Information

7: The Cellular Basis of Inheritance - Biology

7: The Cellular Basis of Inheritance - Biology



We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

  • 7.1: Sexual Reproduction
    Most eukaryotes undergo sexual reproduction. The variation introduced into the reproductive cells by meiosis appears to be one of the advantages of sexual reproduction. Meiosis and fertilization alternate in sexual life cycles. The process of meiosis produces genetically unique reproductive cells called gametes, which have half the number of chromosomes as the parent cell. Fertilization, the fusion of haploid gametes from two individuals, restores the diploid condition.
  • 7.2: Meiosis
    Sexual reproduction requires that diploid organisms produce haploid cells that can fuse during fertilization to form diploid offspring. The process that results in haploid cells is called meiosis. Meiosis is a series of events that arrange and separate chromosomes into daughter cells. During the interphase of meiosis, each chromosome is duplicated. In meiosis, there are two rounds of nuclear division resulting in four nuclei and usually four haploid daughter cells.
  • 7.3: Errors in Meiosis
    The number, size, shape, and banding pattern of chromosomes make them easily identifiable in a karyogram and allow for the assessment of many chromosomal abnormalities. Disorders in chromosome number, or aneuploidies, are typically lethal to the embryo, although a few trisomic genotypes are viable. Because of X inactivation, aberrations in sex chromosomes typically have milder effects on an individual.

Thumbnail: Image of the mitotic spindle in a human cell showing microtubules in green, chromosomes (DNA) in blue, and kinetochores in red. (Public Domain; Afunguy).


Chapter 7: Introduction to the Cellular Basis of Inheritance

Figure 7.1 Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves Monterey pine trees produce seeds from which Monterey pine seedlings emerge and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same. While many single-celled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method.

Sexual reproduction is the production by parents of haploid cells and the fusion of a haploid cell from each parent to form a single, unique diploid cell. In multicellular organisms, the new diploid cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the haploid cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and fertilization to reproduce.


Chapter 7: Introduction to the Cellular Basis of Inheritance

Figure 7.1 Each of us, like these other large multicellular organisms, begins life as a fertilized egg. After trillions of cell divisions, each of us develops into a complex, multicellular organism. (credit a: modification of work by Frank Wouters credit b: modification of work by Ken Cole, USGS credit c: modification of work by Martin Pettitt)

The ability to reproduce in kind is a basic characteristic of all living things. In kind means that the offspring of any organism closely resembles its parent or parents. Hippopotamuses give birth to hippopotamus calves Monterey pine trees produce seeds from which Monterey pine seedlings emerge and adult flamingos lay eggs that hatch into flamingo chicks. In kind does not generally mean exactly the same. While many single-celled organisms and a few multicellular organisms can produce genetically identical clones of themselves through mitotic cell division, many single-celled organisms and most multicellular organisms reproduce regularly using another method.

Sexual reproduction is the production by parents of haploid cells and the fusion of a haploid cell from each parent to form a single, unique diploid cell. In multicellular organisms, the new diploid cell will then undergo mitotic cell divisions to develop into an adult organism. A type of cell division called meiosis leads to the haploid cells that are part of the sexual reproductive cycle. Sexual reproduction, specifically meiosis and fertilization, introduces variation into offspring that may account for the evolutionary success of sexual reproduction. The vast majority of eukaryotic organisms can or must employ some form of meiosis and fertilization to reproduce.


Welcome to the Living World

- It is the process of polymerisation of amino acids to form a polypeptide based on the sequence of codons in mRNA.

- It takes place in ribosomes. Ribosome consists of structural RNAs and about 80 types of proteins.

- Ribosome also acts as a catalyst (23S rRNA in bacteria is the enzyme- ribozyme) for the formation of peptide bond ( peptidyl transferase enzyme in large subunit of ribosome).

- Translation includes 4 steps:

  1. Charging of tRNA
  2. Initiation
  3. Elongation
  4. Termination

1. Charging (aminoacylation) of tRNA

· Formation of peptide bond needs energy obtained from ATP.

· For this, amino acids are activated (amino acid + ATP) and linked to their cognate tRNA in presence of aminoacyl tRNA synthetase.Thus, the tRNA becomes charged.

· In this, small subunit of ribosome binds to mRNA at the start codon (AUG).

· Now large subunit binds to small subunit to form initiation complex.

· Large subunit consists of aminoacyl tRNA binding site (A site) and peptidyl site (P site).

· The initiator tRNA (which carries methionine) binds on P site. Its anticodon (UAC) recognises start codon AUG.

· Second aminoacyl tRNA binds to the A site of ribosome. Its anticodon binds to the second codon on the mRNA and a peptide bond is formed between first and second amino acids in presence of peptidyl transferase.

· First amino acid and its tRNA are broken. This tRNA is removed from P site and second tRNA from A site is pulled to P site along with mRNA. This is called translocation.

· These processes are repeated for other codons in mRNA.

· During translation, ribosome moves from codon to codon.

· When a release factor binds to stop codon, the translation terminates.

· The polypeptide and tRNA are released from the ribosomes.

· The ribosome dissociates into large and small subunits.

A group of ribosomes associated with a single mRNA for translation is called a polyribosome (polysomes).

An mRNA has additional sequences that are not translated (untranslated regions or UTR). UTRs are present at both 5’-end (before start codon) and 3’-end (after stop codon). They are required for efficient translation process.


Welcome to the Living World

In eukaryotes, gene expression occurs by following levels:

1. Transcriptional level (formation of primary transcript).

2. Processing level (splicing, capping etc.).

3. Transport of mRNA from nucleus to the cytoplasm.

4. Translational level (formation of a polypeptide).

The metabolic, physiological and environmental conditions regulate gene expression. E.g.

ú In E. coli, the beta-galactosidaseenzyme hydrolyses lactose into galactose & glucose. In the absence of lactose, the synthesis of beta-galactosidase stops.

ú The development and differentiation of embryo into adult are a result of the expression of several set of genes.

If a substrate is added to growth medium of bacteria, a set of genes is switched on to metabolize it. It is called induction.

When a metabolite (product) is added, the genes to produce it are turned off. This is called repression.

§ “Each metabolic reaction is controlled by a set of genes”

§ All the genes regulating a metabolic reaction constitute an Operon.E.g. lac operon, trp operon, ara operon, his operon, val operon etc.

Lac Operon in E. coli

- The operon controlling lactose metabolism.

- It is proposed by Francois Jacob & Jacque Monod.

a) A regulatory or inhibitor (i) gene: Codes for repressor protein.

b) 3 structural genes:

i. z gene: Codes for b galactosidase. It hydrolyses lactose to galactose and glucose.

ii. y gene: Codes for permease. It increases permeability of the cell to b -galactosides ( lactose).

iii. a gene: Codes for a transacetylase.

- Genes in the operon function together in the same or related metabolic pathway.

- If there is no lactose (inducer), lac operon remains switched off. The regulator gene synthesizes mRNA to produce repressor protein. This protein binds to the operator region and blocks RNA polymerase movement. So the structural genes are not expressed.

- If lactose or allolactose is provided in the growth medium , it is transported into E. coli cells by the action of permease. Lactose (inducer) binds with repressor protein. So repressor protein cannot bind to operator region. The operator region becomes free and induces the RNA polymerase to bind with promoter. Then transcription starts.

- Regulation of lac operon by repressor is called negative regulation.


Polynucleotides are the polymer of nucleotides. DNA & RNA are polynucleotides. A nucleotide has 3 components:

1. A nitrogenous base.

2. A pentose sugar (ribose in RNA & deoxyribose in DNA).

3. A phosphate group.

Nitrogen bases are 2 types:

> Purines: It includes Adenine (A) and Guanine (G).

> Pyrimidines: It includes Cytosine (C), Thymine (T) & Uracil (U). Thymine (5-methyl Uracil) present only in DNA and Uracil only in RNA.

A nitrogenous base is linked to the OH of 1' C pentose sugar through an N-glycosidic linkage to form nucleoside.

A phosphate group is linked to OH of 5' C of a nucleoside through phosphoester linkage to form nucleotide (or deoxynucleotide).

In RNA, each nucleotide has an additional –OH group at 2' C of the ribose (2’- OH).

2 nucleotides are linked through 3’-5’ phosphodiester bond to form dinucleotide.

When more nucleotides are linked, it forms polynucleotide.

> Friedrich Meischer (1869): Identified DNA and named it as ‘Nuclein’.

> James Watson & Francis Crick (1953) proposed double helix model of DNA. It was based on X-ray diffraction data produced by Maurice Wilkins & Rosalind Franklin.

> DNA is made of 2 polynucleotide chains coiled in a right-handed fashion. Its backbone is formed of sugar & phosphates. The bases project inside.

> The 2 chains have anti-parallel polarity, i.e. one chain has the polarity 5’𔾷’ and the other has 3’𔾹’.

> The bases in 2 strands are paired through H-bonds forming base pairs (bp).

A=T (2 hydrogen bonds) C≡G (3 hydrogen bonds)

> Purine comes opposite to a pyrimidine. This generates uniform distance between the 2 strands.

> Erwin Chargaff’s rule: In DNA, the proportion of A is equal to T and the proportion of G is equal to C.

[A] + [G] = [T] + [C] or [A] + [G] / [T] + [C] =1

v Ф 174 (a bacteriophage) has 5386 nucleotides.

v Bacteriophage lambda has 48502 base pairs (bp).

v E. coli has 4.6x10 6 bp.

v Haploid content of human DNA is 3.3x10 9 bp.

Length of DNA = number of base pairs X distance between two adjacent base pairs.

Number of base pairs in human = 6.6 x 10 9

Hence, the length of DNA = 6.6 x10 9 x 0.34x 10 -9

In E. coli, length of DNA =1.36 mm (1.36 x 10 -3 m)

∴ Therefore the number of base pairs

PACKAGING OF DNA HELIX

§ In prokaryotes (E.g. E. coli), the DNA is not scattered throughout the cell. DNA is negatively charged. So it is held with some positively charged proteins to form nucleoid.

§ In eukaryotes, there is a set of positively charged, basic proteins called histones.

§ Histones are rich in positively charged basic amino acid residues lysines and arginines.

§ 8 histones form histone octamer.

§ Negatively charged DNA is wrapped around histone octamer to give nucleosome.

§ A typical nucleosome contains 200 bp.

Therefore, total number of nucleosomes in human =

6.6 x 10 9 bp 200 = 3.3x 10 7

§ Nucleosomes constitute the repeating unit to form chromatin. Chromatin is the thread-like stained bodies.

§ Nucleosomes in chromatin = ‘beads-on-string’.

§ Chromatin is packaged → chromatin fibres → coiled and condensed at metaphase stage → chromosomes.

§ Higher level packaging of chromatin requires non-histone chromosomal (NHC) proteins.

· Euchromatin: Loosely packed and transcriptionally active region of chromatin. It stains light.

· Heterochromatin: Densely packed and inactive region of chromatin. It stains dark.


Welcome to the Living World

· Replication is the copying of DNA from parental DNA.

· Watson & Crick proposed Semi-conservative model of replication. It suggests that the parental DNA strands act as template for the synthesis of new complementary strands. After replication, each DNA molecule would have one parental and one new strand.

· Matthew Messelson & Franklin Stahl (1958) experimentally proved Semi-conservative model.

Messelson & Stahl’s Experiment

> They grew E. coli in 15 NH4Cl medium ( 15 N = heavy isotope of nitrogen) as the only nitrogen source. As a result, 15 N was incorporated into newly synthesised DNA (heavy DNA or 15 N DNA).

> Heavy DNA can be distinguished from normal DNA (light DNA or 14 N DNA) by centrifugation in a cesium chloride (CsCl) density gradient.

> E. coli cells from 15 N medium were transferred to 14 NH4Cl medium. After one generation (i.e. after 20 minutes), they isolated and centrifuged the DNA. Its density was intermediate (hybrid) between 15 N DNA and 14 N DNA. This shows that in newly formed DNA, one strand is old ( 15 N type) and one strand is new ( 14 N type). This confirms semi-conservative replication.

> After II generation (i.e. after 40 minutes), there was equal amounts of hybrid DNA and light DNA.

Taylor & colleagues (1958) performed similar experiments on Vicia faba(faba beans) using radioactive thymidine to detect distribution of newly synthesized DNA in the chromosomes. It proved that the DNA in chromosomes also replicate semi-conservatively.

The Machinery and Enzymes for Replication

· DNA replication starts at a point called origin (ori).

· A unit of replication with one origin is called a replicon.

· During replication, the 2 strands unwind and separate by breaking H-bonds in presence of an enzyme, Helicase.

· Unwinding of the DNA molecule at a point forms a ‘Y’-shaped structure called replication fork.

Watson-Crick model for semiconservative DNA replication

· The separated strands act as templates for the synthesis of new strands.

· DNA replicates in the 5’𔾷’ direction.

· Deoxyribonucleoside triphosphates (dATP, dGTP, dCTP & dTTP) act as substrate and provide energy for polymerization.

· Firstly, a small RNA primer is synthesized in presence of an enzyme, primase.

· In presence of an enzyme, DNA dependent DNA polymerase, many nucleotides join with one another to primer strand and form a polynucleotide chain (new strand).

· During replication, one strand is formed as a continuous stretch in 5’ 3’ direction (Continuous synthesis). This strand is called leading strand.

· The other strand is formed in small stretches (Okazaki fragments) in 5’ 3’ direction (Discontinuous synthesis).

· The Okazaki fragments are then joined together to form a new strand by an enzyme, DNA ligase. This new strand is called lagging strand.

· If a wrong base is introduced in the new strand, DNA polymerase can do proof reading.

· E. coli completes replication within 18 minutes. i.e. 2000 bp per second.

· In eukaryotes, the replication of DNA takes place at S-phase of the cell cycle. Failure in cell division after DNA replication results in polyploidy.


7: The Cellular Basis of Inheritance - Biology

Biology 109 - Fundamentals of Biology

3 Units
Degree Applicable, CSU, UC

54 hours lecture

An introduction to the fundamentals of biology including concepts of cellular and molecular biology, genetics, reproduction, evolution, plant & animal biodiversity, morphological & physiological adaptation, ecology, ecosystems, and environmental sustainability. This course is intended for non-majors.

Spring 2021 Semester:

Section 91920 - ONLINE (8-Week, 4/12/21 - 6/6/21)
Section 91901 - ONLINE (8-Week, 4/12/21 - 6/6/21)

Section 98278 - ONLINE (8-Week, 6/14/21 - 8/8/21)
Section 98281 - ONLINE (8-Week, 6/14/21 - 8/8/21)
Section 98294 - ONLINE (8-Week, 6/14/21 - 8/8/21)
Section 98295 - ONLINE (8-Week, 6/14/21 - 8/8/21)

In this class, we use the Open Educational Resources (OER) Textbook "Concepts of Biology" provided by OpenStax and Rice University. This is a huge advantage to students since this textbook is available for FREE. To download the complete book, visit http://openstax.org/details/books/concepts-biology

This is the complete chapter list for this course, including links to the individual PDF chapters.


Chapter 1: Introduction to Biology

Figure 1.1 This NASA image is a composite of several satellite-based views of Earth. To make the whole-Earth image, NASA scientists combine observations of different parts of the planet. (credit: modification of work by NASA)

Viewed from space, Earth offers few clues about the diversity of life forms that reside there. The first forms of life on Earth are thought to have been microorganisms that existed for billions of years before plants and animals appeared. The mammals, birds, and flowers so familiar to us are all relatively recent, originating 130 to 200 million years ago. Humans have inhabited this planet for only the last 2.5 million years, and only in the last 200,000 years have humans started looking like we do today.

Introduction to Interactive Learning

The goal of interactive learning is to promote engagement with and retention of the concepts and information being studied. The interactive learning activities in this open textbook support self directed practice and are not recorded assessments. Use the interactive learning activities to:


5.6 Chromosomal Inheritance Overview

Genetic heterochromia – a condition that causes a person’s eyes to be 2 different colors – is a result of mutated alleles in genes that control the distribution of melanin in the iris. With uneven distribution of this pigment molecule in each eye, each eye appears a different color.

While genetic heterochromia is a visually-striking condition, it is hardly the most severe genetic disorder that can be transmitted via chromosomes. In fact, genetic disorders of all kinds can be inherited on the chromosomes you get from your parents. Some of these disorders are caused by mutated alleles in specific genes, while others are caused by a disruption in the distribution of chromosomes during meiosis. The AP test will likely ask you about genetic disorders in one way or another. So, stick with us as we cover everything you need to know about chromosomal inheritance and genetic disorders!

Since we’ve already covered much of this information in previous videos, let’s quickly review chromosomal inheritance.

Chromosomal inheritance refers to the fact that genes, each one made of many nucleotides that carry genetic information in their sequence, are carried together on chromosomes. Each chromosome is duplicated during DNA replication, leading to two sister chromatids bound at the centromere.

In diploid organisms, each chromosome has a homolog from a different parental source that carries different alleles of the same set of genes. The process of meiosis first separates these homologous chromosomes, before separating the sister chromatids. This results in gametes that are haploid and carry only 1 copy of each chromosome. You can review this process in our video on section 5.1.

Keep in mind that different species have a different number of chromosomes, carrying a different number of genes. For example, the fruit fly has 4 chromosomes carrying 14,000 genes. By contrast, humans have 23 chromosomes carrying 22,000 genes. But, don’t think that chromosome number or number of genes is an indication of complexity. The adder’s tongue fern, for instance, has over 700 chromosomes.

In our previous video on section 5.3 we covered Mendelian genetics and showed how specific alleles can cause specific phenotypes. Unfortunately, specific alleles can also cause genetic diseases, as well. In fact, genetic disorders caused by mutated alleles often show the exact same genotypic and phenotypic ratios we would expect to see in any other genetic trait.

Let’s consider an autosomal recessive disorder. Just like an autosomal recessive trait, the only offspring that actually show the symptoms of an autosomal recessive disorder are those that receive two mutated, disease-causing alleles. Offspring that only receive one mutated allele are known as “carriers” – they have the ability to pass the disorder onto the next generation, but do not show symptoms themselves. So, in a cross between two carrier parents, 25% of the children will be affected by the disorder, 50% of the children will be carriers of the disorder, and only 25% of the children will be completely free of disease-causing alleles.

A good example of an autosomal recessive disorder is cystic fibrosis. The affected gene in cystic fibrosis normally codes for a protein involved with transporting chloride ions across cell membranes throughout the body. When both alleles for this gene produce dysfunctional versions of the protein, chloride ions build up in the intermembrane spaces and cause the formation of a thick mucous layer in the lungs, pancreas, and other body tissues. This leads to a myriad of symptoms, including difficulty breathing and blocked pancreatic ducts. While the disease used to be fatal within the first few months of life, modern treatments and medications have greatly extended the life of cystic fibrosis patients and future genetic therapies may be able to cure the disorder!

Now, let’s take a quick look at an autosomal dominant disorder like Huntington’s disease. As with normal complete dominance, even 1 dominant allele leads to disease symptoms. So, if a heterozygous affected parent and a normal, homozygous recessive individual create offspring, half of the offspring will be affected, while half of the offspring will not inherit any disease alleles. In a disease like Huntington’s disease which has a slow onset of symptoms, an affected individual may not know they have the disease until late in life. This is why genetic testing and pedigree analysis can be useful tools!

For the AP test, you will need to understand how to read a pedigree and determine the type of genetic condition that is present. Let’s start with the basics.

A square represents a male, while a circle represents a female. Horizontal lines between two individuals represent a pair of individuals making offspring, whereas vertical lines and horizontal lines above individuals represent sibling relationships. This pedigree shows the exact genotype of each individual. This is helpful in determining the exact type of inheritance pattern of a genetic disorder. Keep in mind that some pedigrees only show phenotype, which can be a little more difficult to decipher.

A pedigree looking at a genetic disease will also show you the affected individuals. This is how you know what genotypes cause a particular disorder. In this case, we can easily see that the only affected individual is a homozygous recessive one. Since none of the heterozygotes are affected, we know that this must be a recessive disorder. Since both males and females can be carriers, we also know that this is an autosomal disorder – if the disorder was on the Y chromosome, females could not carry it, and if the disorder was on the X chromosome, males could either be affected or not have the disease at all. Thus, we know that this particular pedigree shows an autosomal recessive disorder.

Let’s take a look at another example. This pedigree offers a few clues that can help us quickly identify the inheritance pattern of this disorder. First, we see that a cross between an affected male and a regular female produces many affected offspring. However, this pedigree gives us a second clue. A cross between two wild-type individuals produces no affected individuals.

So, let’s test a couple of different options. The affected individuals could be homozygous recessive. In this scenario, the wild-type individual would carry a dominant allele. Since affected offspring were produced, we would have to assume that the wild-type individual was heterozygous. With this arrangement of alleles, we would expect the offspring to show a 1:1 ratio of affected to wild type. The ratios are close to this, so we can’t rule it out. However, this could also be an autosomal dominant disorder, since similar ratios would be seen if the first affected male was heterozygous. Therefore, this pedigree could show either autosomal recessive or autosomal dominant patterns of inheritance. Our final clue, the fact that the trait is seen in every generation, suggests this pedigree shows a dominant trait. But, more testing or a larger pedigree would be needed to confirm this.

Finally, let’s see what a pedigree looks like when it shows a sex-linked pattern of inheritance. This pedigree shows a recessive condition carried on the X-chromosome. We can assume that this is an x-linked, recessive condition because males cannot be carriers, only affected or unaffected. This makes sense because each male gets only 1 X-chromosome. Further, we also see that an affected male and wild-type female produce only carrier females and healthy males. This suggests that the Y chromosome is not involved and that the females must inherit the broken allele on the X-chromosome from their father.

While the genetic disorders we’ve looked at previously all involved mutated alleles, other genetic disorders are not caused by mutated alleles. Rather, they are caused by abnormal meiosis events that lead to too many chromosomes in the resulting gamete. These are known as nondisjunction events, and they can happen in meiosis I or meiosis II. If a gamete with 2 copies of the same chromosome merges with a normal chromosome during fertilization, this can lead to 3 copies of the same chromosome in a single zygote – a condition known as trisomy. Trisomy 21, for example, leads to the symptoms seen in patients with Down Syndrome.

However, there are many different kinds of nondisjunction events that can lead to a wide variety of genetic disorders. For example, if a nondisjunction event leads to a gamete without a certain chromosome, this can lead to only 1 copy of the chromosome in the offspring. Turner’s syndrome is one example, where patients have only 1 X chromosome instead of 2. A similar genetic disorder is Klinefelter’s disorder, a condition caused by 2 X chromosomes and one Y chromosome.

In humans, these nondisjunction events often lead to genetic disorders. This is not true of all organisms, especially when the nondisjunction leads to organisms that have more than 2 copies of the genome. For example, nearly 80% of flowering plants evolved from a nondisjunction event that doubled the ploidy level of the resulting offspring, leading to a new species. However, many agriculture crops are produced using nondisjunction events that lead to sterile seed. Farmers sell these sterile fruits and vegetables so people can’t simply replant the seeds they buy and grow the specific varieties of fruit that farmers have carefully bred over the years for various traits.


Watch the video: OpenStax Concepts of Biology Ch 7 The Cellular Basis of Inheritance (August 2022).