Ch 1: DNA Flashcards
Describe the structure of DNA
- deoxyribonucleic acid are large macromolecules composed of two long strands of nucleotide subunits that are wound around each other to form a double helix
Distinct chemical features
- 5-carbon deoxyribose sugar
- negatively charged phosphate group
- organic nitrogen containing compound (base)
- types:adenine, thymine, cytosine, guanine
- A and T bond together with 2 hydrogen bonds (purine)
- C and G bond together with 3 hydrogen bonds (pyrimidine)
What is DNA replication?
DNA replicates by a semi-conservative mechanism where one of the strands in the newly formed molecule is new and the other is the original strand
What is the process of DNA replication?
- the enzyme DNA helicase unzips the helix of double-stranded DNA by breaking the weak hydrogen bonds between the nucleotides and thus exposing the nucleotide bases
- the replication fork (junction between the unwound single strands of DNA and the intact double helix) moves along the parental strand so that there is a continuous unwinding of the parental DNA strands
- free nucleotides within the nucleus attach to their complementary base pairs with the help of the enzyme DNA polymerase. DNA ligase seals the new short stretches of nucleotides into a continuous strand that rewinds
- two double helix DNA molecules with one parental strand and one new strand are produced
Define genome and genomics
genome: the sum of all DNA in the cell of an organism genomics: study of the genomes of organisms
WHat is a chromosome?
all the cell’s DNA and associated proteins in a grainy substance called chromatin, highly condensed and wrapped around histone proteins when a eukaryotic cell divides, joined by a centromere
Define karyotype and what does it contain?
a display of the number and appearance of the chromosomes of an organism or cell observed at metaphase in humans: - 23 pairs of chromosomes (46 in total) - 22 are homologous automosomes (chromosomes that are the same in females and males of a species not including sex chromosomes) - the 23rd pair are heterosome (non-identical chromosomes pairing up at meiosis) called sex chromosomes as they determine the sex of an individual
What are features of somatic cells?
somatic or body cells are diploid, containing 23 (2n) pairs of chromosomes - one chromosome of a pair comes from the male parent and the other chromosome of a pair comes from the female parent
What are features of eukaryotic chromsomes?
- along the length of each DNA molecule, particular genes code for different proteins that can determine particular characteristic/trait - the location of a gene on a chromosome is its locus, in homologous chromosomes the corresponding gene (alleles are alternative forms of the same gene) is found at the same locus - a normal diploid organism has 2 alleles for each gene
Describe prokaryotic chromosomes
- they are generally circular and lie in direct contact with the cytoplasm, they are similar to chromosomes in mitochondria and chloroplasts.
- chromosomes are often joined to the plasma membrane at a single point
- nucleoid: the region within a prokaryotic cell that contains the genetic material also are where chromosomes can be found
- additional numerous rings of DNA (plasmids) may also be present in the cytoplasm, nonessential genes are encoded here. Plasmids can replicate independently of the main chromosome and are important tools in genetic engineering as they can be easily transferred from one bacterium to the next
- prokaryotic cells are generally haploid and contain very little repetitive and non-coding DNA. Most do not have histones
What does eukaryotic cell division involve?
a number of phases resulting in nuclear division
- mitosis - nuclear division that maintains the parental no. of chromosomes for daughter cells, it is the basis of bodily growth and asexual reproduction and
- meiosis - cellular division in which the chromosome no. of a cell is halved to the haploid no., it is the basis of gamete formation in adults and spore formation in plants) and
- cytoplasmic division (cytokinesis - division of the cytoplasm)
What is the cell cycle?
the sequence of events from one cell division to another
- M - nuclear division, stage between this is interphase
- C - cytokinesis
- G1 - active growth, cell growth before DNA replication
- G0 - cells have withdrawn from the active cell cycle, a non-proliferating state in which cells undergo an extended G1 but are not preparing to replicate DNA and divide i.e. nerve cells
- S - synthesis of DNA (replication)
- G2 - cell prepares for division
What is the process of mitosis in eukaryote cells?
Interphase
chromosomes are not visible and cannot be clearly distinguished under any microscope. Centrioles are vsisble in many animal cells. As the cell leaves interphase the chromatin threads became shorter and thicker and are now visible under a light microscope
Prophase
chromatin threads condense and become visible as double strands containing 2 chromatids linked by a centromere. A spindle forms made up of microtubules, originating from the centrioles and the necleolus disappears from view and the nuclear membrane breaks down
Metaphase
chromosomes move to the centre of the cell and line up along the equator/metaphase plate
Anaphase
spindle fibres attach to the chromatids and help to pull them apart. Chromatid separate at the centromere and move to opposite poles of the spindle as the spindle fibres contract. Chromatids are now chromosomes
Telophase
chromosomes de-condense as the chromatin unwinds and becomes less visible. A new nuclear envelope forms, nucleoli reform and the spindle disassembles
Cytokinesis
cytoplasm divides. In plant cells they divide with the formation of a cell plate. The cell wall fuse with parts of the spindle, forming the cell plate. Cellulose is deposited at this site, forming a wall that divides the parent cell into 2 daughter cells each one with a plasma membrane. In animal cells, the division is called cleavage
What is binary fission?
method by which prokaryotic cells reproduce
- involve DNA replication, chromosome segregation and cytokinesis
- leads to the production of 2 daughter cells with the same no. of chromosomes as the parental cell
- Process:
- single DNA strand is replicated - each copy attaches to a different part of the cell membrane
- when the cell begins to pull apart, the replicate and original chromosomes are separated, a wall forms across the cell and divides it into 2 cells of identical genetic composition
Describe the process of meiosis
Meiosis I
Prophase I - chromosmes condense, nucleus disappears and it spindle forms with centrioles, if present, at opposite ends. Homologous chromosomes lie side by side via synapsis. A pair of homologous chromosomes are called bivalent. The chromatids remain in contact as chiasmata Metaphase I - nuclear envelope breaks down and the homologous chromosomes move together to the equator of the spindle
Anaphase I - homologous pairs move towards opposite poles of the spindle, the disjunction occurs independently of other chromosome pairs Telophase I -the spindle breaks down, the cell starts to separate across its middle and nuclear envelopes form around the 2 new nuclei, cytokinesis completes the first stage of meiosis
Meiosis II
Prophase II - new spindle forms at right angles to the first one
Metaphase II - chromosomes move to the equator of the spindle
Anaphase II - chromatids separate and move apart from each other, the chromatids become the chromosomes of the daughter cells. When they reach the poles, the cells enter telophase II Telophase II - spindle apparatus disappears, the chromosomes de-condense and new nuclear envelopes and nucleoli form 4 haploid cells form
What is fertilisation?
male and female haploid sex cells (gametes) fuse to produce a diploid zygote, containing a different combination of DNA to that of either parent
What is apoptosis?
- a genetically programmed series of events that lead to cell death as a result of dismantling of the internal contents of the cell by various enzymes including caspases.
- the apoptotic bodies formed are cleared by phagocytes
- all cells are pre-programmed to age and die. Cell death is a vital and formative process that is essential for development, shaping organs and tissues
What can occur when apoptosis fails?
problems rannging from developmental defects to cancer can result -
- cancer occurs when apoptosis fails and cells do not die as they should, cancer is normally controlled by preventing division when damage has occurred to DNA. When the damage cannot be repaired, apoptosis is initiated and the cell is disposed of
