DNA, mitosis, stem cells Flashcards
mtDNA
Mitochondrial DNA, 5-10 mtDNA molecules in mitochondria
1% of total DNA
Found in Mitochondria
forms circular DNA molecule
37 genes, 24 for making tRNA, 13 for making enzymes necessary for cellular respiration
DNA structure
Is a polymer (molecule of many repeating units) made of nucleotides in form of strands Two strands twist into double helix 2-3m long 1 millionth of m thick Order of bases determines genetic code.
nucleotide
Small repeating units
composed of: deoxirybose, phosphate group, nitrogenous base
Sugar molecule of one nucleotide bonds phosphate group of another one
two strands join by specific nitrogenous bases being attracted to one another by weak hydrogen bonds
Up to 2 million codes
average chromosome is made of 140 million base pairs
Types of nitrogenous bases
adenine Thymine cytosol guanine Uracil (RNA)
Chromosomes
super-coiled structures
Made when cells divide
large enough to see with light microscope
46 in each human cell
Each chromosome is made up of sections of DNA that code for particular proteins
each section called a gene.
Chromatin
DNA when cell isn’t splitting
Forms tangled network in nucleus
Histone
Protein in which DNA wraps around to form nucleosome
RNA
Ribonucleic Acid
Sugar molecule is Ribose not Deoxyribose, ribose has 1 more oxygen atom
RNA is single stranded
Has uracil no thymine
Strand able to fold on itself, forming hydrogen bonds between complementary bases
types of RNA
mRNA
rRNA
tRNA
mRNA
messenger RNA
Made in nucleus and takes genetic code into cytoplasm allowing genetic code into cytoplasm allowing genetic code to be ‘read’ by ribosomes
rRNA
Ribosomal RNA
60% of ribosome mass, other is protein
Ensures correct alignment of mRNA, tRNA and ribosome
also has enzymatic bonds between amino acids
tRNA
Transfer RNA
contains 70-90 nucleotides
Each tRNA is able to carry specific amino acid, therefore plays vital role in protein synthesis
start codon
AUG
Methionine
Stop codon
UAG
Transcription
process by which genetic instructions are copied from DNA to RNA
Transcription triggered by chemical messengers entering nucleus from cytosol and bind to relevant genes on DNA
Helicase enzyme seperates strands, usually 17 bases at time
RNA Polymerase then copies on one strand to create a strand of mRNA wt identical information as parent strand.
Often many polymerase follow first to make many copies at once
template strand
Strand that is copied
coding strand
Other strand
bases on strand will be identical to mRNA (except for uracil and thymine)
Translation
production of protein using info coded in mRNA molecule
In cytosol, ribosome attaches to one end of mRNA at start codon
ribosome then moves along mRNA 3 bases at a time
As ribosome reads codons on mRNA, tRNA with complementary bases join to mRNA
amino acids carried by tRNA are joined so protein is assembled wt amino acids in correct sequence
Gene expression
process of copying info from DNA onto mRNA, then translating the message into a series of amino acids to form protein
Cell is producing mRNA based on the cell’s purpose.
cell can produce up to 150 000 proteins per second
Genes used are ‘switched on’
genes not used are ‘switched off’
Factors affecting gene expression
age of cell Time of day signals from other cells Environment of the cells whether or not cell is dividing
Lipid and carbohydrate synthesis
require no genes, however require enzymes
Enzymes are proteins
thus indirectly genes control synthesis of lipids and carbohydrates
Epigenetic
inheritance of factors that make genes more or less likely to be expressed
Changes in gene expression resulting from mechanisms other than changes in the genes
genome
Hereditary info coded in DNA
Epigenome
Sum of all factors determining where, when and which genes are expressed
therefore controls what proteins are synthesised
Epigenetic factors tell types of cells how to behave
Epigenome can change due to exposure to environment stimuli. These don’t change DNA but interfere transcription and translation
chromatin
For DNA to fit in nucleus, must be tightly coiled
when cell not dividing DNA forms tangled network
DNA and histones referred to as chromatin
Gene expression may change if way DNA is wrapped around Histone
Types of gene expression based on histone
change of amino acids in histone
Acetylation
DNA methylation
Histone Methylation
Change in amino acids in Histone
histone shape will change
Modified shape is copied when no new molecules are formed
ensures stem cells changed into specific cells remains that specific cell
Acetylation
addition of acetyl group to histone protein
Reduces attraction of DNA and histone, relaxing structure of chromatin
promotes transcription by allowing RNA polymerase access
acetylation enhances gene expression
DNA methylation
Occurs by adding methyl group to DNA or Histone
occurs where cytosine is adjacent wt guanine, known as CpG sites (cytosine-phosphorus-guanine)
Methylation prohibits gene expression by restricting access to RNA polymerase
Histone methylation
Increase/decrease transcription of genes. Depending on where methyl group attaches and how many attach
if methylation causes chromatin to relax, transcription increases
need for cell reproduction
Cells don’t grow, therefore more cells needed
new cells needed to replace old, dead, damaged cells
More wear and tear on cell, the shorter the life span
phases of cell cycle
G1 S G2 M G0
G1 phase
Cell produces new proteins, grows and carries out its normal tasks
phases finishes when DNA begins duplication
S phase
Synthesis phase
DNA molecules in nucleus form exact copies of each other
G2 phase
Second growth phase
M phase
Mitotic phase
Cell divides into two daughter cells
G0 phase
When cells leave the cell cycle and stop dividing
mitosis
Cell division
division of nucleus
Ensures each body cell receives exact same hereditary material (DNA) as parent cell
process is continuous, doesn’t occur in steps
Stages of mitosis
Prophase
Metaphase
Anaphase
Telophase
Interphase
period between cell divisions
Goes through G1, S and G2 phases
prophase
Two pairs of centrioles and opposite poles are visible, microtubules begin to radiate from them
nucleolus disappears and nuclear membrane begins to breakdown
Chromatin threads of DNA become tightly packed into chromosomes
coiling of DNA makes it easier to distribute DNA to daughter cells
Metaphase
chromatid pairs line up on the equator of the spindle
Centromere attaches to a spindle fibre
Anaphase
Each pair of chromatids separates at the centromere
chromosomes are pulled towards opposite poles of cell
Telophase
Two sets of chromosomes form tight groups at each pole
nuclear membrane forms around each group, nucleolus appears in each new nucleus
Spindle fibre disappear
chromosomes gradually uncoils to become chromatid
Cytogenesis
division of cytoplasm during telophase
Furrow develops between two nuclei, gradually deepens until it cuts cytoplasm into two parts, each with own nucleus
This along with mitosis results in 2 daughter cells
differentiation
Process by which cells become specialised
during mitosis, different genes become activated, making stem cells into specialised cells.
Specialised cells can divide therefore replaced by other means
when stem cells undergo mitosis, daughter cells might be stem cell (stem cell proliferation) or differentiate
Stem cells
cells that undergo differentiation
Not specialised for any role
capable of repeated division by mitosis
Potential to develop into any cell type means can provide unlimited source of cells for repair
stem cell types
Totipotent stem cells
pluripotent stem cells
Multipotent stem cells
totipotent stem cells
Potential to create any type of cell necessary for embryonic development
pluripotent stem cells
Can give rise to any of the cells in the body
embryonic stem cells are pluripotent
Multipotent stem cells
potential to form a number of different types of cells
Eg. Blood stem cell makes red, white and platelets
