Module 2 - Cell Division

What is the cell cycle?

organisms need new cells to grow + repair tissues in body 
consists of three main phases:
interphase - occupies majority of cycle as cells prepare for division 
mitosis - when the nucleus of cell divides into two
cytokinesis - when whole cell and cytoplasn divides in two to produce two identical cells 

What happens during interphase?

during phase cells carry out usual functions whilst preparing for cell division
consists of three phases:
G1 phase - cell grows and makes new proteins to replicare organelles 
S phase - when DNA is replicated
G2 phase - when cell continues to grow + replicated DNA checked for errors
following mitosis cell may enter G0 where may differentiate

What are cell cycle checkpoints?

cell must past various checkpoinys in order to progress to next stage where checkpoints assess whether each process has been accurately completed - ensures genetic damageis detected + repaired - prevent uncontrolled cell division so important in preventing cancers
three checkpoints include:
G1 checkpoint - checks cell has chemicals needed for replication + for any damage to DNA + has enough enery reserve
G2 checkpoint - checks DNA has been replicated without errors (if errors occurs cell tries to repair them)
metaphase checkpoint - each chromosome (sister chromatids) checked to ensure attached to spindle 
regulated by CDKs which only allow cell cycle to progress if sufficient cell growth + accurate DNA replication has occured

What are chromosomes?

thread-like structures located inside nucleus - consist of DNA tightly coiled around proteins called histones 

How many chromosomes are in each cell?

have total of 46 chromosomes divided into 23 pairs within each cell
first 22 pairs called autosomes which are identical in both males and females - 23rd pair known as sex chromosomes which differs as male have XY chromosomes and female have XX chromosomes 

What pairs are chromosomes known as?

each pair of chromosomes consist of two homologous chromosomes known as homologous pairs - 1 chromosome in pair is a paternal chromosome inherited from individuals fathers + other maternal chromosome inherited from mother
although chromosomes are identical in terms of genes they carry - may contain different alleles 

What is the structure of chromosomes?

may contain one or two chromatids - during interphase DNA is replicated meaning chromosome goes from having one chromatid to having two - two chromatids in each chromosome held together by centromere where two chromatids within a chromosome known as sister chromatids because they are genetically identical 

What are diploid cells?

contain two copies of each chromosome one copy from each parent
total number of chromosomes described as 2n with n representing one set of chromosomes eg humans have 46 chromosomes in each body cell 23 from mother and 23 from father
diploid number for human cells is 2n=46

What are haploid cells?

contain one copy of each chromosome
total number of chromosomes described as n eg human sex cells have 23 chromosomes so n=23

What happens in the phases that aren’t interphase?

mitosis - nucleus divides + cytokinesis - whole cell and cytoplasm divides

What is the importance of mitosis?

type of cell division in which parent cell divides to produce two gentically identical daughter cells - each daughter cell contains same DNA as original parent cell 
used by multicellular organisms for many processes including:
growth - unicellular zygotes can divide by mitosis to form multicellular organisms which need to get bigger
replacement/repairing of damaged or dead tissues - cells constantly dying and being replaced using mitosis
asexual reproduction - some organisms reproduce using mitosis to form genetically identical offspring eg strawberries + yeast
development of body plans - mitosis is used to form different part of an organism 
production of stem cells - stem cells divide by mitosis

What are the stages of mitosis and what does it form?

Prophase - Metaphase - Anaphase - Telophase
cell divides to form two genetically identical daughter cells - needed for growth and repair of body tissues as well as asexual reproduction

What happens during prophase?

chromosomes condense (become shorter and thicker) + visible under microscope - each chromosome made of two chromatids 
centrioles (bundles of protein) migrate to opposite poles of cell + spindle fibres form between centrioles - each centriole starts forming spindle fibres
nucleolus disappears + nuclear envelope starts to break down leaving chromosomes free in cytoplasm

What happens during metaphase?

chromosomes line up at equator of cell
each chromosome attaches to spindle fibres by their centromere
at metaphase checkpoint each chromosome checked to ensure attached to spindle

What happens during anaphase?

centromeres divide to separate each pair of sister chromatids
spindle fibres contract + shortens to pull the chromatids to opposite poles of cell
each chromosome pulled by its centromere causing them to take on a ‘V’ shape

What happens during telophase?

chromatids reach opposite poles of cell where they uncoil to become long + thin chromosomes again 
nuclear envelope forms around each set of chromosomes to form two nuclei and nucleolus starts to reform 

What is cytokinesis?

after steps of mitosis cytoplasm divides in process known as cytokinesis producing two genetically identical daughter cells 

How can mitosis be observed?

under a light microscope using suitable part of plant eg root tips must be sprouting - a suitable dye such as acetic orcein will stain chromosomes and help make stages of mitosis more visible
can add more later from seneca possibly