Genetic Processes - 1 Flashcards
What is genetic material?
Material in an organism that stores information.
What is DNA? What does it stand for?
Molecules which carry genetic information in cells. Deoxyribonucleic Acid.
What is a gene?
A portion of DNA that carries the information which codes for a specific trait.
Traits of chromosomes
Found in eukaryotic cell nuclei
Contain genes
Most occur in sets; humans have 23 pairs
Chromosomes in mitosis
Before replication, each chromosome is single stranded (one chromatid)
After duplication, the chromosomes are made up of two identical sister chromatids held together by a centromere
During mitosis, sister chromatids will separate from each other
One chromatid will end up in each daughter cell
IMPORTANT NOTE. Sister chromatids are still counted as one chromosome, just as one chromatid is counted as one chromosome. When your cells are getting ready to divide, they have 46 chromosomes and 92 chromatids, because there are two chromatids in each chromosome (double the normal amount)
Why does DNA replicate?
DNA replicates so that, during mitosis, newly formed cells each receive a complete copy of genetic information.
What are karyotypes?
A picture of the number and appearance of chromosomes in the nucleus of a eukaryotic cell.
Differing human karyotypes: sex chromosomes
Human females have two X chromosomes, whereas males have one X and one Y.
Biological sex is assigned at birth and refers to someone’s physical anatomy/genetic characteristics.
What is intersex?
Intersex is the term for people who are not born with a typical reproductive or sexual anatomy.
Some of their cells may have XX while others may have XY, which is called mosaic genetics
What is the purpose of mitosis?
The purpose of mitosis is to maintain genetic continuity. This means the number of chromosomes in each daughter cell stays the same.
What are the parts of interphase?
G1 – Cellular contents, excluding chromosomes, are duplicated
S – Each of the 46 chromosomes is duplicated
G2 – The cell “double checks” for errors in the duplicated chromosomes, fixing any mistakes
What are diploid cells?
The number of chromosomes in body cells is twice the haploid number, or 2n. In humans, this number is 46.
What are haploid cells?
The number of chromosomes in a gamete is called the haploid number, or the n. In humans, this number is 23.
What is a gamete?
A gamete is a reproductive cell of an animal or plant. In animals, female gametes are called ova or egg cells, and male gametes are called sperm.
What are the parts of mitosis, and what happens in each stage?
Prophase
Centrioles move to opposite poles of the cell
Chromatin condenses and shortens into chromosomes
Spindle fibres form between the centrioles
Nuclear membrane begins to disappear
Metaphase
Spindle fibres attached to centromeres pull chromosomes into place
Chromosomes line up across the equator of the cell
Centromeres duplicate
Anaphase
Chromatids separate at their centromeres
Single-stranded chromosomes are now pulled to opposite poles by spindle fibres contracting
Telophase
Two nuclear envelopes form
Single stranded chromosomes uncoil to become chromatin
What happens in cytokinesis?
Organelles are distributed between daughter cells and cell membrane pinches inward
2 Daughter cells are formed
What is the purpose of meiosis?
Purpose is to produce haploid gametes, which each hold half the amount of chromosomes as normal
They unite during sexual reproduction
(the number of chromosomes in each cell is half of the parent’s cell)
Produces 4 cells which are not identical
Sexual vs asexual reproduction
Asexual reproduction
Gives rise to offspring which are completely genetically identical to the parent
Often produces offspring rapidly
Sexual Reproduction
Genetic information from two cells is combined to produce a new organism
Requires more time and energy from the organism
Offspring are genetically different from parents
Offspring are better able to adapt to changing environments
What is a homologous pair (of chromosomes)?
Matching pairs of chromosomes that carry information for the same genes. Similar, but not genetically identical.
What is a sister chromatid?
Refers to either of the two chromatids formed by the replication of a single chromosome; identical pair
What are the stages of meiosis?
(Meiosis I)
Prophase I
Metaphase I
Anaphase I
Telophase I
(Meiosis II)
Prophase II
Metaphase II
Anaphase II
Telophase II
What happens during prophase I in meiosis?
Chromatin condenses and becomes visible
Nuclear membrane disappears
Chromosomes come together in homologous pairs
Each chromosome is referred to as sister chromatids, and all four are referred to as a tetrad because there is four chromatids in a pair of homologous chromosomes
Pieces of the homologous pairs break off and exchange segments with other strands.
This process is called crossing over or genetic recombination
Centrioles duplicate
What happens in metaphase and anaphase I in meiosis?
Metaphase I
Centrioles move to the poles
Spindle fibres attach to the centromeres
Homologous pairs are pulled to the equator by spindle fibres
Anaphase I
Spindle fibres contract, pulling homologous pairs apart
Double stranded pairs are pulled to opposite poles. They are not being pulled into single stranded chromosomes.
Only one chromosome from each pair will be in each cell
What happens in telophase I in meiosis?
The homologous pair of chromosomes are fully separated
The nuclear membrane re-appears
Centrioles and spindle fibres disappear
Sister chromatids stay together, forming two haploid cells. The original cell had 4 chromosomes, and 2 were pulled into each cell.
How does meiosis II look?
Meiosis II proceeds like mitosis, but with half the number of chromosomes.
What is the final product of meiosis?
Cells are not identical because of independent assortment
Homologous chromosomes could have split in any order
The more chromosomes in a cell, the more possibilities come from independent assortment
For any diploid organism, the number of combinations is 2^n.
Also different from genetic recombination
What is meiosis in males and females called?
When meiosis occurs in males it is called spermatogenesis because sperm are produced.
When meiosis occurs in males it is called Oogenesis because ova are produced.
What are autosomal chromosomes?
chromosomes which carry genes for non-sexual characteristics.
What do the sex chromosomes do?
determine an individual’s biological sex and carry genes which code for sexual characteristics.
The Y chromosome carries genes for facial hair, while the X chromosome codes for many non-sexual characteristics. This is because humans are automatically born female (e.g., all “sex characteristics” are added on by the Y chromosome)
What is nondisjunction?
occurs when a homologous pair fails to properly separate during cell division.
This means both move to the same side of the cell, resulting in one daughter cell with an extra chromosome and one with a missing chromosome.
Cells with too much or not enough information cannot function properly.
While nondisjunction can happen in mitosis, it is much more serious in meiosis.
Resulting offspring will have either 45 or 47 chromosomes in each cell.
Trisomy
Having an extra chromosome is known as trisomy, because individuals will have 3 homologous chromosomes in place of a pair
Monosomy
Having a missing chromosome is known as monosomy, because individuals will have one chromosome in place of a pair of a homologous pair.
Down Syndrome
Trisomy results in the disorder Down Syndrome, where there is an extra 21st chromosome. It occurs in 1 in 600 babies.
The chance of giving birth to a child with down syndrome increases as a mother gets older, because the occurrence of nondisjunction in the ova increases with age.
Turner Syndrome
Monosomy where the only sex chromosome inherited is the X chromosome is known as Turner Syndrome.
1 in 3000 female births, with most babies being miscarried.
Klinefelter Syndrome
Trisomy also results in Klinefelter Syndrome, where a baby is born with two X chromosomes and one Y chromosome.
Child appears male at birth but produces female hormones as they mature
Can develop breasts/have little facial hair
1 in 500 male births, with children being sterile.
Heredity definition
The passing of traits from a parent to their offspring.
What is heredity?
You inherit half of your chromosomes from your mother and half from your father (23 each). Your physical traits are a result of this interaction.
Biological traits are coded for by genes which are located on the chromosomes in your body.
A trait is the particular version of a characteristic that is inherited, such as hair or eye colour.
The idea that traits are inherited (heritable) has been known for centuries, and early communities would selectively mate for certain desirable traits to eliminate offspring with undesirable traits.
What was blending theory?
This was the idea that traits from parents would blend together completely. This cannot be true, because it would mean that variation does not occur, but we see that it does.
Who is Gregor Mendel?
Preformed classic experiments using garden pea plants.
Provided the basis of our current understanding of inheritance.
What were Mendel’s experiments?
His first experiment was to crossbreed pure tall and pure short breeds. These plants were known as the parental generation (P)
This experiment is called a monohybrid cross because it focuses on only one inherited characteristic.
His hypothesis was that the offspring of the parental generation was going to be of medium height.
What he found instead was that all offspring, called first filial generation (F1) of the P generation was tall.
His conclusion was that the trait of being tall is dominant, while shortness is recessive, because the recessive trait was not seen at all in the first generation of offspring.
Medel’s Second Experiment:
The offspring of the F1 generation was called F2.
¾ of the F2 ended up being short, even though all of the F1 generation was tall.
His conclusion from this was that offspring inherit both the dominant and recessive traits of their parents, but only the dominant factor is expressed. The recessive trait is still there and has a chance to come out in another generation.
Hereditary characteristics are determined by distinct genes that occur in pairs. These paired factors separate from one another and are distributed into different sex cells.
What is the principal of dominance?
When an organism is crossed for a pair of contrasting traits, it shows only the dominant trait.
What is the law of segregation?
From Mendel’s data, he came to the following conclusions:
For each characteristic, an organism carries two factors; one from each parent.
Parent organisms donate only one copy of each gene in their gametes. During meiosis, the two copies of each gene separates.
What are alleles?
Alleles are different forms of a gene
Different alleles arise by mutation
E.g., The gene for eye colour has different alleles: brown or blue
Dominant alleles are indicated with capital letters, recessive alleles are indicated with lower case letters.
You have two alleles for each trait, one from both parents.
If you inherit a dominant allele from one parent and a recessive allele from your other, the dominant allele will mask the recessive allele. This means you will only show the dominant allele.
If you are showing a recessive trait, it means you inherited two copies of it. If you are showing a dominant trait, you could have inherited one or both copies of it.
Heterozygous vs Homozygous
Homozygous
Having two identical alleles for a gene
AA/aa
Heterozygous
Having two different alleles for a gene
Aa
Genotype vs Phenotype
Genotype
The exact alleles an individual carries.
Someone with brown eyes may have BB or Bb as their genotype.
Phenotype
The observable traits in an offspring.
The phenotype of someone with brown eyes is just brown eyes.
What is the law of independent assortment?
Alleles of different genes assort independently of one another during gamete formation, so different traits are inherited independently of each other.
In other words, the ability to have a dominant allele for one trait doesn’t affect your ability to receive a dominant allele in another, because each chromosome with each gene assorts itself randomly.
What is a Punnett Square?
A Punnett Square is used to predict the proportions of genotypes in offspring resulting from a cross of two individuals. Can focus on one trait, or on many.
Steps to solving a monohybrid cross problem
Determine the possible gametes from each parent
Draw a Punnett Square for each monohybrid cross
Execute the cross
Monohybrid cross
A monohybrid cross focuses on one inherited characteristic.
Fertilization
two gametes joining together to form one cell (zygote) with 2n chromosomes.
Somatic cells
any cells in the body which are not gametes.
Reproduction
producing offspring
Genetic Recombination (/Crossing over)
the rearrangement of DNA sequences by the breakage and rejoining of chromosomes or chromosome segments.
Independent assortment
the alleles of two (or more) different genes get sorted into gametes independently of one another.
Gametogenesis
the process by which gametes are formed for sexual reproduction.
Non disjunction
the failure of the chromosomes to separate, which produces daughter cells with abnormal numbers of chromosomes
