2.6 - cell division

main phases of the cell cycle

1. interphase

2. mitosis

3. cytokinesis

interphase

• cells prepare for cell division

• consists of G1 phase, S phase and G2 phase

G1 phase

cell grows and makes new proteins to replicate organelles

S phase

when DNA is replicated

G2 phase

cell continues to grow and replicated DNA is checked for errors

Purpose of cell cycle checkpoints?

assess whether the processes at each phase of the cycle have been accurately completed

G1 checkpoint

checks that the cell has the chemicals needed for replication and for any damage to the DNA

G2 checkpoint

checks that DNA has been replicated without any errors

metaphase checkpoint

Each chromosome is checked to ensure it is attached to the spindle

structure of chromosomes

consists of a thread of DNA tightly coiled around histone proteins

23rd pair of chromosomes

sex chromosomes

homologous pairs

consists of two homologous chromosomes, one from the mother and one from the father

sister chromatids

identical copies of a single replicated chromosome that are joined together by a common centromere

diploid cells

cells that contain two copies of each chromosome, one copy from each parent

haploid cells

cells that contain one copy of each chromosome

mitosis

a type of cell division in which a parent cell divides to produce two genetically identical daughter cells

uses of mitosis

• growth

• replacement of damaged or dead tissues

• asexual reproduction

• development of body plans - forming different parts of an organism

• production of stem cells

prophase

• chromosomes condense

• centrioles migrate to opposite poles of the cell and each centriole starts forming spindle fibres

• the nucleolus disappears and the nuclear membrane begins to break down

metaphase

• chromosomes line up in the equator of the cell

• each chromosome attaches to the spindle fibre by their centromere

• at the metaphase checkpoint, each chromosome is checked to ensure it is attached to the spindle

anaphase

• centromeres split to separate each pair of sister chromatids

• spindle fibres contract and pull chromatids to opposite poles of the cell

telophase

• the chromatids reach the opposite poles of the cell, where they uncoil

• a nuclear membrane forms around each set of chromosomes to form two nuclei and the nucleolus starts to reform

cytokinesis

the cytoplasm divides to produce two genetically identical daughter cells

meiosis

a type of cell division in which a parent cell divides to form 4 haploid cells that are genetically distinct from each other

difference between meiosis 1 and 2

in 1, the homologous chromosomes are separated, whereas in 2, the chromatids are separated

prophase 1

• chromosomes condense and homologous chromosomes pair up

• centrioles migrate to opposite poles of the cell where each centriole starts forming spindle fibres

• the nucleolus disappears and the nuclear membrane breaks down

metaphase 1

• chromosomes line up along the equator off the cell in their homologous pairs

• each chromosome attaches by their spindle fibre

anaphase 1

• homologous chromosome pairs are separated and pulled to opposite poles of the cell

telophase 1

• the chromosomes reach opposite poles of the cell, where they uncoil

• a nuclear membrane forms around each set of chromosomes and the nucleolus starts to reform

• the cytoplasm divides to form two cells

prophase 2

• chromosomes condense

• centrioles migrate to opposite poles of the cell and spindle fibres begin to form at each centriole

• nucleolus disappears and the nuclear membrane breaks down

metaphase 2

• chromosomes line up at the equator of the cell

• each chromosome attaches to the spindle fibre by their centromere

anaphase 2

• centromeres divide and separate each pair of chromatids

• the spindle fibres contract, pulling the chromatids to opposite poles of the cell

telophase 2

• the chromatids reach the opposite poles of the cell where they uncoil

• a nuclear envelope forms around each set of chromosomes to form two nuclei and the nucleolus starts to reform

• the cytoplasm divides and 4 cells are produced

roles of meiosis

• production of haploid gametes - allows sexual reproduction to take place

• creates genetic variation - this increases diversity, allowing natural selection to take place

two events that lead to genetic variation

• crossing over

• independent assortment

crossing over

• occurs during prophase 1 of meiosis - the chromatids of each chromosome twists around one another, forming a chiasmata

• when the chromosomes are separated during anaphase 1, the chromatids break at the chiasmata and then reconnect to the chromatid from the homologous chromosome, which swaps alleles

independent assortment

• occurs during metaphase 1

• pairs of homologous chromosomes line up along the cell's equator. However, whether the paternal or maternal chromosomes appears on the left or right is completely random. As a result, which chromosomes end up in each daughter cell is also random

specialised cells

cells with certain features that allow them to carry out a particular function

tissue

a group of similar cells working together to carry out a particular function

organ

a group of tissues working together to carry out a particular function

organ system

a group of organs working together to carry out a particular function

squamous epithelium

• provides thin lining for many organs such as lungs

• made of squamous epithelial cells,

• one cell thick

ciliated epithelium

• lines organs such as trachea,where it can sweep mucus away from the lungs

• made of ciliated epithelial cells and goblet cells

cartilage

• connective tissues that acts as a cushion between bones

erythrocytes

• RBC - responsible for carrying oxygen around the body

• flattened biconcave shape - increases SA

• no nucleus or organelles - provides more room for haemoglobin

• flexible - fit through narrow capillaries

neutrophils

• types of WBC that help to defend the body against pathogens

• flexible cell membrane - allows cell to engulf pathogens

• many lysosomes - contain digestive particles to break down engulfed particles

• multi-lobed nucleus - allows cell to deform to squeeze through small gaps

stem cells

undifferentiated cells that can develop into other types of cell. They are used for growth, development and tissue repair

features of stem cells

• can divide by mitosis to produce more undifferentiated cells

• stem cells can differentiate into specialised cells

totipotent stem cells

can differentiate into any type of cell and go on to form whole organisms

pluripotent stem cells

can differentiate into most cell types, but can’t form whole organisms

multipotent stem cells

can only differentiate into a few different types of cells

unipotent stem cells

can only differentiate into one type of cell

embryonic stem cells

• found in early stages of embryo development

• stem cells are first totipotent, and after 7 days they are pluripotent

adult stem cells

• found in some adult tissues

• stem cells are multipotent or unipotent

• stem cells in the bone marrow replace worn out RBC and neutrophils

plant stem cells

• found in meristem tissue or meristems at the tips of roots and shoots

• stem cells are pluripotent

uses of stem cells in research and medicine

• testing new drugs for toxicity and side effects

• studying development of organisms

• identifying causes of disorders