HL BIO - gene and division quiz
Goal of cell division
To pass down genetic information as well as organelles to the next generation.
Parent/mother cell:
the cell that produces the copy
Daughter cells:
the cells produced from the parent cell
Cell division in prokaryotes
Reproduce asexually using binary fission (two-splitting)
The DNA is replicated semi-conservatively. two DNA loops attach to the membrane. membrane elongates, the septum is formed and pinches off (cytokinesis), forming two separate cells. The daughter cells are genetically identical (clones).
Cell division in Eukaryotes
Mitosis: a type of cell division that produces 2 genetically identical daughter cells to the parent cells.
Meiosis: A type of cell division in sexually reproducing organisms that produces sex cells (ex. sperm and egg)
Why do cells divide
Growth: increasing cell numbers through mitosis
Asexual reproduction: some can
Tissue repair: damaged, infected or dead cells need to be replaced
Embryonic development: a fertilized egg (zygote) will undergo mitosis and differentiation in order to develop into an embryo
Interphase
80% of the time is spent in this phase
G0 - Resting phase where the cell has left the cycle and has stopped dividing. Still carry out normal functions.
G1 - Rapid growth and metabolic activity (protein synthesis)
S - DNA synthesis and replication
G2 - Centroles replicate, DNA is condensed and coiled into chromosomes, and chloroplasts & mitochondria replicate
Sister Chromatids
duplicated chromosomes attached by a centromere. After anaphase they separate and are called chromosomes.
Centromere
part of a chromosome that links sister chromosomes.
Kinetochore
a protein structure attached to the centromere where astral microtubules attach to
Astral spindle michrotubules
spindle fibres
Centrioles
organize spindle microtubules. In animal cells, two centrioles are held by a protein mass referred to as a centrosome.
Chromatin
the uncoiled form of DNA
Supercoiling of chromosomes
Chromatin (DNA associated with histones) begins to condense during G2 phase. Chromosome condenses after duplication by winding around histones (specific proteins). Nucleosomes (histone wrapped in DNA) coil in a cylindrical fibre called a solenoid, and then it supercoils into the visible structure seen under a microscope.
Why do chromosomes supercoil
Chromosomes need to be stored compactly to fit within the nuclei of cells. This problem becomes more apparent when chromosomes need to be short and compact enough to separate and move to each end of the cell during mitosis.
Mitosis Prophase
Centrosomes move to opposite poles and spindle fibres start moving and attach to kinetochore. DNA supercoils (chromatin condenses and becomes sister chromatids). Nuclear membrane disappears.
Mitosis Metaphase
spindle fibres attach to each chromatid at the centromere. this causes sister chromatids to line up along metaphase plate (centre of cell). Centrosomes at opposite poles.
Mitosis Anaphase
Motor proteins push spindle fibres to opposite directions causing centromere to split and sister chromosomes move to opposite ends.
Mitosis Telophase
chromosomes at each pole. Nuclear membrane reappears around each set of chromosomes. Nucleoli reappear. Chromosomes uncoil de-condense into chromatin (now not visible under microscope).
Cytokinesis in animals
ring of contractile protein immediately inside the plasma membrane at the equator pulls the plasma membrane inwards creating the cleavage furrow. Eventually pinched apart to form two daughter cells.
Cytokinesis in plants
membrane-enclosed vesicles migrate to centre of cell from golgi. vesicles fuse to form tubular structures that merge to form two layers of plasma membrane (cell plate). cell plate develops until it connects with the existing cell's plasma membrane. Two daughter cells now receive pectins and stuff in the lumen between them from the vesicles by exocytosis to form the middle lamella (gluing the cells together). Both daughter cells secrete cellulose to form new adjoining cell walls.
Cyclins and control of cell cycle
Cyclins are a family of proteins that go through repeated increases and decreases in concentration that control the progression of cells during the cell cycle (discovered by Timothy Hunt in '82). They bind to cyclin-dependant kinases (enzymes) and become active and attach phosphate groups to other proteins in the cell. This attachment triggers other proteins to become active and carry out tasks.
Cyclin in different cell phases
G1: tells the cell to replicate DNA
Mitotic: binds to a CDK to form a mitosis-promoting factor (MPF) and triggers chromosome condensation, fragmentation of the nuclear membrane, and spindle formation. Levels peak in metaphase and by anaphase, destruction of cyclins starts.
G0: Some cells may not divide (dormant or aging/deteriorating). cells enter G0 from G1, and dormant cells (quiescent) may re-enter G1 at a later time. Normally cells will only divide a finite amount of times, but specialized cells will often permanently enter G0 cause of differentiation (ex. neurons). Cells that cannot divide are called amitotic.
Tumours
Abnormal growth of tissue (either benign or malignant). Malignant tumours are named after part of body of primary tumour.
Metastasis
Cancer cells spreading from primary to secondary tumour(s).
Oncologist
Cancer doctor
Mutation
Change in an organism's genetic code. A mutation in the base sequence of certain genes can result in cancer. If mutation occurs in a proto-oncogene (gene that controls the cell cycle) it can become an oncogene (which overproduces a protein that will cause uncontrolled cell division, leading to cancer).
Mutagens
Agents that cause gene mutations. Not all mutations cause cancer but everything that causes a mutation has the risk of cancer. ex. carcinogens (chemicals that cause mutations), high energy radiation, short-wave UV light, some viruses.
Ras
Ras gene codes for a G protein. Normally, growth factor binds to cell surface receptor and the G protein will trigger a cascade of protein kinases (normal cell division).
If mutated, G protein can trigger kinase cascade without growth factor (uncontrolled cell division).
Tumour-suppressor genes
encode for proteins that inhibit cell division or promote apoptosis (controlled cell death). This happens if mutated DNA is being replicated. Mutations in these genes cause proteins to be deactivated or non-functional and therefore will be free to replicate (uncontrolled cell division).
p53
p53 gene codes for p53 protein which is a transcription factor which promotes the formation of another protein that will stop the cell cycle or trigger apoptosis.
A mutated p53 gene won't produce p53 and therefore cells will continue to divide that weren't supposed to.
To become a tumour-causing cell,
several mutations must occur in the same cell. The probability of this happening in a single cell is very small.
Factors (other than mutagens): greater the number of cells greater the chance of mutation. the longer the life span the greater the chance of mutation
Miotic Index
Ratio between number of cells in mitosis in a tissue and the total number of observed cells.
MI = number of cells in mitosis / total number of cells
Chemotherapy
system-wide effects beyond primary tumour. given through intravenous injection or orally. kills cancer cells or shrinks/destroys tumour. Can impact other healthy tissues. side effects: hair loss, mouth sores, nausea, fatigure.
Radiation
High-energy invisible light waves damage cancer cells, causing tumours to shrink. Can kill cancer cells for weeks to months after initial treatment. Can be given through injection into the tumour (internal radiation) or a beam outside the body (external beam radiation). side effects: nausea, mouth sores, throat problems, fatigue
Sexual reproduction
joining of gametes (male and female reproductive cells) is called fertilization, and the cell produced is called a zygote. Gametes are haploid n (half the number of chromosomes as parent cells). After fertilization, zygote cell is diploid with total of 2n chromosomes. diploid number of humans is 46.
Meiosis
Reduction of the nucleus to form haploid gametes
1. Genetic Reduction
2. Genetic Rcombiation
Genetic Reduction
produces haploid daughter cells with half the number of chromosomes of the diploid parent cell
Genetic Recombination
gametes have different combinations of alleles and therefore offspring are genetically different from their parents. Increases genetic variation
Meiosis 1 vs Meiosis 2
1: Seperates the homologous pairs of chromosomes into sister chromatids. Reduction division (2n -> n)
2: Separates the sister chromatids
Meiosis Interphase
S-phase of interphase, DNA replication happens. chromosomes and their copies are attached to each other at the centromere (sister chromatids). After S-phase, more growth takes place to get ready for meiosis.
Meiosis Prophase 1
the homologous chromosomes associate with each other to form bivalents or tetrad (4 chromosomes) in a process called synopsis. DNA supercoils, centrioles migrate, nuclear membrane dissolves, crossing over between non sister chromatids can take place and this results in recombination of alleles (which is a source of genetic variation)
Crossing Over
DNA of one maternal and one paternal non-sister chromatids break at the same place then join together, making the homologous segments trade places and create hybrid chromosomes (called recombinant chromosomes) with new combinations of maternal and parental genes. The point of crossing over is called the chiasma which should appear the central space.
Meiosis Metaphase 1
Bivalents (2 sister chromatids) line up at the equator. Random orientation occurs where each bivalent aligns independently hence the daughter nuclei get a different mix of chromosomes. 2^n possible orientations in metaphase 1 and 2 (genetic variation). Alleles have 50% chance of moving to a particular pole and the direction bivalent aligns don't affect other bivalent alignments. Allele combinations always equally possible.
Meiosis Anaphase 1
Homologous pairs are separated and pulled to opposite poles. spindle fibres contract. Reduction division: bivalent split and half the chromosomes move to each pole.
Meiosis Telophase 1
Nuclei are now haploid and not diploid and each contains one pair of sister chromatids for each of the species' chromosomes. Cytoplasm begins to divide. New nuclei form. Chromosomes decondense. If crossing-over happened then the sister chromatids wont be exact copies.
Meiosis Prophase 2
Nuclear membrane dissolves, chromosomes condense, no crossing over, centrioles move to opposite poles perpendicular to previous
Meiosis Metaphase 2
Pairs of sister chromatids align at the equator. spindle fibres attach at centromeres. random orientation again but not as much as metaphase 1 because there is only a difference between chromatids where crossing over takes place.
Meiosis Anaphase 2
The sister chromatids are separated and now chromosomes are pulled to opposing poles. spindle fibres contacts and centromeres are split.
Meiosis Telophase 2
Four new haploid nuclei are formed. nuclear membrane reforms. chromosomes decondense. cytokinesis begins, dividing the cells. The end result is four haploid gamete cells. fertilization of these cells will produce a diploid zygote. due to crossing over, the four nuclei are likely to be genetically different.
Mendel's Law of Independent Assortment
The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present.
This only holds true for unlinked genes (genes on different chromosomes)!
Random Orientation vs Independent Association
Random: Refers to the behaviour of homologous pairs of chromosomes or pairs of sister chromatids in meiosis.
Independent: Refers to the behaviour of alleles of unlinked genes as a result of gamete production (meiosis). Due to the random orientation of the chromosomes, the alleles of these unlinked genes have become independently assorted into the gametes.
Spermatogenesis
The formation of sperm gametes. Follows meiosis perfectly and continues until death. 1 spermatogonium creates 4 haploid cells.
Oogenesis
Formation of egg gametes. Unequal division of the cytoplasm which creates something called a polar body which will eventually degenerate (happens in both meiosis 1 and 2). 1 oogonium creates 1 haploid cell.
Non-Disjunction and Down Syndrome
Non-Disjunction: failure of homologous chromosome pairs or sister chromatids to separate during meiosis. Both chromosomes move to the same pole. Produces gametes that have too many/few chromosomes. Monosomy is one less and Trisomy is one more. Klinefelter's syndrome is caused by XXY. Turner's syndrome is caused by only one X.
Karyograms
Diagram or photograph of the chromosomes present in a nucleus arranged in homologous pairs of decreasing length.
Nature of Science
The risk of having a trisomy like down syndrome increases in older mothers. Prenatal tests are advisable. Amniocentesis or chorionic villus samples can be taken and a karyotype can be made.
Gene Mutation
change to the base sequence of a gene and may occur spontaneously as a result of errors in DNA replication or caused by mutagens.
Substitution
One base in the coding sequence of a gene is replaced by a different base (ex. mispairing of G to T). This is the most significant type of mutation as this can result in the change of a single amino acid in the polypeptide.
Insertion
A nucleotide is inserted so there is an extra base in the sequence of the gene. Requires a break to be made in the sugar-phosphate backbone of the DNA molecule (major change)
Deletion
A nucleotide is removed so there is one less base and this requires two breaks in the sugar-phosphate backbone. The removal of a base/nucleotide is also called point mutation.
Mutations in gametes
can be passed down to offspring
Same-sense mutation (silent)
No effect on the amino acid produced because of the redundancy of the genetic code.
Mid-sense mutation
change in base sequence caused one different amino acid to be produced (e.g. sickle cell anemia). May not have much effect if the new amino acid has a similar structure and chemical properties.
Nonsense mutation
change in base sequence has caused a STOP codon to be produced, so the polypeptide produced is shortened (e.g. cystic fibrosis)
consequences of base substitutions
usually negative or neutral. no effect in non-coding DNA.
same-sense: no affect on polypeptide but allow for a second mutation to change the codon into one for a different amino acid.
mis/non-sense: severe or even lethal effects.
Single-nucleotide polymorphisms (SNPs)
occur in noncoding regions of DNA. can be associated with some diseases. this correlation allows scientists to look for SNPs to figure out if someone is prone to developing a disease.
frameshift mutations
change the reading frame for every codon from the mutation onwards in the direction of transcription and translation. (ex. minor insertions and deletions)
consequences of insertions and deletions
minor: can result in total loss of function. they are frameshift mutations
major: almost always result in the code for polypeptides to stop functioning
of multiples of three nucleotides: not frameshift but still severe consequences cuz more than one amino acid will be added/removed. changes to the structure of protein therefore affecting function.
BRCA1
BRCA1 gene codes for BRCA1 protein in humans. it is a tumour suppressor gene but its actual function is DNA repair. if it mutates there is an increased risk of other mutations due to the lack of DNA repair. signs: tumour formation/cancer in breasts, ovaries, and prostate. 20k alleles.
HTT
Huntington's disease causes severe nerve damage but doesn't show symptoms until you're about 40. cause of this disease is when HTT gene (found on chromosome 4) has an insertion mutation. CAG are added to gene (trinucleotide repeat expansion). mHtt (resulting mutated protein) causes the symptoms of the disease. the CAG trinucleotide repeats the code for glutamine so there is more of it in the protein than normal. The more repeats, the more severe symptoms (more than 40, then they are affected).
CCR5
Leukocytes (white blood cells) attack infections. Chemokines are the chemical signals for the leukocytes. there are proteins on the leukocytes that act as receptors to pick up chemokines. Co-receptor molecule called C-C chemokine receptor type 5 (CCR5) helps form the receptor. HIV-1 uses these receptors as an entry point to infect leukocytes called CD4 cells. those who have a working set of CCR5 on chromosome 3 are at risk if exposed to HIV-1. HIV-positive people cant fight off infections because their leukocytes are destroyed by the virus (AIDS). Delta 32 mutation is a deletion mutation (32 nucleotides removed) of the CCR5 receptor gene. 32 isn't a multiple of 3, so this causes a frameshift and a stop codon. ribosomes will read stop codon and stop making the protein (cannot produce functioning CCR5 doorway that HIV needs to infect leukocytes). This mutation is beneficial for prevention of HIV-1, but can make you more susceptible to other types of infection (like West Nile Virus).
Causes of gene mutation
mutation risk is increased with the help of mutagens:
1. Ionizing radiation increases the mutation rate
2. Chemical substances can cause changes in DNA
mutations can be random though and can happen during DNA replication because of errors that aren't corrected (rare).
Randomness in mutation
can be unpredictable. cannot be directed to intended outcome. mutations can occur anywhere in the base. the position of the base in a genome also affects the change of mutation. more than one copy in a gene sequence increases chances of mutation. uncoiled DNA also increases chances. satellite DNA is a noncoding DNA with really high mutation rates. mutation hotspots are places where cytosine is followed by guanine (CpG sites).
consequences of mutations in germ cells
rise to gametes. genes can be passed to offspring. a new allele produced in a male's sperm could contribute that mutation to the zygote of a child. if a female child has inherited the mutation, the germ cells in her ovaries contain the mutation and can pass it down to the next generation.
consequences of mutations in somatic cells
won't be passed down since it's eliminated when a person dies. mutations in these cells are associated with cancer and tumours. genes that regulate cell division are called proto-oncogenes because mutations can change them into oncogenes, which are cancer-causing genes.
mutations as a source of genetic variation
mutation changes one allele into another. mutation is the original source of genetic variation because mutations increase the number of alleles of genes in a population.
Lactose intolerance (mutation as a source of genetic variation)
early humans only drank milk as infants. by the time they reached adulthood, their bodies stopped being able to break down lactose. still true for most people today. as time progressed, we started harvesting and consuming milk/dairy products on a daily basis. dairy-based agriculture societies show a higher amount of the genetic code that allows us to digest lactose. this adaptation is an advantage that increases survivability.
Allele
a variant of a gene
a pair of homologous chromosomes have the same
pattern of banding