results in different combinations of {chromosomes / alleles} (in gametes) (1