Reproduction

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Apuntes de la lección

Sexual Reproduction

Sexual reproduction involves the fusion of the nuclei of two gametes to form a zygote (fertilised egg cell), producing offspring that are genetically different from each other.
Fertilisation is the fusion of gamete nuclei; because each gamete comes from a different parent, there is variation in the offspring.
A gamete is a sex cell: in animals, sperm and egg (ovum); in plants, pollen nucleus and egg (ovum).
Gametes contain half the number of chromosomes of normal body cells, so they have a haploid nucleus (one copy of each chromosome).
In humans, a normal body cell contains 46 chromosomes, but each gamete contains 23 chromosomes.
When male and female gametes fuse, the zygote contains the full 46 chromosomes (half from each parent) and is described as diploid.

During sexual reproduction, a sperm fertilizes an egg.

Asexual Reproduction

Asexual reproduction produces genetically identical offspring from one parent.
It does not involve gametes or fertilisation, so there is no mixing of genetic information.
The offspring are clones – genetically identical to the parent and to each other.
Many plants reproduce asexually.
Bacteria produce exact genetic copies of themselves by a type of asexual reproduction called binary fission.

Comparing Sexual and Asexual Reproduction

Sexual reproduction involves two parents; asexual reproduction involves one parent.
Sexual reproduction produces offspring that are genetically different; asexual reproduction produces genetically identical offspring.
Sexual reproduction involves gametes and fertilisation; asexual reproduction does not.
Sexual reproduction creates genetic variation in offspring; asexual reproduction creates no genetic variation (except mutations).
Asexual reproduction can produce many offspring quickly; sexual reproduction is generally slower and produces fewer offspring.

The Role of Meiosis

Cells in reproductive organs divide by meiosis to form gametes (sex cells).
Meiosis is a reduction division: the chromosome number is halved from diploid to haploid.
The chromosome number must be halved so that after fertilisation the zygote has the correct diploid number.
Meiosis involves two divisions, producing four haploid daughter cells.
Meiosis produces genetic variation by forming new combinations of maternal and paternal chromosomes in each gamete.
When gametes fuse randomly at fertilisation, each offspring is genetically different from any other.

Meiosis

DNA and the Genome

The genome is the entire set of genetic material of an organism.
The Human Genome Project was completed in 2003 after a 13-year project to sequence the whole human genome.
A gene is a section of DNA that codes for a particular sequence of amino acids, which form proteins.
DNA (deoxyribonucleic acid) is found in the nucleus of a cell and is a polymer made of two strands coiled into a double helix.
DNA is made of repeating subunits called nucleotides; each nucleotide has a common sugar and phosphate group with one of four bases attached.
The four bases are Adenine (A), Cytosine (C), Thymine (T) and Guanine (G).
Bases pair up by complementary base pairing: A–T and C–G, held together by weak hydrogen bonds.

DNA, genes and chromosomes: genes are sections of chromosomes, and chromosomes are made of DNA.

Protein Synthesis

A gene is a section of DNA with a particular sequence of bases that codes for a particular sequence of amino acids.
A sequence of three bases codes for a single specific amino acid.
The order of bases controls the order of amino acids joined together in a polypeptide chain, which folds to form a protein.
Transcription occurs in the nucleus: DNA unwinds, and RNA polymerase makes a complementary mRNA copy of the gene.
Translation occurs in the cytoplasm: mRNA attaches to a ribosome, which reads the code in groups of three bases (codons).
tRNA molecules bring specific amino acids to the ribosome; their anticodons pair with complementary codons on mRNA.
Peptide bonds form between amino acids until a stop codon is reached, completing the polypeptide chain.

Genetic Variants and Mutations

A mutation is a random change to the DNA base sequence.
Genetic variants are different versions of genes caused by mutations.
Mutations in non-coding DNA can affect phenotype by influencing the binding of RNA polymerase, altering the quantity of protein produced.
Mutations in coding DNA may change the sequence of amino acids, altering the shape and function of the protein.
Insertions, deletions and substitutions are types of mutation; insertions and deletions have a knock-on effect on later base triplets.
Mutations can be caused by ionising radiation (gamma rays, X-rays, UV) and certain chemicals (e.g. tar in tobacco).
Sickle cell anaemia is caused by a gene mutation that changes haemoglobin, making red blood cells stiff and sickle-shaped.

Key Definitions in Inheritance

A gene is a short length of DNA on a chromosome that codes for a particular characteristic.
Alleles are different versions of the same gene; we inherit two alleles for each gene, one from each parent.
The genotype is the combination of alleles an organism has for a characteristic.
The phenotype is the observable characteristics of an organism.
A dominant allele only needs to be inherited from one parent for the characteristic to show.
A recessive allele must be inherited from both parents for the characteristic to show.
Homozygous means two identical alleles for a gene; heterozygous means two different alleles.

Predicting Genetic Inheritance

Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene.
A Punnett square shows the possible combinations of alleles in offspring and can be used to work out ratios and probabilities.
In a cross between two heterozygous parents (Tt × Tt), the expected ratio is 3:1 (dominant : recessive).
The probability of an offspring showing the dominant phenotype is 75%; the recessive phenotype is 25%.
Family pedigree diagrams trace the inheritance of a characteristic through generations and can be used to work out the probability of inheriting a genetic disorder.
In pedigree diagrams, males are squares and females are circles; affected individuals are often shaded.

Inheritance of Sex and Other Patterns

Sex is determined by an entire chromosome pair: females have XX and males have XY.
The father determines the sex of the child because only he can pass on a Y chromosome.
Sperm cells determine the sex of the offspring.
Codominance occurs when both alleles in a genotype are expressed in the phenotype (e.g. blood group inheritance).
Sex-linked characteristics are controlled by alleles on the sex chromosomes; most are on the X chromosome.
Males are more likely to show sex-linked recessive conditions (e.g. red-green colour blindness, haemophilia) because they have only one X chromosome.
Polygenic inheritance involves characteristics controlled by more than one gene, giving a wide range of phenotypes (e.g. eye colour).

Variation

Variation is the differences between individuals of the same species.
Continuous variation shows many small degrees of difference and can be measured on a scale (e.g. height, mass); graphs give a smooth bell curve.
Discontinuous variation shows distinct differences with no in-betweens (e.g. blood group, sex, tongue rolling); graphs give a step-like shape.
Variation can be caused by genes, the environment, or a combination of both.
Genetic variation arises from meiosis and the random fusion of gametes at fertilisation.
Environmental variation is caused by factors outside the organism, such as climate, diet, lifestyle and accidents.
Discontinuous variation is usually caused by genetic variation alone; continuous features often result from genetic and environmental causes together.

A group of dogs of different breeds, illustrating genetic variation within a species.

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Preguntas de práctica

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1.How many chromosomes are contained in a human gamete?
Easy
- A46
- B48
- C24
- D23
2.When the gametes fuse together at fertilisation, the new cell that is produced is genetically different to each of the parents' cells. What is the new cell called?
Easy
- AEmbryo
- BZygote
- CGamete
- DClone
3.In 1953, scientists Watson and Crick published their work on the structure of DNA. What term is used to describe the shape of DNA?
Easy
- ADouble helix
- BSingle strand
- CPolypeptide
- DNucleotide
4.What is the definition of the word allele?
Medium
- AA section of DNA that codes for a protein
- BThe physical appearance of an organism
- CA variation of a gene
- DThe combination of alleles in an organism
5.What is a genotype?
Medium
- AThe physical appearance of an organism
- BA section of DNA that codes for a protein
- CThe alleles present in an organism for a particular gene
- DA change in the DNA sequence
6.Which statement correctly describes asexual reproduction?
Easy
- AIt involves the fusion of gamete nuclei
- BIt produces genetically identical offspring from one parent
- CIt always requires two parents
- DIt produces offspring that are genetically different from each other
7.Which of the following statements about meiosis are correct? (Select all that apply)
Medium
- AIt produces four daughter cells
- BIt produces gametes
- CIt produces genetically identical cells
- DThe daughter cells contain half the number of chromosomes as the parent cell
- EIt occurs during asexual reproduction
8.Which of the following are advantages of sexual reproduction? (Select all that apply)
Medium
- AIt produces genetic variation in the offspring
- BIt requires only one parent
- CIt produces offspring that may be better adapted to a changing environment
- DIt is a fast method of reproduction
- EIt produces genetically identical offspring

Más preguntas

9.In humans, a normal body cell contains 46 chromosomes and each gamete contains 23 chromosomes.
Easy
True or false?
10.In DNA, adenine always pairs with guanine and cytosine always pairs with thymine.
Easy
True or false?
11.A recessive allele only needs to be inherited from one parent for the characteristic to show up in the phenotype.
Easy
True or false?
12.Match each genetic term to its correct definition.
Medium
- Gene
- Allele
- Genotype
- Phenotype
- The alleles present in an organism for a particular gene
- A section of DNA that codes for a particular characteristic
- The observable characteristics of an organism
- A variation of a gene
13.Place the stages of protein synthesis in the correct order.
Medium
- DNA unwinds and the hydrogen bonds between complementary bases break
- A complementary copy of the gene is made as mRNA
- mRNA leaves the nucleus through a nuclear pore
- mRNA attaches to a ribosome in the cytoplasm
- tRNA molecules bring amino acids to the ribosome and a polypeptide is formed
14.Meiosis produces four daughter cells.
Easy
True or false?
15.Mitosis is the type of cell division used to make gametes.
Easy
True or false?
16.Which type of cell division produces daughter cells that contain half the number of chromosomes as the parent cell?
Medium
- AMitosis
- BMeiosis
- CBinary fission
- DFertilisation
17.Which base pairs with adenine (A) in DNA?
Medium
- AGuanine (G)
- BCytosine (C)
- CThymine (T)
- DUracil (U)
18.How many bases code for one amino acid?
Medium
- A2
- B3
- C4
- D6
19.Which of the following statements describe asexual reproduction? (select all that apply)
Medium
- AIt does not involve gametes or fertilisation
- BIt requires two parents
- CIt produces genetically identical offspring
- DIt produces offspring that are genetically different from each other
- EIt involves the fusion of gamete nuclei
20.Match each reproductive process to the correct description.
Medium
- Sexual reproduction
- Asexual reproduction
- Binary fission
- Fertilisation
- Fusion of gamete nuclei to form a zygote
- Reproduction involving one parent and no gametes
- A type of asexual reproduction in bacteria
21.Place the stages of meiosis in the correct order.
Medium
- Chromosomes double themselves and line up in the centre of the cell
- The cell divides for the first time
- The cell divides for the second time
- Four haploid daughter cells are produced
22.Which of the following are advantages of asexual reproduction? (select all that apply)
Medium
- AOnly one parent is required
- BOffspring are genetically identical to the parent
- CIt produces genetic variation in the offspring
- DIt can produce many offspring quickly
- EIt requires two parents
23.Which of the following statements about meiosis is correct?
Medium
- AIt produces two diploid daughter cells
- BIt produces four haploid daughter cells
- CIt produces two haploid daughter cells
- DIt produces four diploid daughter cells
24.A human body cell contains 46 chromosomes. How many chromosomes are there in a human sperm cell?
Medium
- A46
- B23
- C92
- D22
25.Meiosis produces four daughter cells that are genetically identical to each other.
Easy
True or false?
26.Which of the following statements about DNA structure are correct? (select all that apply)
Medium
- ADNA is a polymer made of nucleotides
- BThe two strands of DNA form a double helix
- CAdenine pairs with guanine
- DThe sugar and phosphate groups form the backbone of each strand
- EThe bases pair by forming strong covalent bonds
27.Match each component of a nucleotide to its correct description.
Medium
- Deoxyribose sugar
- Phosphate group
- Base
- Hydrogen bond
- Part of the backbone of DNA
- One of four types: A, T, C or G
- Holds complementary base pairs together
28.In a sample of DNA, 30% of the bases are adenine. What percentage of the bases are guanine?
Medium
- A20%
- B30%
- C40%
- D70%
29.What is a gene?
Easy
- AA section of DNA that codes for a particular sequence of amino acids
- BA type of protein
- CA whole chromosome
- DA molecule of mRNA
30.Which of the following correctly describes the role of mRNA in protein synthesis?
Hard
- AIt carries the genetic code from the nucleus to the ribosome
- BIt carries amino acids to the ribosome
- CIt forms the structure of the ribosome
- DIt permanently replaces DNA in the nucleus
31.Which of the following statements about the genetic code is correct?
Medium
- AEach amino acid is coded for by a sequence of three bases
- BEach amino acid is coded for by a sequence of two bases
- CEach amino acid is coded for by a sequence of four bases
- DEach base codes for one amino acid
32.Which of the following statements about RNA are correct? (select all that apply)
Medium
- ARNA is single-stranded
- BRNA contains the base uracil instead of thymine
- CRNA contains the sugar ribose
- DRNA is double-stranded
- ERNA contains the base thymine
33.What is a mutation?
Medium
- AA random change in the DNA base sequence
- BA type of protein
- CA change in the phenotype of an organism
- DA section of DNA that codes for a protein
34.A mutation in the non-coding DNA just in front of a gene makes it more difficult for RNA polymerase to bind. What effect will this have?
Hard
- ALess mRNA will be transcribed and less protein will be produced
- BMore mRNA will be transcribed and more protein will be produced
- CThe gene will code for a different amino acid sequence
- DNo protein will be produced at all
35.Which of the following best explains why a mutation in coding DNA can affect the function of an enzyme?
Hard
- AIt changes the amino acid sequence, which can alter the shape of the active site
- BIt changes the number of chromosomes in the cell
- CIt always stops the enzyme from being produced
- DIt changes the type of sugar in the DNA backbone
36.Which of the following are examples of environmental variation? (select all that apply)
Medium
- AScarring from an accident
- BWeight gain from an inactive lifestyle
- CBlood group
- DSpeaking with a particular accent
- EEye colour
37.Which of the following is an example of discontinuous variation?
Medium
- ABlood group
- BHeight
- CMass
- DFinger length
38.Which of the following statements about mutations is correct?
Medium
- AMost mutations do not alter the protein or only alter it slightly
- BAll mutations cause a change in the phenotype
- CMutations only occur in non-coding DNA
- DMutations always lead to the development of new alleles
39.A student completes Table 1 comparing mitosis and meiosis. Which of the following statements describe meiosis rather than mitosis? (Select all that apply)
Medium
- AHow gametes are made
- BProduces four daughter cells
- CHow fertilised egg cells divide
- DDaughter cells contain half the number of chromosomes as the parent cell
- ERequired for growth
40.The DNA in one human body cell is made up of 6 000 million nucleotides. Each nucleotide is 0.34 nanometres (nm) long. What is the length of the DNA in the cell, in millions of nanometres?
Medium
- A2040 million nm
- B204 million nm
- C20 400 million nm
- D2040 million cm
41.Complete the sentences about reproduction using the correct terms.
Easy
Sexual reproduction involves the fusion of the nuclei of two ____ to form a ____, which is also known as a fertilised egg cell.
42.Complete the sentences about the structure of DNA by choosing the correct base in each case.
Medium
In DNA, adenine pairs with ____ and cytosine pairs with ____.
43.What is the term used to describe an organism that has two identical alleles for a particular gene?
Medium
44.What is the name of the process by which bacteria produce exact genetic copies of themselves?
Medium
45.The DNA in one human body cell is made up of 6 000 million nucleotides. Each nucleotide is 0.34 nanometres (nm) long. Calculate the length of the DNA in the cell in millions of nanometres.
Medium
46.Complete the sentence about a change in the DNA code.
Medium
If the DNA code changes this is called a ____ and can result in a different protein being made.
47.Using the equation: Probability of a girl with haemochromatosis = probability of haemochromatosis × probability of being a girl. If the probability of a child having haemochromatosis is 0.25, what is the probability that the couple's next child will be a girl with haemochromatosis?
Medium
48.A gene codes for a protein that is 150 amino acids long. How many bases are needed to code for this protein?
Medium
49.Explain why offspring produced by sexual reproduction show more variation than offspring produced by asexual reproduction.
Medium
50.Briefly describe how a protein is synthesised from the instructions coded in the DNA.
Hard
51.Explain why DNA must first be transcribed into mRNA before it can be translated into a protein.
Hard
52.Explain how a mutation in non-coding DNA could affect an organism's phenotype.
Hard
53.Explain how a mutation in coding DNA could lead to a different amino acid being coded for and how this might affect the function of an enzyme.
Hard
54.The DNA in one human body cell is made up of 6 000 million nucleotides. Each nucleotide is 0.34 nanometres (nm) long. Calculate the length of the DNA in the cell in millions of nanometres.
Medium
- A2244 million nm
- B2448 million nm
- C1836 million nm
- D2040 million nm
55.Using the equation: Probability of a girl with haemochromatosis = probability of haemochromatosis × probability of being a girl. If the probability of a child having haemochromatosis is 0.25, what is the probability that the couple's next child will be a girl with haemochromatosis?
Medium
- A2.125
- B0.125
- C-0.875
- D1.125
56.A gene codes for a protein that is 150 amino acids long. How many bases are needed to code for this protein?
Medium
- A540
- B495
- C450
- D405

Tarjetas

What is sexual reproduction?

The process involving the fusion of the nuclei of two gametes to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other.

What is a gamete?

A sex cell, such as sperm and egg cells in animals, or pollen nucleus and ovum in plants.

How many chromosomes are in a human gamete?

23 chromosomes.

What is a zygote?

A fertilised egg cell, formed when the male and female gametes fuse. It contains the full 46 chromosomes, half from each parent.

What is asexual reproduction?

The process resulting in genetically identical offspring being produced from one parent, without gametes or fertilisation.

What is binary fission?

A type of asexual reproduction in bacteria where they produce exact genetic copies of themselves.

What is meiosis?

A type of cell division that produces four haploid daughter cells (gametes) with half the number of chromosomes, increasing genetic variation.

Why is meiosis important?

It produces gametes (e.g., sperm and egg cells) and increases genetic variation by forming new combinations of maternal and paternal chromosomes.

What is the genome?

The entire set of genetic material of an organism.

What is a gene?

A section of DNA that codes for a particular sequence of amino acids, which form proteins.

What is the structure of DNA?

A polymer made of two strands coiled around each other to form a double helix.

What is a nucleotide?

The individual subunit of DNA, consisting of a common sugar and phosphate group with one of four different bases attached to the sugar.

What are the complementary base pairs in DNA?

Adenine pairs with Thymine (A-T) and Cytosine pairs with Guanine (C-G).

What is the role of mRNA in protein synthesis?

mRNA is a transcript copy of a gene that carries the code from the DNA in the nucleus to the ribosome in the cytoplasm for translation.

What is transcription?

The process where DNA is transcribed to produce an mRNA molecule, occurring in the nucleus.

What is translation?

The process where mRNA is translated by a ribosome to produce an amino acid sequence, occurring in the cytoplasm.

What is a mutation?

A rare, random change in the sequence of DNA bases in a gene or a chromosome.

What is an allele?

A variation of the same gene. We inherit two alleles for each gene, one from each parent.

What is genotype?

The combination of alleles that control each characteristic.

What is phenotype?

The observable characteristics of an organism, such as eye colour or blood type.

What does homozygous mean?

When the two alleles of a gene are the same (e.g., both dominant or both recessive).

What does heterozygous mean?

When the two alleles of a gene are different.

How is sex determined in humans?

Females have XX sex chromosomes and males have XY. The father determines the sex of the child because only he can pass on a Y chromosome.

What is codominance?

When both alleles within a genotype are expressed in the phenotype. An example is human blood group inheritance.

What is meant by sex-linked characteristics?

Characteristics controlled by alleles found on the sex chromosomes, usually the X chromosome. Males are more likely to show recessive sex-linked conditions.

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