Cell Cycle and Cell Division Prokaryotic Cells Bacteria

Cell Cycle and Cell Division

Prokaryotic Cells - Bacteria l l E. coli is a bacteria that resides in our colon and can be beneficial to us – our major source of vitamin K that is necessary for proper coagulation Have only 1 copy of their DNA, considered to be haploid, any change in the sequence may be readily seen in the phenotype – observable characteristics

Binary Fission l l l E coli can grow and duplicate every 20 minutes DNA is attached to the cell membrane and after it duplicates, it separates as the cell enlarges When cell is double in size, the cell divides by adding a plasma membrane and cell wall between the 2 cells

Escherichia coli Genome l l DNA is circular and encodes ~ 3000 proteins Bacteria with unusual growth properties have been identified – contain mutations that changed the phenotype l l Rifampicin growth because of mutation in the RNA polymerase that allows for m. RNA synthesis while in presence of the drug Bacteria can pass DNA back and forth between one another l Mixed 2 mutant types together and found that bacteria can grow on media without either amino acid

Gene Transfer Bacterial mating or conjugation – can be achieved in bacteria that contain plasmids – small, circular ds. DNA that is separate from rest of chromosome l Plasmids can replicate independently from the bacterial chromosome l F plasmid or fertility plasmid allows for mating and gene transfer l

Bacterial Mating l l l Bacteria with F plasmid (donor) will encounter a bacteria without the F plasmid (recipient) and establish a cytoplasmic bridge to pass on the F plasmid Now both are donor bacteria Method of passing on antibiotic resistance

Integration l The plasmid can integrate into the chromosome and when bacteria conjugate they can pass on chromosomal genes l Increases genetic variation

Transformation l l Bacteria can pick up DNA from surroundings Take advantage of this in molecular biology labs to replicate DNA fragments of choice

DNA in the Eukaryotic Cell A cell’s DNA is divided into a set of chromosomes – each being a long linear DNA associated with proteins to help fold into chromatin l Also associated with proteins involved in gene expression, DNA replication and DNA repair l

Chromosomes l With exception of the sex chromosomes (X and Y) humans have 2 similar copies of each chromosome l l l one from our mother (♀) one from our father (♂) Called homologous chromosomes One instance of non-homologous chromosomes are males because of their X and Y Chromosomes are similar but not identical

Chromosomal Bands l l Set of human chromosomes is called a karyotype The staining patterns can allow geneticists to identify areas of abnormalities

Types of Chromosomes l l Mitotic chromosomes are those that are visible during mitosis – condensed Interphase chromosomes are loose and string -like - chromatin

Components of Chromosomes l 3 important DNA sequences needed for accurate DNA copying, complete genome to new daughter cells l DNA replication origin – a place to start DNA replication, multiple sites l Centromere – a place to attach the mitotic spindle for chromosome separation l Telomere – at end of linear DNA to prevent the chromosome from getting shorter in every round of replication

Important Parts During Cell Cycle

Telomeres l l Telomerase is the enzyme responsible for adding DNA sequence to the end of the chromosome to act as a template to allow complete DNA replication Also protects the DNA from degradation

Chromatin vs Chromosome l l Chromatin is a complex of protein and DNA – very dispersed Chromosome is the compact form of DNA so that it is protected during mitosis

Nucleosomes l l Nucleosome – “bead-on-a-string” look to DNA The string is the DNA and the bead is the nucleosome core particle that the DNA is wrapped around – the protein is histone 50 nm

Histone Octmer Nucleosome is the histone proteins and the adjacent linker DNA – threads the histones together l Histone is core around which the DNA coils l Disc shaped with 2 copies of H 2 A, H 2 B, H 3 and H 4 l

l l l Proteins with a large proportion of positively charged amino acids (Lys and Arg) Helps to bind the negatively charged DNA 5 types in 2 main groups l Nucleosomal histones Small and coil DNA into nucleosomes l H 2 A, H 2 B, H 3 and H 4 l H 3 and H 4 highly conserved l l H 1 histones l Larger and less conserved Histones

Interphase Chromosomes l l While not as condensed as mitotic chromosomes, there areas more tightly packed than others Depends on the genes that are being expressed Most highly condensed chromatin is called heterochromatin The more extended chromatin is called euchromatin and is either being transcribed or easily available for transcription

X Chromosome Females have 2 X chromosomes and males have 1 X and 1 Y chromosome l One X becomes inactive and highly condensed – Barr body l Some cells have the paternal X off and some have the maternal X off l l This happens during early development and once off, all cells that arise from that cell will have it off l This is then passed down (inherited) in all the cells that arise from that cell

Barr Body Formation

Tortoise Shell and Calico Cats

Cytoskeleton’s Role l l Mitotic spindle – microtubules used to separate the chromosomes – formation starts in late G 2 Contractile ring – myosin and actin filaments used to separate the 2 daughter cells – formation starts in M phase

Organelles Fragment Chromosomes divide as discussed in Chapter 6 l Mitochondria and chloroplasts will divide and double their numbers l Membrane bound organelles fragment to increase the likelihood to be divided evenly between 2 daughter cells l

Cell Cycle The orderly sequence of events which a cell duplicates its contents (DNA and organelles) and then divides in 2 l Fundamental task is to copy and pass on its genetic information to daughter cells l

Cell Cycle l New cells need to get a copy of the entire genome in the process – most cells double their mass as well

4 Phases of the Cell Cycle l Mitosis or M phase has 2 steps l l Nucleus divides – mitosis Cell divides in 2 cytokinesis

4 Phases of the Cell Cycle l Period between mitosis is called interphase – has 3 phases l l l Synthesis (S) phase is when DNA is replicated Gap 1 (G 1) phase is phase between M and S phase Gap 2 (G 2) phase is phase between S and M phase

G 1 and G 2 Phases l l l Portion of the cell cycle that allows for the synthesis of proteins, lipids and other molecules needed for cell division Duplicates the organelles and keeps the cells from getting smaller on each division At end of G 2, the DNA condenses into chromosomes from its normal chromatin structure

5 Stages of Mitosis l Continuous event but divided into 5 stages l l l Prophase – chromosomes condenses and mitotic spindle forms outside the nucleus Prometaphase – nuclear membrane breaks down, attach mitotic spindle to chromosomes Metaphase – mitotic spindle lines up chromosomes along the equator of the cell Anaphase – sister chromatids separate and move to opposite ends Telophase – nuclear envelope reassembles and cell is ready for cytokinesis

Prophase Centrosome is duplicated in late S phase and as prophase starts they separate and move to opposite poles of the cell using motor proteins and ATP hydrolysis l Each centrosome organizes its microtubules which then interact to form the mitotic spindle which undergoes dynamic instability l

Spindle Poles l l During prophase some of the microtubules become stabilized and form the mitotic spindle Some microtubules interact and are called polar microtubules and the centrosome is called the spindle pole

Prometaphas e l l Starts with the break up of the nuclear envelope because of the disassembly of the intermediate filaments in the nuclear lamina Spindle microtubules will bind to the chromosomes at a structure called the kinetochore which form on chromosomes during late prophase on the centromere

Kinetochore l l l Kinetochore protein assembles on centromere Microtubules probe into the area and when encounters a kinetochore will attach – now called the kinetochore microtubule A microtubule from each pole will bind to the chromosome l Humans bind 20 to 40 microtubules per kinetochore

3 Classes of Microtubules

Metaphase l l Chromosomes line up at the metaphase plate that is halfway between spindle poles Chromosomes are constantly under tension from both kinetochore microtubules

Anaphase l l The connection between chromatids is cut by proteolytic enzymes – now called daughter chromosome – pulled to the spindle pole Process of chromosome segregation

Anaphase A and B l l Anaphase A – separate the chromosomes to the spindle pole Anaphase B – the spindle poles separate further separating the chromosomes

Telophase l l l Nuclear envelope reforms around each group of chromosomes Nuclear lamina reforms Chromosomes de-condense – return to chromatin

Cytokinesis Separation of the new nuclei and cytoplasmic organelles between 2 new daughter cells l Starts in anaphase but is not complete until after 2 new nuclei are formed l Caused by the contractile ring of actin and myosin l

Cleavage Furrow l l l Evidence of cytokinesis is by the presence of the cleavage furrow Dependent on the location of the mitotic spindle Usually forms around the equator of the cell exception is during early development

Contractile Ring l l l Array of actin and myosin filaments that assembles during anaphase Force to divide cells comes from the actin moving over the myosin getting continuously smaller Disassembles when cell is divided into 2

Meiosis l l l Process of getting a cell with half the number of chromosomes – cell would be haploid ‘Normal’ cells would have a full set of chromosomes from the mom and the dad and are called diploid Fertilization will return the haploid sex cell to a diploid cell by uniting sperm and egg

Meiosis l l Germ cells or gamates (in plants they may be spores) contain one set of chromosomes Egg is large and nonmotile Sperm is small and motile Diploid cells formed by fertilization

Meiosis Involves the duplication of the chromosomes and then 2 successive cell divisions to yield the haploid cells l Can get genetic recombination between the chromosomes prior to the first cell division since they are in such close proximity – allows for new phenotypes l

Mitosis vs Meiosis l In mitosis all 46 chromosomes line up along the metaphase plate and 1 copy of each gets taken to the new daughter cell l l Each cell gets both ♀ and ♂ (46 chromosomes) In meiosis the chromosomes find their homologous pair and these line up at the metaphase plate – get parental chromosome shuffling during the division l l First division the cell gets either the ♀ or ♂ chromosome but not both (23 sister chromatids) Second division the cell gets a single copy of the chromosome (23 chromosomes)

Difference #1 l l Mitosis – chromosomes line up in the metaphase plate Meiosis – homologous pairs line up together

Reassortment

Genetic Recombination DNA can undergo rearrangements, caused by genetic recombination l General recombination – between any pair of homologous DNA sequences l l 2 copies of same chromosome l Crossing over of chromosomes during meiosis l No nucleotides get altered – no gain or loss of nucleotides at the cross over point

Genetic Recombination

Holliday Junction l The Holliday junction is the area of cross-over that occurs during recombination

Difference #2 ⋇ l l Mitosis – has only one division Meiosis – has 2 divisions l l First division is to separate homologous pairs Second division is to separate sister chromatids

Bivalent Chromosomes l l l Homologous pairs form a structure called bivalent – consists of 4 chromatids Genetic recombination can occur at this time – mixing of the ♀ and ♂ genes by crossing over – one arm or portion of arm can be exchanged due to close proximity Allows for offspring having a novel assortment of genes

Remainder of Meiosis I l Very similar to mitosis from this point on l Nuclear membrane disassembles l Mitotic spindle attaches to the bivalent l Line up on metaphase plate l Homologs separate l Nuclear membrane reforms and cells divide

Daughter Cells from Meiosis I Differ from diploid cells l They have 46 chromosomes but both copies come from the same parent with the exception of whethere was any crossing over l Inherited as if a single chromosome l

Meiosis II No DNA replication in this step and no significant interphase l Follows like mitosis but the new daughter cell have only 1 copy of each chromosome – total of 23 l

Meiosis

Nondisjunction l l Occasionally homologs do NOT separate during meiosis This causes one of the haploid cells to have an extra copy of the chromosome and the other cell to have no copy Upon fertilization, one embryo will have 3 copies and the other would have 1 copy (from the ‘normal’ parent) Cause of Down’s syndrome – they have 3 copies of chromosome 21

Gene Expression l l l Hard to believe that a lymphocyte and a neuron contain the same DNA All cells contain the entire genome Differences in cells arises from what genes are expressed where

Gene Regulation l l Differentiation occurs because various cell types have different genes being expressed In the frog, you can take the nucleus from an adult cell and put it in an egg and still get a tadpole