Lecture 2 Flashcards
3 Stages of DNA replication?
Initiation, Elongation, and Termination
What happens during initiation in DNA replication ?
1.enzyme unzips and 2. SSBP
- DNA Replication begins at the origin sites, Enzymes called helicase (needs ATP to do this) unwind and separates the Stranded DNA which creates a replication fork.
- Single Stranded Binding proteins prevent parental strands from renealing. They also protect the DNA from nucleases.
Elongation: Primase, Polymerase, Ligase, tooperiasomerase
Primer Binding: primase, synthesizes a short RNA primer which provides a starting point for DNA synthesis. The primer binds to the template (original) DNA strand. 3’ - 5’ end
- DNA polymerase and other proteins add nucleotides to the growing DNA strand. It reads the template stand and adds complementary nucleotides ATGC to the new strand following the base pairing rules.
The enzyme Ligase: catalyses the formation of phosphodiesters bonds.
Ligase: also connects Okazaki fragments on the lagging (3’ to 5’ strand)
- Leading Strand Synthesis: DNA polymerase synthesises the leading strand continuously in the 5’ to 3’ direction.
Topoisomerase: unwinds overwound DNA strand ahead of the replication fork.
6.Proofreading: Polymerase
Phases of the cell cycle
- G1
checkpoint
S phase
G2
Checkpoint - Mitosis
- Cytokinesis
G1, S, and G2 phases
G1/G2 The cell grows in size, synthesizes new proteins, and carries out normal functions
Checkpoint: make sures there’s no genetic errors before replication
S phase: DNA replication occurs, resulting in duplication of the cells genetic material.
Prophase - mitosis
Prophase: chromatin condenses into visible chromosomes, the nuclear envelope breaks down, mitotic spindle forms.
Prometaphase - mitosis
Pro-metaphase: The nuclear envelope fully disintegrates, and the spindle fibres attach to the chromosomes at the centromere.
Metaphase - mitosis
Metaphase: Chromosomes align at the centre of the cell along the metaphase plate.
Anaphase - mitosis
Anaphase: Sister chromatids separate and move towards opposite poles of the cell, pulled by spindle fibres.
Telophase - mitosis
Telophase: Chromosomes reach the poles, the nuclear envelope reforms around a set of chromosomes, and the spindle fibres dissemble.
Cytokinesis - mitosis
Cytokinesis: The cytoplasm divides resulting in 2 new daughter cells, a contractile ring of actin filaments forms, pinching the cell membrane inward to create a cleavage fellow.
What is mitosis ?
a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth.
What is meiosis ?
the division of a cell’s nucleus to produce gametes, which are reproductive cells like sperm and eggs.
why is Meiosis is a reduction division ?
-> four non-identical haploid cells
what happens to the chromosome number ?
the chromosome number is halved from diploid (46 chromosomes in 23 pairs in humans) to haploid (23 chromosomes in humans)
Summary of meiosis:
Interphase, Phase 1, Phase 2, Cytokinesis
Meiosis consists of one round of DNA replication followed by two rounds of division: meiosis I and II:
Prophase 1 - LZPDD
- Prophase I: LZPDD - Leaping Zebras Pursue Daring Dreams.
Leptotene: Chromosomes condense and become visible.
Zygotene: Homologous chromosomes pair up to form tetrads.
Pachytene: Crossing over occurs (genetic material is exchanged) this forms chiasmata
Diplotene: homologous chromosomes start to sperate but stay attached by chiasmata.
Diakinesis: homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes
Metaphase I:
Tetrads align at the cell’s equator, and spindle fibres attach to the centromeres of each chromosome.
The alignment of maternal and paternal homologues along the metaphase plate is random.
This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. 8 million possible arrangements.
Anaphase I:
Homologous chromosomes separate and move towards opposite poles of the cell, pulled by the spindle fibres.
Telophase I:
Chromosomes reach the poles and the chromosomes become diffuse and the nuclear membrane reforms
Cytokenesis I:
The final cellular division to produce two new haploid cells (the original diploid cell had two copies of each chromosome; the newly formed haploid cells have one copy of each chromosome)
Prophase II:
Each haploid daughter cell prepares for the second round of division. Chromosomes condense again, and the nuclear envelope breaks down.
Metaphase II:
Chromosomes align at the equator of each daughter cell, and spindle fibres attach to the centromeres.
Anaphase II:
Sister chromatids separate and move towards opposite poles of the cell.
