2 Flashcards
Central Dogma of Molecular Biology
A theory stating that genetic information flows only in one direction, from DNA to RNA to protein, or RNA directly to protein.
Chromosome
A single long DNA double helix that is packaged with proteins. The single DNA double helix in a chromosome can have hundreds or thousands of genes.
Packaging
Provides stability and organization
Prokaryotic Chromosomes
Has a single, circular chromosome. A ring of DNA is packaged with certain proteins into a coiled structure. It’s located in the nucleoid and in the cytosol.
Eukaryotic chromosomes
A single long, linear DNA double helix, “packaged” with proteins into a “noodles” chromatin fiber. Usually has multiple linear chromosomes in its nucleus. At the beginning of cell division the chromosomes condense into more compact structures.
What do Eukaryotic cells use chromosomes for?
The cell is actively expressing genes contained in the chromosomes’ DNA to make the proteins (and functional RNA molecules) needed for the cell to function.
Allele
Different variation of a gene, can lead to different variations of a trait
Gene
Specific stretch of DNA, affects traits
How can genes affect an organism’s traits?
Genes affect traits through the actions of the proteins that they encode. The order of nucleotides in a gene determines the order of amino acids in a protein. A protein’s amino acids determines its structure and its function.
Diploid Cells
Have 2 of each kind of chromosome
Haploid Cells
Have 1 of each kind of chromosome
Homologous Chromosomes
Two chromosomes of the same type, they contain the same genes but not always the same alleles
Sister Chromatids
They are two identical copies that result from replicating a chromosome.
Mitotic Cell Division
When a cell divides to make two genetically-identical copies of itself
What is mitotic cell division good for?
- Asexual reproduction of single-called eukaryotic organisms
- Growth and development
- Replacement
- Wound healing and regeneration
Mitosis
the process that divides up the DNA evenly, happens after a cell has replicated its DNA so each new cell gets a complete copy of the original cell’s DNA
Mitotic cell division
When a cell divides to make two genetically-identical copies of itself
What does mitosis do to sister chromatids?
Mitosis splits up sister chromatids so that each new cell will have one complete copy of the original cell’s genetic information (its DNA).
What are the stages of mitosis?
Prophase, prometaphase, metaphase, anaphase, telophase
Mitosis: Prophase
Chromosomes condense. Centrosomes radiate microtubules and migrate to opposite poles.
Mitosis: Prometaphase
Microtubules of the mitotic spindle attach to chromosomes
Mitosis: Metaphase
Chromosomes align in center of cell
Mitosis: Anaphase
Sister chromatids (which become individual chromosomes when the centromere splits) separate and travel to opposite poles
Mitosis: Telophase
Nuclear envelope re-forms and chromosomes decondense
Meiotic Cell Division
A form of cell division that includes only one round of DNA replication but two rounds of nuclear division; meiotic cell division makes sexual reproduction possible.
Gametes
A reproductive haploid cell; gametes fuse to form a diploid zygote. In many species There are two types of gametes: eggs in females, sperm in males
How are gametes produced?
They are produced by a form of cell division called meiotic cell division.
What does meiotic cell division result in?
It results in daughter cells with half the number of chromosomes as the parent cell.
What does meiosis consist of?
It consists of one round of DNA synthesis and two rounds of cell division.
What are the differences between meiotic cell division and mitotic cell division?
- Meiotic cell division results in four daughter cells instead of two
- Each of the four daughter cells produced by meiotic cell division contains half the number of chromosomes as the parent cell instead of the same number (“meiosis” is from the Greek for “diminish” or “lessen.”)
- Each of the four daughter cells produced by meiotic cell division is genetically unique instead of genetically identical. Basically, they are genetically different from each other and the parental cell
What are the two successive cell divisions of meiotic cell division?
Meiosis I and meiosis II, they occur one after the other. Each cell division results in two cells, so that by the end of meiotic cell division a single parent cell has produced four daughter cells.
Meiosis I
Reductional division; the first stage of meiotic cell division, in which the number of chromosomes is halved from 2n to n
Meiosis II
Equational division; the second stage of meiotic cell division, in which the number of chromosomes is unchanged
What happens to homologous chromosomes during meiosis I?
Homologous chromosomes separate from each other, reducing the total number of chromosomes by half.
What happens to sister chromatids during meiosis II?
Sister chromatids separate, as in mitosis
Meiosis I: Prophase I
Chromosomes condense and homologous chromosomes pair.
Synapsis
The aligned and precise pairing of homologous chromosomes in prophase I of meiosis.
What happens during the process of crossing over between DNA molecules?
Homologous chromosomes of maternal origin and paternal origin exchange DNA segments. It results in exchange of genetic material.
What is the result from crossing over DNA molecules?
The recombinant chromatids are those that carry partly parental and partly maternal segments. By creating new combinations of genes in a chromosome, crossing over increases genetic diversity.
Chiasma, plural “Chiasmata”
A crosslike structure within a bivalent (paired homologous chromosomes) constituting a physical manifestation of crossing over.
What happens at the end of prophase I?
The chromosomes are fully condensed and have formed chiasmata, the nuclear envelope has begun to disappear, and the meiotic spindle is forming.
Meiosis I: Prometaphase I
The nuclear envelope breaks down and the meiotic spindles attach to kinetochores on chromosomes.
Meiosis I: Metaphase I
Homologous pairs line up in center of cell, with bivalents (pair of homologous chromosomes) oriented randomly with respect to each other. The random alignment of maternal and paternal chromosomes in metaphase I further increases genetic diversity in the product of meiosis.
Meiosis I: Anaphase I
The two homologous chromosomes of each bivalent separate as they are pulled in opposite directions. The centromeres do not split and the two chromatids that make up each chromosome remain together.
How is meiosis I: anaphase I different from anaphase of mitosis?
In mitosis the centromeres split and each pair of chromatids split
Centromere
A constricted region of a chromosome and plays a key role in helping the cell divide up its DNA during division (mitosis and Meiosis). More specifically, it’s the region where the cell’s spindle fibers attach.
How does metaphase I: anaphase I end?
It ends as the chromosomes arrive at the poles of the cell. Only one of the two homologous chromosomes goes to each pole, so in human cells there are 23 chromosomes (the haploid, the single set, number of chromosomes)
What is meiosis I sometimes called?
It’s sometimes called the reductional division because it reduces the number of chromosomes in daughter cells by half.
Meiosis I : Telophase I and Cytokinesis
The chromosomes may uncoil slightly and a nuclear envelope briefly reappears. The process of cytokinesis divides the cytoplasm, producing two separate cells. The chromosomes do not completely decondense so telophase I blends into the first stage of the second meiotic division.
Is there any DNA synthesis between the two meiotic divisions?
There are no DNA synthesis between the two meiotic divisions.
What does the second meiotic division resemble?
It resembles mitosis
Meiosis II : Prophase II
The nuclei have the haploid number of chromosomes. The chromosomes recondense to their maximum extent. Toward the end, the nuclear envelope begins to disappear and the spindle begins to be set up.
Meiosis II : Prometaphase II
Spindles attach to kinetochores, the chromosomes line up so that their centromeres lie on an imaginary plane cutting across the spindle.
Meiosis II : Anaphase II
The centromere of each chromosome splits. The separated chromatids, now each regarded as a full-fledged chromosome, are pulled toward opposite poles of the spindle.
Meiosis II : Telophase II
The chromosomes uncoil and become decondensed and a nuclear envelope re-forms around each set of chromosomes. The nucleus of each cell now has the haploid number of chromosomes. It’s followed by the division of the cytoplasm in many species
Why is meiosis II also called the equational division?
The cells at the beginning and at the end of the process have the same number of chromosomes.
Comparison of mitosis and meiosis
During meiosis I, maternal and parental homologous separate from each other, whereas during meiosis II, sister chromatids separate from each other which is similar to mitosis. Mitosis occurs in all eukaryotes. Meiosis is present in most, but not all eukaryotes.
Somatic cells
All body cells besides sex cells
Germline cells
Sex cells (eggs and sperm)
How can meiosis increase genetic variation?
By recombination and independent assortment, each gamete contains a different set of DNA. This produces a unique combination of genes in the resulting zygote.
Recombination (crossing over)
Occurs during prophase I.
It refers to the exchange of DNA between paired homologous chromosomes (one from each parent) that occurs during the development of egg and sperm cells (meiosis). This process results in new combinations of alleles in the gametes (egg or sperm) formed, which ensures genomic variation in any offspring produced. The process occurs when two chromosomes (one from mother and one from father) line up and parts of the chromosome can be switched. The chromosomes contain the same genes, but may have different forms of the genes (also called alleles). The mother’s form of a gene could be moved to the father’s chromosome, and vice versa. Different combinations of different gene forms (alleles) are then potentially passed down to offspring. This genetic variation helps increase the diversity of a species.
In simpler term,
Homologous chromosomes - 1 inherited from each parent - pair along their lengths, gene by gene. Breaks occur along the chromosomes, and they rejoin, trading some of their genes. The chromosomes now have genes in a unique combination.
Independent assortment
The process where the chromosomes move randomly to separate poles during meiosis. A gamete will end up with 23 chromosomes after meiosis, but independent assortment means that each gamete will have 1 of many different combinations of chromosomes.
What does the reshuffling of genes into unique combinations increase?
It increases the genetic variation in a population and explains the variation we see between siblings with the same parents.
Nondisjunction
Occurs when chromosome fail to segregate during meiosis; when this happens, gametes with an abnormal number of chromosomes are produced.
What is the nondisjunction of meiosis 1 and 2?
Meiosis 1- when homologous chromosomes fail to separate
Meiosis 2- when sister chromatids fail to separate
Aneuploidy
The presence of an abnormal number of chromosomes in a cell, when the number of chromosomes don’t equal 46. It occurs during meiosis when pairs of chromosomes don’t complete the process of cell division and fail to separate.
What causes aneuploidy?
Most result from nondisjunction (when chromosomes fail to separate) of chromosomes during meiosis. It originates from oocytes (the egg)
Genotype
The underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism
Homozygous
When the two alleles match, homo = same
Heterozygous
When the two alleles don’t match, hetero = different
How do sexually-reproducing diploids make gametes?
They use meiosis to divide up their chromosomes to make gametes.
Haploid Gamete
Has one of each kind of chromosome
Which of the two homologous chromosomes go into a gamete?
Which of the two chromosomes from each homologous pair that goes into a particular gamete is random. Reminder: the chromosomes carry the alleles
Relation of meiosis to inheritance of alleles from a parent
Crossing over and law of segregation. During meiosis, each parent’s two copies of each allele are separated from each other, then the gamete receives one copy of each allele from each parent (for a total of two alleles).
For an individual with a known genotype for a gene, predict possible alleles of that gene in the individual’s gametes.
If Ana has one A allele and one B allele then she will have the probability of 1/2 (50% chance) that one of Ana’s gametes (eggs) will have the A allele for the ABO gene, and a probability of 1/2 that one of her gametes will have the B allele for the ABO gene.
If two individuals of a known genotype for a gene have an offspring, what would the probability that their offspring will have a given genotype?
Olivia has the genotype AB and Kadeem has the genotype BB. What is the probability that their child will have type B blood? You need to combine two known probabilities. It’s an independent event because they don’t affect each other. Multiply their individual probabilities:
Probability Olivia will give child allele B and Kadeem will give their child allele B = probability that Olivia will give child allele B x Probability that Kadeem will give child allele B
(Olivia) 1/2 x (Kadeem) 1 = (overall) 1/2
Classic Mendelian Trait
Traits that are passed down by dominant and recessive alleles of one gene.
Mendelian inheritance
The inheritance of traits controlled by a single gene with two alleles (recessive and dominant), one of which may be completely dominate to the other.
Recessive Trait
Only occurs (happens) when an individual only has alleles associated with that trait for the gene affecting that trait
Dominant Trait
Only occurs (happens) as long as an individual has at least one of the alleles associated with that trait for the gene affecting that trait
Connect dominant and recessive patterns of inheritance to molecular mechanisms and
central dogma.
Dominant and recessive patterns of inheritance relate to central dogma because central dogma is when the DNA contains the information needed to produce all of our proteins and RNA is a messenger that Carrie’s this information to the ribosomes to create the proteins, it explains the process of gene expression. The molecular mechanisms behind a recessive or dominant phenotype is:
1. For a dominant allele produces a dominant phenotype in individuals who have one copy of the allele, which can come from just one parent.
2. For a recessive allele to produce a recessive phenotype, the individual must have two copies, one from each parent
Molecular mechanisms
A “system” or interconnected network of biochemical interactions that provide a logic circuit as to how cells process a signal to decide on an output.
Sex-linked Trait
Refers to characteristics (or traits) that are influences by genes carried on the sex chromosomes. The twenty-third chromosome is the sex determining chromosome.
Sex-linked trait connected to patterns of inheritance to molecular mechanisms and central dogma
Female: XX Male: XY
Female can only pass down and X, Male can pass down a X or a Y
Sex depends on whether or not they get two Xs or one X and one Y.
They have a 50/50 percent chance of being female or male. The male sperm determines the gender. The gamete of sperm is either X or Y.
Patterns of inheritance- fathers can’t pass X-linked traits to their sons, only able to pass on X chromosomes traits to their daughters and Y chromosomes traits to their sons. Mothers can pass x-linked genes to both.
X-linked Genetic disorders
The alleles for certain conditions are X-linked. These diseases are much more common in men than they are in women due to their X-linked inheritance pattern. Recessive X-linked traits appear more often in males than females because, if a male receives a “bad” allele from his mother, he has no chance of getting a “good” allele from his father (who provides a Y) to hide the bad one. Females will often receive a normal allele from their fathers, preventing the disease allele from being expressed.
Hemizygous
A chromosome in a diploid organism is hemizygous when only one copy is present.
Codominance
A type of inheritance in which two versions (alleles) of the same gene are expressed separately to yield different traits in an individual
Incomplete dominance
Occurs when neither trait is truly dominant over the other. Both traits can be expressed in the same regions, resulting a blending of two phenotypes. Ex. A white and black dog producing a gray offspring.
Pedigree
Circle- female
Square- male
If the trait is dominant, one of the parents must have the trait. Dominant traits will not skip a generation. If the trait is recessive, neither parent is required to have the trait since they can be heterozygous.
Autosomal
Non-sex chromosomes
Linked Genes
They are located close together in the same chromosome and do not follow patterns of independent assortment. They are usually (but not always) inherited together
