genetics Flashcards
What is the error rate of the DNA replication?
About 1 in every 10^5 nucleotides
What is the error rate in in the synthesis of a complete DNA molecule?
1 in 10^10
What is a check point?
Regulated transition point in the cell cycle were negative signals can block progression to the next phase.
Sometimes include the signaling mechanisms that monitor cell cycle events and transmit the information to the control system.
What are the 3 main checkpoints during a cell cycle?
- ** G1 / S → the cell growth checkpoint **,
Checks if the cell is big enough, sufficient nutrient, synthesized all the proper proteins before the s phase.
Also checks for DNA damage - if there his negative “stop” signals, the cell enters resting phase (G0) until its or until it dies. - ** G2 → DNA synthesis check point **
Checks for adequate cell size and makes sure the DNA has been replicated correctly.
If everything is good - mitosis
If not - the cell goes through self destruction mechanism and dies - ** M→ the spindle assembly checkpoint **
Occurs between metaphase to anaphase transition point
Checks to make sure there is chromosomal attachment to the spindle fibers from both poles.
Cell cycle control system
Controls the timing and coordination of cell cycle events.
Nucleotides
The monomers composing the nucleic acid
* Pentose - five carbon sugar, ribose/ deoxyribose
* nitrogenous base - carbon 1
* phosphate group - carbon 5
Phosphodiester bond
A condensation reaction
Nucleotide chains are formed by this reaction
The phosphate group is linked to the sugar
The backbone has directionality
5’ end → a free phosphate group
3’ end → a free OH group
DNA
Deoxyribose nucleic acid
* Contains all the information the cell needs to function & has the instruction on how to duplicate it
* polynucleotide located in the cell’s nucleus
* contain the instructions of protein synthesis
* the genome is formed by the number of DNA molecules and their size.
Deoxyribose
his sugar group is missing an oxygen on carbon 2
What is the structure of DNA?
- Double strand molecule
- the 2 strand are connected between the nitrogenous bases hydrogen bonds by y complementary base pairing:
- adenine → thymine (2 bonds)
- guanine → cytosine (3 bonds)
Maintains a constant distance of except 3 carbon rings between the polynucleotide chains
Allows the formation of the double helix structure - anti-parallel = 5’ end will always face the 3’ end
Primary structure of DNA
The linear sequence of nucleotides forming each strand
Linked by phosphodies for bonds
Secondary structure of DNA
The anti-parallel orientation of the strands (base pairing)
Linked by hydrogen bonds
Tertiary structure of DNA
3D structure
The double helix
How many base pairs each turn in the double helix contains?
10- 10.5 base pairs
Direction of the DNA twist
Right handed - clockwise screwing motion moves the helix upward
Left handed - anti clockwise screwing motion moves the helix upward
DNA packing
Refer to the level of compression and arrangement in which the DNA is found (depends on what stage in the cell cycle the DNA is found )
- DNA molecules are always wrapped around by histones
8 histones are packed by a DNA molecule = nucleosome - Chromatin = chain composed of many nucleosomes
- Chromosome = organized packing of chromatin → seen only when the cell prepares for divisions
Why is DNA drawn to histone?
They are drawn to each other by electrostatic force
Histones are positively charged proteins
DNA is negatively charged
How many Chromosome does the human cell contains?
46
Divided to 2 sets of 23 → one set from each parent
22 are autosomes
1 sex chromosome
Sex chromosome
X or Y
Female - XX
Male - XY
Homologous chromosomes
The 23 chromosomes pair.
Each pair contains genetic information that codes for the same characteristics.
The genes themselves are different
Sister chromatid
2 identical copies of the same chromosome ( nucleotide sequence is identical)
Happens after the duplication of DNA
Each duplicated chromosome Contains 2 double stranded DNA.
The two sister chromatids are linked together is a joining structure known as the centromere
Genetic code
3 letter code →” triplet code”
# universal
# has punctuations →
3 codes act as ‘full stops’ ( mark the end of a polypeptide coding sequence.
Some act as ‘start signals’ ( code TAC for methionine )
# redundant (some amino acids are encoded by more them one triplet ).
Cell cycle
Sequence of events that leads to the reproduction of the cell.
S phase - duplication (mainly chromosomes, other organelles are replicated throughout the cell cycle )
g1,g2 - growth
M phase - equal distribution
What are the stages of eukaryotic cell cycle?
Gap phases provide additional films for cell growth & serve as regulatory transition
G1 → gap phase, cellular contents (excluding chromosomes) are duplicated.
It has a swim or drown decision
S → 46 chromosomes duplicated
G2→ “double checking” the duplicated chromosomes for error, repairing
Mitosis → nuclear division
Cytokinesis → cell division (the membrane)
# G0 → prolonged non-dividing state
# interphase → G1 + S + G2
DNA replication
The process of the creation of another identical copy of all the DNA in the cell.
- happens only when the cell is close to dividing
- semi-conservative model of replication → the original strand and the new strand are joined together to form a new DNA molecule.
- started of the ori (Origen of replication), the enzymes recognize the area and know to start breaking the hydrogen bonds there
Conservative replication
A completely new double helix is formed based on the “old” strand.
The old molecule remains completely separate from the new ones.
Semi conservative replication
Each new molecule is A combination of one old strand and one complete new.
- hydrogen bonds between the base pairs broken and this ‘unzips’ or unwinds the DNA double helix to form two single polynucleotide DNA strands that can act as template for the new strand.
Dispersive replication
Each DNA molecule contains both old and new parts that are scattered randomly throughout the polynucleotide chain.
Helices
Is an enzyme that breaks hydrogen bonds
DNA polymerase
Is an enzyme that is responsible for the new formation of the new strand.
- directionality: always synthesizes from 5’ to 3’
- add nucleotides to leading & logging DNA strands
Leading strands
→ 5’ to 3’, complementary to 3’ to 5’
→ replicated continuously
→ synthesized toward the replication fork
Legging strand
→ 5’ to 3’
→ opposite direction of the unzipping, synthesized away from the replication fork
→ not continuous
Copy a segment of unzipped DNA and then return to the beginning
→ create fragments of DNA called Okazaki fragments. They will link to one complete strand with the assistance of DNA ligase.
DNA ligase
Enzyme
# catalyzes the linkage of two neighboring nucleotides with covalent phosphodiester bonds during DNA replication to form the sugar - phosphate backbone of the newly synthesized DNA molecule
RNA primer
Short piece of RNA.
Binds to the 3’ end of the strand
Starting point of the replication, DNA polymerase cannot start without it.
DNA repair
Proofreading error-checking mechanisms.
DNA primase
- Enzyme
- generate RNA primers
What are the differences between the DNA of eukaryotic and prokaryotic?
** Location: **
# eukaryotic: nucleus + mitochondria + chloroplasts
# prokaryotic: a region in the cytoplasm called nucleoid region
** DNA packing: **
# eukaryotic: linear ( circular in the mitochondria & chloroplasts)
# prokaryotic: circular
** DNA quantity: **
# eukaryotic: a lot of DNA molecules in the nucleus. Bigger molecule, more bases
# prokaryotic: one double stranded DNA and a few plasmids
** DNA replication: **
# eukaryotic: a lot of ori’s, slower rate
# prokaryotic: one ori, faster rate, more mutations
Telomeres
The ends of the chromatids in chromosomes.
- ‘sealed’ with protective structure of the DNA
- prevent losing vital information during DNA replication
Telomerase
An enzyme that acts on the telomeres of chromosomes.
Preserve the length of chromosomes during cell division.
# retire primarily in cancer cells and reproductive cells.
Cell cycle
The sequence of events that occur in living cells and prepare the cell for division.
→ the life of the cell, from the moment its created to it’s division
Parent cell
The cell which performs the division.
Daughter cells
The new cells created from the division.
They are equal to each other in the genetic material
How long does the human cell division take?
24 hours
# 10-12 → S phase
# 11-13 → for growth ( G1 & G2)
#1 hour for the M phase
Mitosis
The process of nuclear division by which a genetically identical daughter nuclei are produced ( also identical to the parent nuclei)
Segregation of the 2 sets of chromosomes that are that are present in the cell after S phase.
What are the stages of mitosis?
- Prophase
- Metaphase
- Anaphase
- Telophase
Mitotic spindle
Pulls the sister chromatid apart and moves a complete set of chromosomes to each pole of the cell, where they are packaged into daughter nuclei.
→ microtubule fibers coming out of the centrosome, they span the length of the cell and they bind to the centromers and split the chromatids.
Kinetochore
Each chromatid carries it.
# A multiprotein complex that attaches the chromatid to microtabules connected to a spindle pole.
# proteins in the kinetochore help generate forces that drive chromosome movement.
Prophase
Chromosomes condense and become visible
# spindle fibers emerge from the controsomes
# nuclear envelope breaks down
# controsomes move toward opposite poles.
Metaphase
Chromosomes are lined up at the metaphase plate
# each sister chromatid is attached to a spindle fiber originating from opposite poles.
Anaphase
centromers split in two
# sister Shrovetide are pulled toward opposite poles
# certain spindle fibers begins to elongate the cell
Telophase
Chromosomes arrive at opposite poles and begin to condense
# nuclear envelope material surrounds each set of chromosomes
# the mitotic spindle breaks down
Cytokinesis
A cleavage furrow separates the daughter cells
Stem cell
A cell that can divide (by mitosis) an unlimited number of times.
→ every new cell has the potential to stay a stem cell or to develop info a specialized cell (blood cell or muscle cell, process called differentiation ).
RNA
Ribonucleic acid
- polymer composed of nucleotide monomers linked by phosphodiester bonds (covalent)
- single strand nucleic acid
- it’s Penrose is ribose, has oxygen on carbon 2
The central dogma
exception: reverse transcriptase ( RT ) enzyme - can take RNA and convert it to DNA (common in viruses).
“DNA makes RNA and RNA makes protein”
** DNA <—> RNA → protein **
DNA is in the nucleus, can’t leave. While, protein is synthesized in ribosomes outside of the nucleus.
RNA can move from the nucleus to the cytoplasm.
What are the 3 main types of RNA relevant for protein synthesis?
mRNA → messenger RNA, involved in the transcription of DNA
tRNA → transfer RNA, has an important role in the translation component of protein synthesis
rRNA → ribosomal RNA, crucial role in ribosome structure.
Transcription
The process by which DNA makes mRNA
Translation
The process by which the message carried by mRNA is decoded to make protein
RNA polymerase
Cars break the hydrogen bonds and synthesize the RNA molecule by the base of the DNA template (doesn’t need primer).
Build from 5’ to 3’
Promoters
Specific sequence which mark when transcription starts
Terminators
Specific sequences which mark where transcription ends.
Transcription unit
The DNA stretch transcribed to RNA
Genes
each DNA molecule contain many
# each gene encodes for a sequence of amino acid ( future protein)
#same specie will include the same genes
Not all the sequences in the DNA are genes
Alleles
Different alleles are responsible for different traits of the same character.
Same gene, different nucleotide sequence.
The process of transcription
- ** Initiation = ** the RNA polymerase recognizes the promoter, and binds to the DNA. He starts breaking the hydrogen bonds and that reveals the nucleotides.
- ** Elongation = ** extension of the molecule. RNA polymerase link nucleotides to synthesize RNA based on the DNA nucleotide sequence.
- ** Termination = ** the end of the transcription process, the RNA polymerase reached the terminator and releases itself from the DNA.
No Okazaki fragments are formed during RNA synthesis (only one strand is transcribed),
Anti-sense strand
The template strand of the DNA
Serves as a template for specific mRNA synthesis
Sense strand
The DNA strand which contain the gene sequence
Post transcriptional modifications
Before the RNA exits the nucleus it does some modifications in order to survive in the cytoplasm.
5’ cap → addition of 1 guanine nucleotide to the 5’ end.
Polyadenylation ( poly-a-tail) → addition of 50 - 250 adenine nucleotides to the 3’ end
Introns → removed from the chain and stay in the nucleus
RNA splicing
The process of removing major non-coding parts of the RNA chain.
Introns - removed and stay in the nucleus
Exons - leave the nucleus, and stay in the mRNA molecule
Ribosome
The organelle were translation is carried out (in the active site)
Composed of 2 subunits and rRNA:
Prokaryotic 70S → large- 50s, small - 30s
Eukaryotic 80s → large- 60s, small- 40s
Codon
3 nucleotides group in the mRNA
Each codon encodes for one amino acid.
64 combinations of genetic code overl
Start codon
AUG
Methionine
Stop codons
UAA →u are annoying
UAG → u are gone
UGA → u go away
tRNA
Transfer RNA
- small RNA molecule
- distinctive folded structure with 3 hairpin loops (shape: three-leafed clover)
- one hairpin loop contain a sequence called anticodon ( recognize & decode mRNA codon)
- has a region at the end where amino acid binds
- 61 kinds of tRNA
Each tRNA translates a specific mRNA codon to a specific amino acid.
The process of translation
- ** initiation ** = the ribosomes connects and reads the mRNA until it gets to the AUG (methionine), where he can start translating.
- ** elongation** = the ribosome reads on the mRNA making the polypeptide sequence with the help of the tRNA (anticodon)
- ** termination ** = the ribosome reads the ‘stop’ codon and stop coding the mRNA
