DNA replication: mitosis & meiosis Biology Human Biology

DNA replication Mitosis Meiosis DNA replication Interphase Prophase Keywords Keywords Mitosis Meiosis DNA replication Interphase Prophase Metaphase Anaphase Telophase Chromatin Chromatids Chromosomes Centromere Centriole Spindle fibres Cytokinesis

Genetic information Each species has a specific number of chromosomes, composed of a particular kind of DNA Example: - Humans have 46 chromosomes Dogs have 78 chromosomes Dandelions have 16 chromosomes Strawberries have 14 chromosomes

Prokaryotes vs. eukaryotes Single celled Asexual reproduction Binary fission Eukaryotes Multicellular Mitosis Sexual reproduction Meiosis Apoptosis

Prokaryotes

Asexual reproduction – binary fission DNA replication of the plasmid The plasmid (single chromosome) is duplicated The 2 chromosomes segregate - each copy of the chromosome attaches to a different part of the cell membrane Cytokinesis The cell begins to divide - a wall forms across the cell and divides it into two The end result is 2 (genetically) identically daughter cells

Eukaryotes

Mitosis Cell division is responsible for growth, replacement and repair. The result of mitosis is the formation of two genetically identical cells – the parent cell divides to form 2 identical daughter cells

Stages of mitosis Interphase is a period when the cell functions as normal, but is also preparing for cell division. The chromosomes are strung out in long chromatin threads within the nucleus. Each chromatin thread is replicated over a period of time. All the various organelles are also being replicated at this time. Mitosis can be divided into 5 stages: Interphase Prophase Metaphase Anaphase Telophase

Prophase Centrioles become visible in the cytoplasm Chromatin condenses into chromosomes Each chromosome consists of 2 identical strands called chromatids These are attached to each other at a point call the centromere Nuclear membrane disappears centrioles chromosome –consisting of 2 chromatids

Metaphase During metaphase the chromatids line up at the equator of the cell. The centromere of each pair is attached to a spindle fibre spindle fibres chromosomes – spindle fibre attached at the centromere centrioles

Anaphase During anaphase each pair of chromatids separate at the centromere The chromatids are pulled apart towards the opposite poles of the cell by the spindle fibres spindle fibre chromatid centriole

Telophase During telophase the 2 sets of chromosomes form tight groups at each pole The spindle disappears Nuclear membranes and nuclei form Centrioles divide Chromosomes uncoil and disappear Cytokinesis occurs during this phase Cytokinesis is the division of the cytoplasm centriole cytokinesis group of chromosomes

Stages of mitosis

Interphase – beginning of cell cycle

Just before prophase – chromosomes have been replicated

Meiosis There is another form of cell division called meiosis Meiosis does not produce identical daughter cells that are copies of the original parent cell Instead, meiosis produces the gametes (sex cells) After meiosis, each gamete ends up with only half the number of chromosomes

Homologous pair of chromosomes Meiosis Meiosis Mitosis The process of meiosis involves 2 nuclear divisions, but the chromosomes duplicate only once The first division of meiosis is similar to mitosis However, during metaphase I the chromosomes pair off, rather than all of them lining up singly. Metaphase I Metaphase Homologous pair of chromosomes Metaphase II

Meiosis I & meiosis II

Diploid vs haploid cells Term Definition Diploid The number of chromosomes present in the body cells (somatic cells) of an individual (2n) Haploid The number of chromosomes present in the gametes (germline cells) (n)

Mitosis vs. meiosis

The importance of meiosis Meiosis maintains chromosome number Half from each diploid parent only; a complete set on fertilisation Meiosis allows for recombination of alleles Pairing of different alleles from different parents increases variation Meiosis allows for the separation of alleles One allele is found on each chromosome in a pair. Separation allows alleles to move independently, thus increasing variability

What’s so special about meiosis? Meiosis plays an important role in maintaining genetic diversity There are a number of events that happen during meiosis that ensure the next generation has as much variation as possible Random fertilisation Crossing over Independent assortment Mutations Non-disjunction

Random fertilisation Fertilisation of gametes is a random process Each fertilisation event produces a unique zygote

Crossing over Crossing over occurs during prophase I. This ensures genetic diversity in offspring Homologous chromosomes pair up Similar sections cross over and swap places All gametes have different genetic content Linked genes are those found on the same chromosome and usually inherited together are separated and can be inherited independently

Crossing over

Crossing over

Random assortment After meiosis, each gamete has only one set of genes. They unite randomly at fertilization, combining their genes to form a different genetic combination from either parent. This is called random assortment Sometimes it is also called independent assortment

(½)23 Random assortment Humans have 23 pairs of chromosomes: there is an equal chance that the gametes will get either one of the pair. ½ chance of getting maternal chromosome 1 ½ chance of getting maternal chromosome 2 And so on ... Therefore that chance of getting ALL the maternal chromosomes is (½)23 or ½ x ½ x ½ x ½ x ..... 23 times

Random assortment

Mutations

Chromosomal mutations Duplication Inversion Deletion Insertion Translocation

Gene or point mutations

Results in too many chromosomes in one gamete and not enough in another.

Hybrid vigour increases the survival of the offspring of the cross between the two different parents

Apoptosis Programmed cell death Cell death is vital to the health of an organism Metamorphosis Apoptosis is important during metamorphosis Example: tadpole into a frog. As a tadpole changes into a frog, the cells in the tadpole tail are induced to undergo apoptosis; as a consequence, the tail is lost. Tissue maintenance In eukaryotes, cell death exactly balances cell division, otherwise their tissues and organs would grow or shrink.

Apoptosis Cell volume increases and the cell membrane bulges outwards Nucleus fragments as enzymes destroy it Apoptic bodies form Apoptic debris is engulfed by macrophages