SESSION 3 Flashcards
Describe the steps of a cell cycle
Before a cell can divide all the genetic information is duplicated during the S phase of the cell cycle. Each chromosome duplicates, creating the classical X- shape; each chromosome now consists of two identical sister chromatids that are touching in a structure called a centromere
In order to grow somatic cells need to divide. After cellular content is duplicated (G1 phase), and the DNA is duplicated (S phase) and double checked (G2 phase), the cell divides in a highly organised process called mitosis
Describe the process of mitosis
Prophase:
Chromosomes become visible as threadlike structure
As they continue to condense, they are seen as double structures, with sister chromatids joined at a single centromere
Metaphase:
Chromosomes become aligned at equator of cell
Anaphase:
Centromeres divide, and chromosomes move toward opposite poles
Telophase:
Christens decondense; nuclear membrane forms
Cytokinesis:
Formation of a cleavage sorrow at the equator of the cell
The cell divides into two, the cytoplasm divides, and distributing organelles to the daughter cell
What is the role of mitosis?
Essential for growth and cell replacement
Some cells retain the capacity to divide throughout their life cycle, whereas other do not divide in adulthood
Embryonic stem cells have unlimited proliferative capacity
Describe the process of meiosis
Chromosomes replicate in interphase
Meiosis 1 -> reduces the chromosome number
Prophase 1:
chromosomes become visible, homologous chromosomes pair, and sister chromatids become apparent
Recombination takes place
Metaphase 1:
Paired chromosomes align at equator of the cell
Anaphase 1:
Paired homologous chromosomes separate
Members f each chromosome pair more to opposite ends
Telophase I:
Chromosomes uncoil become dispersed
Cytokinesis:
Cytoplasm divides, forming two cells
Meiosis 2 -> begins with haploid cells
Prophase II:
Chromosomes re- co, charted
Metaphase II:
Unpaired chromosomes become aligned at equator of cell
Anaphase II:
Centromeres separate
Daughter chromosomes, which were sister chromatids, pull apart
Telophase II:
Chromosomes until, nuclear envelop re forms
Meiosis ends
Cytokinesis:
The cytoplasm divides, forming daughter cells
What is the role of meiosis?
Meiosis is the process of cell Division during which one cycle of chromosomal replication is followed by two successive cell divisions to produce haploid cells
The main consequence of meiosis is that is creates genetic diversity which is achieved in two ways:
- random assortment of maternal and paternal chromosomes during meiosis I
- crossing over
Chromosomes line up along the equator in their homologous pairs in meiosis 1
Describe the structure of sperm
Acrosome- large modified lysosomes- contains hydrolytic enzymes -digests path to egg at fertilisation
Middle piece - contains many mitochondria for aerobic respiration to provide ATP to tail
Tail- Contains microtubules- arise from pair of centrioles - have a 9 + 2 arrangement
Describe spermatogenesis
Occur sin the testes
Cells called spermatogonia line the tubules of the testes and divide by mitosis, producing daughter cells called spermatocytes
Spermatocytes undergo meiosis, and the 4 haploid cells that result are called spermatic
Each spermatic develops into a mature sperm, during which the haploid nucleus becomes condensed and forms the head of the sperm.
In the cytoplasm, a neck and a whip- like tail develop and most of the remaining cytoplasm is lost
Describe the structure of an egg
Corona radiate- follicle cells surrounding secondary oocyte
Zona pellucid- jelly- like larger secreted by follicle cells
Microvilli- absorb nutrients from follicle cells
Describe the process of oogenesis
The production of gametes is called oogenesis and takes place in the ovaries
Cells in the ovary are known as oogonia
Females are born with all the primary oocytes they will ever have
Oocytes remain in meiosis I until a female undergoes puberty
Unfertilised oocytes are lost during menstruation
What is stage G0 in the cell cycle? And why does it exist?
G0 phase- cell cycle arrest
Neurones cells- cell is neither dividing nor preparing to divide
The G0 phase is a period in the cell cycle in which cells exist in a quiescent state
Mature cells that don’t need to divide
Temporary when signals are correct they enter G1
What is a nucleolus?
A small dense spherical structure in the nucleus
The brain of the nucleus
Contains rRNA in the form of heterochromatin
Its main function is to rewrite RNA and combine it with proteins- resulting in the formation of incomplete ribosome
Why is it that the nucleolus is present in the nucleus during interphase, but absent during most of mitosis?
During interphase the nucleolus is present because DNA is not condensed form and therefore the nucleolus is there and is involved in rRNA production
Mitosis DNA is condensed and therefore the 5 or so chromosomes involved in the nucleus condense to be divided
How do the autosomal find each other in meiosis 1?
All the telomeres of the chromosomes get attached to the nucleus envelop- anchoring them to a particular spot the search area is restricted
Homologous recombination- breaks the double stranded DNA into single strands which search fro the homologous chromosomes
How do sex chromosomes find each other in meiosis I?
Specific sequences coding for ribosomal DNA which must be conserved in both males and females
Find each other using repetitive sequencing of ribosomal RNA
Identical sequencing
Sequence similarity
Sequence identity is important for finding each other
Sequences that are near identical for X and Y to find each other - par1 and par2
Define aneuploidy
An error in cell division that results in the daughter cells having the wrong number of chromosomes.
In some cases there is a missing chromosomes, while in others there is an extra
