life science

Translation

Occurs within the nucleus

Double stranded DNA unwinds and unzips when weak hydrogen bonds break

One DNA strand is used as a template to form mRNA

Using free RNA nucleotides from the nucleoplasm

The mRNA is complimentary to the DNA

This process is controlled by enzymes

The code for the protein synthesis is copied onto mRNA

Translation

mRNA moves from nucleus to ribosome in the cytoplasm

The anticodon on the tRNA matches the codon on the mRNA

The tRNA brings the amino acid to the ribosome

Amino acids are joined together by peptide bonds to form the required protein

Prophase 1

Chromosomes shorten and become visible as chromatids joined by a centromere

Nuclear membrane and nucleus disappear

Spindle fibres start to form

Crossing over takes place at the chiasma

Spindle fibres continue to form

Metaphase 1

Spindle fibres extend across the whole cell

Homologous chromosomes line up at the equator of the spindle fibres in their homologous pairs

Each homologous chromosome of each homologous pair lies on either side of the equator

Each centromere of every chromosome attaches to the spindle fibres

Anaphase 1

Spindle fibres shorten and pull each chromosome of each chromosome pair to opposite poles of the cell

Telophase 1

The chromosomes reach the poles

Each pole has half the number of chromosomes than the original cell

The cell membrane constricts and divides the cytoplasm to form two cells

Prophase 2

The cells formed in meiosis 1 divide again

Spindle fibres start to form

Metaphase 1

Individual chromosomes line up at the equator of each cell with the centromere attached to the spindle fibres

Anaphase 2

Spindle fibres shorten

Centromeres are split

Daughter Chromosomes/chromatids are pulled to the opposite poles of each cell

Telophase 2

The daughter chromosomes/chromatids reach the poles

Each pole has half the chromosomes present that the original cell

Four haploid daughter cells are formed

The four haploid daughter cells are genetically different from each other

Crossing over

In homologous chromosomes-the chromatids overlap at the loci called the chiasma/chiasmata

Genetic material is exchanged resulting in different genetic material combinations of maternal/paternal chromosomes

Random arrangement of chromosomes

Occurs in meiosis 1 & 2

Each Homologous pair of chromosomes/Each chromosome lines up at the equator differently

Allowing the gametes to have different combinations of maternal/paternal chromosomes

Events that lead to Down syndrome

Non-disjunction occured

A homologous pair of chromosomes at position 21 failed to separate during anaphase 1 or 2

This results in one gamete having an extra chromosome at position 21

The fertilisation of this gamete with a normal gamete will result in a zygote with 47 chromosomes

An extra chromosome at position 21 will result in Down syndrome

Meiosis

Involves 2 cell divisions

Gives rise to gametes

Haploid cells are produced

2 phases

Mitosis

Involves 1 cell division

Gives rise to somatic cells

Diploid cells produced

1 phase

Meiosis 1

Crossing over occurs in prophase 1

Homologous chromosome pairs separate

Whole chromosomes are pulled to opposite poles of the cell in Anaphase 1

2 haploid daughter cells produced

Meiosis 2

No crossing over in prophase 1

Sister chromatids separate

chromatids pulled to opposite poles in anaphase 2

4 haploid daughter cells produced

Importance of meiosis

Production of haploid gametes

Halving effect of meiosis overcomes doubling affect of fertilisation maintaining a constant number of chromosomes

Mechanism to introduce genetic variation

Crossing over

Random arrangement of chromosomes

Internal fertilisation

Takes place inside females body

No water required

External fertilisation

Takes place outside female body

Water is required

Oviparous

Fertilisation:internal/external

Development of embryo: external to females body

Nutrition : yolk only form of nutrition

Usually present in small quantities

Type of egg :clacereous

Example :snakes birds

Ovovivipary

Fertilisation:internal

Development of embryo: inside female body

Nutrition :yolk

Type of egg: calcareous /leathery

Example :snakes /birds

Vivipary

Fertilisation:internal

Development of embryo:inside female body

Nutrition:young receive nutrition through placenta

No egg

Mammal